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FEATURES
n n n n n n n n n n n n n n
LTC3775 High Frequency Synchronous Step-Down Voltage Mode DC/DC Controller DESCRIPTION
The LTC(R)3775 is a high efficiency synchronous step-down switching DC/DC controller that drives an all N-channel power MOSFET stage from a 4.5V to 38V input supply voltage. A patented line feedforward compensation circuit and a high bandwidth error amplifier provide very fast line and load transient response. High step-down ratios are made possible by a low 30ns minimum on-time, allowing extremely low duty cycles MOSFET RDS(ON) current sensing maximizes efficiency. Alternatively, a sense resistor can be used for higher current limit accuracy. Continuous monitoring of the voltages across the top and bottom MOSFETs allows cycle-by-cycle control of the inductor current, configurable by external resistors. The soft-start function controls the duty cycle during start-up, providing a smooth output voltage ramp up. The operating frequency is user programmable from 250kHz to 1MHz and can be synchronized to an external clock.
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. Protected by U.S. Patents, including 5408150, 5481178, 5705919, 6580258, 5847554, 5055767.
Wide VIN Range: 4.5V to 38V Line Feedforward Compensation Low Minimum On-Time: tON(MIN) < 30ns Powerful Onboard MOSFET Drivers Leading Edge Modulation Voltage Mode Control 0.75%, 0.6V Reference Voltage Accuracy Over Temperature VOUT Range: 0.6V to 0.8VIN Programmable, Cycle-by-Cycle Peak Current Limit Sense Resistor or RDS(ON) Current Sensing Programmable Soft-Start Synchronizable Fixed Frequency from 250kHz to 1MHz Selectable Pulse-Skipping or Forced Continuous Modes of Operation Low Shutdown Current: 14A Typical Thermally Enhanced 3mm x 3mm QFN Package
APPLICATIONS
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Automotive Systems Telecom and Industrial Power Supplies Point of Load Applications
TYPICAL APPLICATION
330F 35V 3.16k 57.6k 4.7F 0.01F 39.2k ILIMT ILIMB VIN TG 0.1F 0.36H VOUT 1.2V 470F 15A 2.5V 2 VIN 5V TO 28V
Efficiency and Power Loss vs Load Current
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 POWER LOSS 1 EFFICIENCY 2 3 VIN = 12V VOUT = 1.2V CONTINUOUS MODE SW FREQ = 500kHz 5
4 POWER LOSS (W)
SENSE LTC3775 INTVCC BOOST SW SS FREQ FB BG PGND MODE/SYNC COMP RUN/SHDN SGND
330pF 3.9nF
10 0 0.01
3775 TA01a
10k
10k
4.7k
0.1
1 10 LOAD CURRENT (A)
0 100
3775 TA01b
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LTC3775 ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW MODE/SYNC RUN/SHDN BOOST
Supply Voltage VIN ......................................................... -0.3V to 40V BOOST ................................................... -0.3V to 46V BOOST-SW............................................... -0.3V to 6V SW ............................................................ -5V to 40V ILIMT .............................................................-0.3V to VIN SENSE.............................................................-5V to VIN INTVCC ......................................................... -0.3V to 6V RUN/SHDN................................................... -0.3V to 6V FB, MODE/SYNC ................................... -0.3V to INTVCC FREQ, ILIMB, SS..................................... -0.3V to INTVCC INTVCC RMS Currents... .........................................50mA Operating Junction Temperature Range (Note 2).................................................. -40C to 125C Storage Temperature Range................... -65C to 150C
16 15 14 13 ILIMT 1 ILIMB 2 FB 3 COMP 4 5 SS 6 FREQ 7 SGND 8 BG 17 PGND 12 SW 11 VIN 10 SENSE 9 INTVCC
UD PACKAGE 16-LEAD (3mm 3mm) PLASTIC QFN TJMAX = 125C, JA = 68C/W, JC = 4.2C/W (NOTE 3) EXPOSED PAD (PIN 17) IS PGND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH LTC3775EUD#PBF LTC3775IUD#PBF TAPE AND REEL LTC3775EUD#TRPBF LTC3775IUD#TRPBF PART MARKING* LDJK LDJK PACKAGE DESCRIPTION 16-Lead (3mm x 3mm) Plastic QFN 16-Lead (3mm x 3mm) Plastic QFN TEMPERATURE RANGE -40C to 85C -40C to 125C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. Consult LTC Marketing for information on non-standard lead based finish parts. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
TG
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LTC3775 ELECTRICAL CHARACTERISTICS
SYMBOL Input Supply VIN IVIN RUN/SHDN Pin VRUN VSHDN VSHDN(HYST) IRUN Error Amplifier VFB VFB VOUT IFB ICOMP f0dB Soft-Start ISS RSS Current Limit ILIMB ILIMT ISENSE VILIMT(MAX) VILIMB(MAX) INTVCC VINTVCC(LINE) ILIMB Source Current ILIMT Sink Current SENSE Pin Input Current Topside Current Limit Threshold (VIN-SENSE) VILIMT = 0.1V
l l l l
The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25C. VIN = 12V, VRUN = 5V, unless otherwise specified.
PARAMETER VIN Supply Voltage Input DC Supply Current VFB = 0.7V (Note 5) VRUN = 0V 1.19 VRUN/SHDN Rising VRUN/SHDN = 0V VRUN/SHDN = 1.5V 0.597 0.5955 CONDITIONS
l
MIN 4.5
TYP
MAX 38
UNITS V mA A
3.5 14 1.22 0.74 140 -1 -4 0.600 0.01 0.01 -50 -0.5 1 -1 25 -1 1.3 -9 90 90 80 4.9 -10 100 100 100 5.2 0.01 -1 3.0 -0.1 0.35 3.6 0.5 4.2 -11 110 1 110 120 5.5 0.1 50 0.603 0.6045 1.25
RUN/SHDN Pin Enable Threshold RUN/SHDN Pin Shutdown Threshold RUN/SHDN Pin Shutdown Threshold Hysteresis RUN/SHDN Pin Source Current
V V mV A A V V %/V % nA mA mA MHz A k A A A mV mV V %/V % V V V
Feedback Pin Voltage Feedback Voltage Line Regulation Output Voltage Load Regulation FB Pin Input Current COMP Pin Output Current Error Amplifier Unity-Gain Crossover Frequency SS Pin Source Current SS Pin Pull-Down Resistance in Current Limit 4.5V < VIN < 38V 1V < VCOMP < 2V (Note 6) VFB = 0.6V Sourcing Sinking (Note 6) VSS = 0V
l
Bottom Side Current Limit Threshold (PGND-SW) VILIMB = 0.5V LDO Regulator Output Voltage INTVCC Line Regulation 7.5V < VIN < 38V
INTVCC Low Dropout Voltage Regulator
VINTVCC(LOAD) INTVCC Load Regulation IINTVCC = 0mA to 20mA INTVCC Regulator Dropout Voltage (VIN - VINTVCC) IINTVCC = 20mA VDROPOUT VUVLO INTVCC UVLO Voltage INTVCC Rising Hysteresis
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LTC3775 ELECTRICAL CHARACTERISTICS
SYMBOL Oscillator fOSC fHIGH fLOW fSYNC tON(MIN) tOFF(MIN) DCMAX VMODE VMODE(HYST) RMODE/SYNC Driver BG RUP TG RUP BG RDOWN TG RDOWN BG, TG t2D TG, BG t1D Bottom Gate (BG) Pull-Up On-Resistance Top Gate (TG) Pull-Up On-Resistance Bottom Gate (BG) Pull-Down On-Resistance Top Gate (TG) Pull-Down On-Resistance Bottom Gate Off to Top Gate On Delay Top Switch-On Delay Time Top Gate Off to Bottom Gate On Delay Synchronous Switch-On Delay Time CL = 3300pF (Note 7) CL = 3300pF (Note 7) 2.5 2.5 1.0 1.5 15 15 ns ns Oscillator Frequency Maximum Oscillator Frequency Minimum Oscillator Frequency External Sync Frequency Range TG Minimum On-Time TG Minimum Off-Time Maximum TG Duty Cycle MODE/SYNC Threshold MODE/SYNC Hysteresis MODE/SYNC Input Resistance to SGND With Reference to Free Running (Notes 6, 8) VMODE/SYNC = 0V (Note 6) fOSC = 500kHz MODE/SYNC Rising
l
The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25C. VIN = 12V, VRUN = 5V, unless otherwise specified.
PARAMETER CONDITIONS RFREQ = 39.2k
l l l
MIN 425 1000
TYP 500
MAX 575 250
UNITS kHz kHz kHz % ns ns % V mV k
-20 30 300 90 1.2 430 50
20
Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LTC3775E is guaranteed to meet performance specifications from 0C to 85C. Specifications over the -40C to 85C operating junction temperature range are assured by design, characterization and correlation with statisitical process controls. The LTC3775I is guaranteed over the full -40C to 125C operating junction temperature range. The junction temperature, TJ, is calculated from the ambient temperature, TA, and power dissipation, PD, according to: TJ = TA + (PD * JA) Note 3: Failure to solder the exposed pad of the UD package to the PC board will result in a thermal resistance much higher than 68C/W.
Note 4: All currents into device pins are positive; all currents out of device pins are negative. All voltages are referenced to ground unless otherwise specified. Note 5: Supply current in normal operation is dominated by the current needed to charge and discharge the external MOSFET gates. This current will vary with supply voltage and the external MOSFETs used. Note 6: Guaranteed by design, not subject to test. Note 7: Rise and fall times are measured using 10% and 90% levels. Delay and nonoverlap times are measured using 50% levels. Note 8: The LTC3775 leading edge modulation architecture does not have a minimum TG pulse width requirement. The TG minimum pulse width is limited by the SW node rise and fall times.
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LTC3775 TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency vs Load Current
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.01 40 0.1 1 10 LOAD CURRENT (A) 100
3775 G01
Efficiency vs Input Voltage
100 1.206 1.204 1.202 VOUT (V) 1.200 1.198 VOUT = 1.2V CONTINUOUS MODE SW FREQ = 500kHz FIRST PAGE CIRCUIT 4 8 12 16 20 INPUT VOLTAGE (V) 24 28
3775 G02
Load Regulation
VIN = 12V SW FREQ = 500kHz FIRST PAGE CIRCUIT
VIN = 12V VOUT = 1.2V SW FREQ = 500kHz FIRST PAGE CIRCUIT PULSE-SKIPPING MODE EFFICIENCY (%)
90 80 70
15A LOAD
1A LOAD 60 50
CONTINUOUS MODE
1.196 1.194 0 2 4
10 12 8 6 LOAD CURRENT (A)
14
16
3775 G03
Line Regulation
1.206 VOUT = 1.2V LOAD = 1A 1.204 FIRST PAGE CIRCUIT FB VOLTAGE (mV) 1.202 VOUT (V) 1.200 1.198 1.196 1.194 603 602 601 600 599 598
FB Voltage vs Temperature
Load Step in Forced Continuous Mode
VOUT(AC) 100mV/DIV
IL 10A/DIV
ILOAD 10A/DIV
4
8
12 16 20 INPUT VOLTAGE (V)
24
28
3775 G02
597 -50 -25
50 25 75 0 TEMPERATURE (C)
100
125
3775 G05
50s/DIV VIN = 12V VOUT = 1.2V LOAD STEP = 0A TO 10A TO 0A MODE/SYNC = 0V SW FREQ = 500kHz FIRST PAGE CIRCUIT
3775 G06
Positive Load Step in Forced Continuous Mode
VSW 20V/DIV VOUT(AC) 100mV/DIV VSW 20V/DIV VOUT(AC) 100mV/DIV
Negative Load Step in Forced Continuous Mode
VOUT(AC) 100mV/DIV
Load Step in Pulse-Skipping Mode
IL 10A/DIV
IL 10A/DIV
IL 10A/DIV
ILOAD 10A/DIV
ILOAD 10A/DIV
3775 G08
ILOAD 10A/DIV
VIN = 12V 5s/DIV VOUT = 1.2V LOAD STEP = 0A TO 10A MODE/SYNC = 0V SW FREQ = 500kHz FIRST PAGE CIRCUIT
3775 G07
5s/DIV VIN = 12V VOUT = 1.2V LOAD STEP = 10A TO 0A MODE/SYNC = 0V SW FREQ = 500kHz FIRST PAGE CIRCUIT
VIN = 12V 50s/DIV VOUT = 1.2V LOAD STEP = 1A TO 11A TO 1A MODE/SYNC = INTVCC SW FREQ = 500kHz FIRST PAGE CIRCUIT
3775 G09
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LTC3775 TYPICAL PERFORMANCE CHARACTERISTICS
Pulse-Skipping Mode Waveform with 0.1A Load
VOUT(AC) 100mV/DIV
Output Short-Circuit Waveform
550 540 SWITCHING FREQUENCY (kHz)
3775 G25
Switching Frequency vs Temperature
IL 20A/DIV 0A LOAD
530 520 510 500 490 480 470 460
IL 2A/DIV
VSW 10V/DIV
3775 G10
VSS 1V/DIV
VIN = 12V 5s/DIV VOUT = 1.2V LOAD = 0.1A MODE/SYNC = INTVCC SW FREQ = 500kHz FIRST PAGE CIRCUIT
20s/DIV VIN = 12V VOUT = 1.2V CSS = 0.01F FIRST PAGE CIRCUIT
450 -50
-25
50 25 0 75 TEMPERATURE (C)
100
125
3775 G11
Duty Cycle vs VCOMP
100 90 80 DUTY CYCLE (%) 70 60 50 40 30 20 10 0 0.6 0.8 1.0 1.2 1.4 VCOMP (V) 1.6 1.8 2.0 VIN = 40V ILIMT (A) 110 VIN = 5V VIN = 12V VIN = 24V 108 106 104
ILIMT vs Temperature
110 108 106 104 ILIMT (A) 102 100 98 96 94 92 -25 50 25 0 75 TEMPERATURE (C) 100 125 90
ILIMT vs Input Voltage
102 100 98 96 94 92 90 -50
4
8
12
16 20 24 28 32 INPUT VOLTAGE (V)
36
40
3775 G12
3775 G13
3775 G14
ILIMB vs Temperature
11.0 10.8 10.6 10.4 ILIMB (A) IRUN (A) 10.2 10.0 9.8 9.6 9.4 9.2 9.0 -50 -25 50 25 0 75 TEMPERATURE (C) 100 125 0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4 -1.6 -1.8
IRUN vs Temperature
35 30 INPUT CURRENT (A) 25 20 15 10 5 0
Shutdown Current vs Input Voltage
-2.0 -50
-25
50 25 0 75 TEMPERATURE (C)
100
125
4
8
12
16 20 24 28 32 INPUT VOLTAGE (V)
36
40
3775 G15
3775 G16
3775 G17
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LTC3775 TYPICAL PERFORMANCE CHARACTERISTICS
Shutdown Current vs Temperature
20 18 SHUTDOWN CURRENT (A) QUIESCENT CURRENT (mA) 16 14 12 10 8 6 4 2 0 -50 -25 50 25 0 75 TEMPERATURE (C) 100 125 8 7 6 INTVCC (%) 5 4 3 2 1 0 3.6 4.0 4.4 5.2 4.8 INTVCC (V) 5.6 6.0
3775 G19
Quiescent Current vs INTVCC
0 -0.1 -0.2 -0.3 -0.4 -0.5 -0.6 -0.7 -0.8 -0.9 -1.0
INTVCC Load Regulation
VIN = 12V
0
10 30 40 20 INTVCC LOAD CURRENT (mA)
50
3775 G20
3775 G18
INTVCC Dropout
0 INTVCC DROPOUT VOLTAGE (V) TA = 25C INTVCC DROPOUT VOLTAGE (V) 0
INTVCC Dropout vs Temperature
LOAD CURRENT = 20mA -0.2
-0.2
-0.4
-0.4
-0.6
-0.6
-0.8
-0.8
-1.0
0
10 30 40 20 INTVCC LOAD CURRENT (mA)
50
3775 G21
-1.0 -50
-25
50 25 0 75 TEMPERATURE (C)
100
125
3775 G22
BG Turn-On Waveform Driving Renesas RJK0301
BG Turn-Off Waveform Driving Renesas RJK0301
BG 1V/DIV
BG 1V/DIV
0V
0V
VIN = 12V 20ns/DIV VOUT = 1.2V LOAD = 1A MOSFET: RENESAS RJK0301
3775 G23
VIN = 12V 20ns/DIV VOUT = 1.2V LOAD = 1A MOSFET: RENESAS RJK0301
3775 G24
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LTC3775 PIN FUNCTIONS
ILIMT (Pin 1): Topside Current Limit Set Point. This pin has an internal 100A pull-down current, allowing the topside current limit threshold to be programmed by an external resistor connected to VIN. See Current Limit Applications. ILIMB (Pin 2): Bottom Side Current Limit Set Point. This pin has an internal 10A pull-up current, allowing the bottom side current limit threshold to be programmed by an external resistor connected to SGND. See Current Limit Applications. FB (Pin 3): Error Amplifier Input. The FB pin is connected to a resistive divider from VOUT to SGND. The feedback loop compensation network is also connected to this pin. COMP (Pin 4): Error Amplifier Output. Use an RC network between the COMP pin and the FB pin to compensate the feedback loop for optimum transient response. SS (Pin 5): Soft-Start. Connect this pin to an external capacitor, CSS, to implement a soft-start function. When the voltage on the SS pin is less than the 0.6V internal reference, the LTC3775 regulates the VFB voltage to the SS pin voltage instead of the 0.6V reference. FREQ (Pin 6): Frequency Set. A resistor connected from this pin to SGND sets the free-running frequency of the internal oscillator. See Applications Information section for resistor value selection details. SGND (Pin 7): Signal Ground. All the internal low power circuitry returns to the SGND pin. All feedback and softstart connections should return to SGND. SGND should be Kelvin connected to a single point near the negative terminal of the VOUT bypass capacitor. BG (Pin 8): Bottom Gate Drive. This pin drives the gate of the bottom N-channel synchronous switch MOSFET. This pin swings from PGND to INTVCC. INTVCC (Pin 9): Internal 5.2V Regulator Output. The gate driver and control circuits are powered from this voltage. Bypass this pin to power ground with a low ESR ceramic capacitor of value 4.7F or greater (X5R or better). SENSE (Pin 10): Topside Current Sensing Input. Connect this pin to the switch node of the converter for top MOSFET RDS(ON) current sensing. Alternatively, this pin can be connected to a sense resistor at the drain of the top MOSFET for more accurate current limit. VIN (Pin 11): Main Input Supply. Bypass this pin to PGND with a low ESR ceramic capacitor of value 1F or greater (X5R or better). SW (Pin 12): Switch Node. Connect this pin to the source of the upper power MOSFET. This pin is also used as the input to the bottom side current limit comparator and the zero-crossing reverse current comparator. TG (Pin 13): Top Gate Drive. This pin drives the gate of the top N-channel MOSFET. The TG driver draws power from the BOOST pin and returns to the SW pin, providing true floating drive to the top MOSFET. BOOST (Pin 14): Top Gate Driver Supply. This pin should be decoupled to SW with a 0.1F low ESR ceramic capacitor. An external Schottky diode from INTVCC to BOOST creates a floating charge-pump supply at BOOST. No other external supplies are required. MODE/SYNC (Pin 15): Pulse-Skipping Mode Enable/Sync Pin. This multifunction pin provides pulse-skipping mode enable/disable control and an external clock input for synchronization of the internal oscillator. Pulling this pin below 1.2V (DC) or driving it with an external logic-level synchronization signal disables pulse-skipping mode operation and forces continuous operation. Pulling the pin above 1.2V enables pulse-skipping mode operation. This pin has an internal 50k pull-down resistor connected to SGND. RUN/SHDN (Pin 16): Enable/Shutdown Input. Pulling this pin above 1.22V enables the controller. Forcing this pin below 1.22V causes the driver outputs to pull low. Pulling this pin below 0.74V forces the LTC3775 into shutdown mode. While in shutdown, the INTVCC regulator and most internal circuitry turns off and the supply current drops below 14A. This pin has an internal 1A pull-up current that allows the LTC3775 to power up if this pin is left floating. PGND (Pin 17): Power Ground (Exposed Pad). The BG driver returns to this pin. Connect PGND to the source of the bottom power MOSFET and the VIN and INTVCC bypass capacitors. PGND is electrically isolated from SGND. The exposed pad of the QFN package is connected to PGND.
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LTC3775 BLOCK DIAGRAM
VIN CVCC
VIN VIN R4 5.2V REG
INTVCC INTVCC
INTVCC 3.6V
OVERTEMP RSET
FREQ MODE/SYNC 50k SS EXT SYNC MODE/SYNC DETECT 1A INTVCC ISS VIN EA LINE FEEDFORWARD MODE IREV OSC
+ -
0.2 * VILIMB PGND
MODE BOOST TG SWITCH LOGIC AND ANTISHOOTTHROUGH SW INTVCC BG PGND
CSS COMP R2 C1 R3 C3 C2 FB
0.6V SS FB
+ + -
PWM
RB
RA
0.66V
MAX
+
+ -
+ -
+ -
0.74V
+ -
R5
RUN/SHDN
SHDN CHIP SHUTDOWN
1.22V
+ -
IRUN 1A
EN REF 0.6V CTLIM 100A ILIMT UVLO RILIMT VIN RSENSE SENSE ILIMB 10A VILIMB RILIMB
+ -
INTVCC
CBLIM
+ -
PGND
INTVCC DB CB QT
QB L
SGND
VOUT COUT
3775 BD
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LTC3775 APPLICATIONS INFORMATION
Operation (Refer to Block Diagram) The LTC3775 is a constant frequency, voltage mode controller for DC/DC step-down converters. It is designed to be used in a synchronous switching architecture with two external N-channel MOSFETs. For circuit operation, please refer to the Block Diagram. The LTC3775 uses voltage mode control in which the duty cycle is controlled directly by the error amplifier output. The error amplifier adjusts the voltage at the COMP pin by comparing the VFB pin with the 0.6V internal reference. When the load current increases, it causes a drop in the feedback voltage relative to the reference. The COMP voltage then rises, increasing the duty cycle until the LTC3775 output feedback voltage again matches the reference voltage. In normal operation, the top MOSFET is turned on when the PWM comparator changes state and is turned off by the internal oscillator. The PWM comparator maintains the proper duty cycle by comparing the error amplifier output (after being "compensated" by the line feedforward multiplier) to a sawtooth waveform generated by the oscillator. When the top MOSFET is turned off, the bottom MOSFET is turned on until the next cycle begins, or if pulse-skipping mode operation is enabled, until the inductor current reverses as determined by the reverse current comparator. Feedback Control The LTC3775 senses the output voltage at VOUT with an internal feedback op amp (see Block Diagram). This is a true op amp with a low impedance output, 80dB of openloop gain and a 25MHz gain-bandwidth product. The positive input is connected to an internal 0.6V reference, while the negative input is connected to the FB pin. The output is connected to COMP which is in turn connected , to the line feedforward circuit and from there to the PWM generator. At steady state, as shown in the Block Diagram, the output of the switching regulator is given the following equation R VOUT = VREF * 1+ A RB
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Unlike many regulators that use a transconductance (gm) amplifier, the LTC3775 is designed to use an inverting summing amplifier topology with the FB pin configured as a virtual ground. This allows the feedback gain to be tightly controlled by external components. In addition, the voltage feedback amplifier allows flexibility in choosing pole and zero locations. In particular, it allows the use of "Type 3" compensation, which provides a phase boost at the LC pole frequency and significantly improves the control loop phase margin. In a typical LTC3775 circuit, the feedback loop consists of the line feedforward circuit, the modulator, the external inductor, the output capacitor and the feedback amplifier with its compensation network. All these components affect loop behavior and need to be accounted for in the loop compensation. The modulator consists of the PWM generator, the output MOSFET drivers and the external MOSFETs themselves. The modulator gain varies linearily with the input voltage. The line feedforward circuit compensates for this change in gain, and provides a constant gain from the error amplifier output to the inductor input regardless of input voltage. From a feedback loop point of view, the combination of the line feedforward circuit and the modulator looks like a linear voltage transfer function from COMP to the inductor input and has a gain roughly equal to 30V/V. It has fairly benign AC behavior at typical loop compensation frequencies with significant phase shift appearing at half the switching frequency. The external inductor/output capacitor combination makes a more significant contribution to loop behavior. These components cause a second order LC roll-off at the output with 180 phase shift. This roll-off is what filters the PWM waveform, resulting in the desired DC output voltage, but this phase shift causes stability issues in the feedback loop and must be frequency compensated. At higher frequencies, the reactance of the output capacitor approaches its ESR, and the roll-off due to the capacitor stops, leaving -20dB/decade and 90 of phase shift.
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LTC3775 APPLICATIONS INFORMATION
Figure 1 shows a Type 3 amplifier. The transfer function of this amplifier is given by the following equation:
- 1+ sR2C1 1+ s(RA + R3)C3 VCOMP = VOUT sRA C1+ C2 1+ s(C1|| C2 )R2 1+ sC3R3
Required error amplifier gain at frequency fC:
A V(CROSSOVER) f f 40 log 1+ C - 20 log 1+ C - 20 log AMOD fESR fLC
2 2
(
(
)(
)
)(
)
(
)
The RC network across the error amplifier and the feedforward components R3 and C3 introduce two pole-zero pairs to obtain a phase boost at the system unity-gain frequency, fC. In theory, the zeros and poles are placed symmetrically around fC, and the spread between the zeros and the poles is adjusted to give the desired phase boost at fC. However, in practice, if the crossover frequency is much higher than the LC double-pole frequency, this method of frequency compensation normally generates a phase dip within the unity bandwidth and creates some concern regarding conditional stability. If conditional stability is a concern, move the error amplifier's zero to a lower frequency to avoid excessive phase dip. The following equations can be used to compute the feedback compensation component values:
fSW = switching frequency fLC = 1 2 LCOUT 1 2 RESR COUT
fLC fP2(RES) + 1+ f 1+ f R2 C C * 20 log RA fC fLC + 1+ f f -f
ESR ESR
fP2(RES) - fZ2(RES) fZ2(RES) fP2(RES) ) 1+ f C LC
where AMOD is the modulator and line feedforward gain and is equal to:
A MOD VIN(MAX ) * DCMAX VSAW = 40 V * 0.95 30 V/ V 1.25V
Once the value of resistor RA and the pole and zero locations have been decided, the values of C1, R2, C2, R3 and C3 can be obtained from the above equations. Compensating a switching power supply feedback loop is a complex task. The applications shown in this data sheet show typical values, optimized for the power components shown. Though similar power components should suffice, substantially changing even one major power component may degrade performance significantly. Stability also may depend on circuit board layout. To verify the calculated component values, all new circuit designs should be prototyped and tested for stability.
GAIN (dB)
fESR =
choose:
fC = crossover frequency = 1 fZ1(ERR) = fLC = 2R2C1 f 1 fZ2(RES) = C = 5 2 RA + R3 C3 fSW 10
C2 VOUT C3 RA RB VREF R3 FB R2 C1 PHASE (DEG)
-1 GAIN +1 -1
1 fP1(ERR) = fESR = 2R2(C1|| C2) fP2(RES ) = 5fC = 1 2R3C3
Figure 1. Type 3 Amplifier Compensation
+
(
)
-
COMP
0 PHASE BOOST
FREQ -90 -180 -270 -380
3775 F01
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LTC3775 APPLICATIONS INFORMATION
Output Overvoltage Protection An overvoltage comparator, MAX, guards against transient overshoots (>10%) as well as other more serious conditions that may overvoltage the output. In such cases, the top MOSFET is turned off and the bottom MOSFET is turned on until the overvoltage condition is cleared. Run/Shutdown The LTC3775 can be put into a low power shutdown mode with quiescent current <14A by pulling the RUN/SHDN pin below 0.74V. The RUN/SHDN pin can also be used as an accurate external UVLO (undervoltage lockout) input with a threshold of 1.22V. The driver outputs stay low if this pin is <1.22V. The external resistive divider R4 and R5 shown in the Block Diagram can be used to set the UVLO level based on VIN. The VIN voltage at which the switching starts is given by the following formula: UVLO (Upper) = 1.22V * (1 + R4/R5) - (1A * R4) The RUN/SHDN pin has an internal 1A pull-up for default turn-on if this pin is left floating. This 1A pull-up current is included in the above UVLO calculation. When RUN/SHDN goes above 1.22V, this pull-up current is increased to 5A. This provides some amount of hysteresis to the UVLO threshold. The lower UVLO level becomes: UVLO (Lower) = 1.22V * (1 + R4/R5) - (5A * R4) So the amount of hysteresis is given by: UVLO (Hysteresis) = 4A * R4
VOUT 0.5V/DIV LTC3775 VIN R4 R5 IRUN 1A RUN 1.22V 4A IL 5A/DIV SWITCHOVER FROM PULSESKIPPING TO CONTINUOUS MODE
Soft-Start The LTC3775 includes a soft-start circuit that provides a smooth output voltage ramp during start-up. The SS pin requires an external capacitor, CSS, to GND with the value determined by the required soft-start time. An internal 1A current source charges CSS. When the voltage on the SS pin is less than the 0.6V internal reference, the LTC3775 regulates the VFB voltage to the SS pin voltage instead of the 0.6V reference. As the SS voltage rises linearly from 0V to 0.6V and beyond, the output voltage, VOUT , rises smoothly from zero to its final value. The total soft-start time can be calculated as: t SOFTSTART = 0.9 * CSS 1A
The SS pin is pulled low in the following conditions: during an LDO undervoltage condition (INTVCC < 3.6V), during shutdown (RUN pin < 1.22V), during an overtemperature condition (TJ > 165C) and during current limit. If either the top or bottom current limit comparator trips, the SS pin is pulled low until the inductor current regulates at around the current limit setting. Once the fault is cleared, SS will start charging up allowing the duty cycle and output voltage to increase gradually. Due to the current limit action on the SS pin, it is important to avoid an overcurrent condition during start-up of the power supply, or VOUT will fail to start up properly.
0.74V
Figure 2. RUN Pin Control
12
+ - + -
TURN OFF TG EN CHIP SHUTDOWN SHDN
3775 F02
VSS 1V/DIV
2ms/DIV VIN = 12V VOUT = 1.2V CSS = 0.01F MODE = 0V SW FREQ = 500kHz
3775 F03
Figure 3. Typical Start-Up Wavform for a Buck Converter Using the LTC3775
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LTC3775 APPLICATIONS INFORMATION
To prevent discharging a pre-biased VOUT, the LTC3775 always starts switching in pulse-skipping mode up to SS = 0.54V, regardless of the mode selected by the MODE/SYNC pin. Thus if VOUT > 0V during power-up, VOUT will remain at the pre-biased voltage (if there is no load) until the SS voltage catches up with VOUT, after which VOUT will track the SS ramp. The LTC3775 reverts to the selected mode once SS > 0.54V. Constant Switching Frequency The internal oscillator can be programmed from 250kHz to 1MHz with an external resistor from the FREQ pin to ground, in order to optimize component size, efficiency and noise for the specific application. The internal oscillator can also be synchronized to an external clock connected to the MODE/SYNC pin and can lock to a range of 20% of the programmed free-running frequency. When locked to an external clock, pulse-skipping mode operation is automatically disabled. Constant frequency operation offers a number of benefits: inductor and capacitor values can be chosen for a precise operating frequency and the feedback loop can be similarly tightly specified. Noise generated by the circuit will always be at known frequencies. Subharmonic oscillation and slope compensation, common headaches with constant frequency current mode switchers, are absent in voltage mode designs like the LTC3775. Thermal Shutdown The LTC3775 has a thermal detector that pulls the driver outputs low if the junction temperature of the chip exceeds 165C. The thermal shutdown circuit has 25C of hysteresis. Current Limit The LTC3775 includes an onboard cycle-by-cycle current limit circuit that limits the maximum output current to a user-programmed level. The current limit circuit consists of two comparators, CTLIM and CBLIM that monitor the voltage drop across the top and bottom MOSFETs respectively. Since the MOSFET 's effective resistance, RDS(ON),
CBLIM EXTEND BG
is low during its on-time, the voltage drop from the drain to source is proportional to the current flow. Alternatively, for better accuracy, the topside current may be monitored with a sense resistor. The benefit of having two comparators is to allow continuous monitoring and cycle-by-cycle control of the inductor current regardless of the operating duty cycle. In high duty cycle operation the top MOSFET, QT , is on most of the time. Thus, a high side comparator is necessary to limit the output current during high duty cycle operation. Architectures that contain only one comparator to monitor the low side MOSFET will not effectively limit the output current during high duty cycle operation. Conversely, during low duty cycle operation, a low side comparator is necessary to limit the output current. Another common current sensing scheme uses a sense resistor in series with the inductor to allow continuous monitoring. However, this scheme restricts the range of VOUT due to the common mode range of the current limit comparator. The LTC3775 does not have this VOUT restriction. Figure 4 shows the current limit circuitry. The top current limit comparator, CTLIM monitors the current through the top MOSFET, QT , when TG is high. If the inductor current exceeds the current limit threshold when QT is on, QT turns off immediately and the bottom MOSFET, QB, turns on. The SENSE pin is the input for CTLIM. For applications where
LTC3775 CTLIM TURN OFF TG
VIN
VIN RILIMT RSENSE
+ -
ILIMT 100A SENSE 10A ILIMB 0.2 * VILIMB
TG (OPT) RILIMB
QT
SGND
3775 F04
Figure 4. LTC3775 Current Limit Circuit
+
PGND BG QB
- +
SW
VOUT
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LTC3775 APPLICATIONS INFORMATION
the upper MOSFET 's RDS(ON) is used to sense current, connect the SENSE pin to the source of QT (the SW node). Alternatively, for accurate current sensing, connect this pin to a sense resistor located at the drain of QT . The reference input of CTLIM is connected to the ILIMT pin. Connect an external resistor, RILIMT , from the ILIMT pin to VIN to set the the current limit threshold. The voltage at the SENSE pin drops as the inductor current increases. CTLIM trips if the voltage at the SENSE pin goes below the voltage at the ILIMT pin causing TG to pull low and turn off QT . The bottom current limit comparator, CBLIM, monitors the current through the bottom MOSFET, QB, when BG is high. If the inductor current exceeds the current limit threshold when QB is on, QB remains on until the current drops below the threshold. The SW pin is the input for CBLIM. The reference input to CBLIM is derived from the voltage at the ILIMB pin. Connect an external resistor, RILIMB, from the ILIMB pin to SGND to set the current limit threshold. The inductor current flows from PGND to SW when QB is on (for a postitive load current). The SW node is therefore a negative voltage. The LTC3775 inverts the voltage at the SW pin before comparing it with the attenuated voltage (5x) at the ILIMB pin. BG stays high once CBLIM trips and TG remains low until the inductor current drops below the threshold. Figure 5 shows typical waveforms during output overload. Current Limit Blanking Time The LTC3775 current limit circuit features a short blanking time following low-to-high and high-to-low transitions at the SW node. This prevents false tripping of the current limit circuit if there is ringing on the SW node. When the top gate, TG, goes high, the topside comparator, CTLIM, waits for 200ns before turning on to monitor the SENSE voltage. Likewise, when the bottom gate, BG, goes high the bottom side comparator, CBLIM, waits for 200ns before turning on to monitor the SW voltage. This means that the minimum TG and BG pulse is slightly more than 200ns during current limit. These blanking times do not, however, limit the duty cycle capability of the control loop. The LTC3775 control loop is capable of operation with a TG on-time as low as 30ns. If a sense resistor is employed on the top side, the LTC3775 automatically lowers the CTLIM blanking time from 200ns to 100ns. The CBLIM blanking time remains at 200ns. The blanking time can be reduced when a sense resistor is used because the SENSE pin connects to the drain of the top MOSFET which rings less than the SW node. The LTC3775 detects that a sense resistor is employed by checking that the SENSE pin stays high (equal to VIN) when BG is high. If the SENSE pin is connected to the SW node, SENSE will be at 0V when BG is high. The Current Sensing Input Pins The SENSE and ILIMT pins are inputs to the top current limit comparator, CTLIM. The top current limit threshold is set by the resistor, RILIMT , connected to the ILIMT pin and the ILIMT pin 100A pull-down current. RILIMT should be placed close to the LTC3775 and the other end of RILIMT should run parallel with the SENSE trace to the Kelvin sense connection underneath the sense resistor, as shown in Figure 6. The sense resistor should be connected to the drain of the top power MOSFET and the VIN node using short, wide PCB traces. Ideally, the top terminal of the sense resistors will be immediately adjacent to the positive terminal of the input capacitor, as shown in Figure 7a. This path is a part of the high di/dt loop formed by the sense resistor, top power MOSFET, inductor and output capacitor.
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IL 20A/DIV 0A LOAD
VSS 1V/DIV
20s/DIV VIN = 12V VOUT = 1.2V CSS = 0.01F FIRST PAGE CIRCUIT
3775 F05
Figure 5. Typical Waveforms During Output Overload
14
LTC3775 APPLICATIONS INFORMATION
Since the current limit comparator contains leading edge blanking, an external RC filter is not required for proper operation. However, an external filter can be designed by adding a capacitor across the SENSE and ILIMT pins (CF in Figure 7a). The filter component should be placed close to the SENSE and ILIMT pins. If RDS(ON) sensing is employed, the Kelvin sense connection should run from the SENSE pin and the RILIMT resistor to the source and drain terminals of the top power MOSFET respectively, as shown in Figure 7b. The external RC filter should not be added since the source terminal is switching.
VIN
The bottom side current limit threshold is set by the resistor, RILIMB, from the ILIMB pin to SGND and the ILIMB pin 10A pull-up current. The voltage at ILIMB is attenuated 5x internally before it is applied to the input of bottom current limit comparator, CBLIM. This voltage must be quiet. Connect RILIMB from the ILIMB pin to a quiet ground near the LTC3775 SGND pin. The other input of CBLIM is connected to the SW pin. The SW pin is also shared with the bottom gate driver and should be connected near the drain of the bottom MOSFET, QB. Pulse-Skipping Mode The LTC3775 can operate in one of two modes selectable with the MODE/SYNC pin: pulse-skipping mode or forced continuous mode. Pulse-skipping mode is selected when increased efficiency at light loads is desired, as shown in Figure 8. In this mode, the bottom MOSFET is turned off when inductor current reverses in order to minimize the efficiency loss due to reverse current flow. As the load current decreases (see Figure 9), the duty cycle is reduced to maintain regulation until the minimum on-time (50ns) is reached. When the load decreases below this point, the LTC3775 begins to skip cycles to maintain regulation. This reduces the frequency and improves efficiency by minimizing gate charge losses. In forced continuous mode, the bottom MOSFET is always on when the top MOSFET is off, allowing the inductor current to reverse at low currents. This mode is less efficient due to switching, but has the advantages of better transient
100 90 VIN = 12V VOUT = 1.2V SW FREQ = 500kHz FRONT PAGE CIRCUIT PULSE-SKIPPING MODE
TO RLIMIT RSENSE TO SENSE PIN
3775 F06
TOP MOSFET DRAIN
Figure 6. Kelvin SENSE Connection for Topside Current Limiting Sensing
VIN CIN LTC3775 ILIMIT RLIMIT CF SENSE TG
3775 F07a
VIN
RSENSE
80 EFFICIENCY (%) SW 70 60 50 40 30 20 10 QT
Figure 7a. External Filter for Topside Current Sensing
VIN CIN LTC3775 ILIMIT TG SENSE
3775 F07b
VIN RLIMIT
CONTINUOUS MODE
0 0.01
0.1
1 10 LOAD CURRENT (A)
100
3775 F08
SW
Figure 7b. Kelvin Connection for Topside RDS(ON) Sensing
Figure 8. Efficiency in Pulse-Skipping/Forced Continuous Modes
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LTC3775 APPLICATIONS INFORMATION
PULSE-SKIPPING MODE FORCED CONTINUOUS
0A DECREASING LOAD CURRENT 0A
0A
0A
0A
0A
3775 F09
Figure 9. Comparison of Inductor Current Waveforms for Pulse-Skipping Mode and Forced Continuous Mode
response at low load currents, constant frequency operation, and the ability to maintain regulation when sinking current. See Figure 8 for a comparison of the efficiency at light loads for each mode. In pulse-skipping mode, the LTC3775 reverse-current comparator, IREV , monitors the SW pin for zero crossing when the bottom gate, BG, is high. It turns off BG if the inductor current reverses and the SW voltage goes above GND. To prevent false tripping due to ringing on the SW node when BG is first turned on, there is a blanking time of 200ns similar to the bottom side current limit blanking. Under certain light load conditions, if the TG on-time is short, the inductor current may reverse during the IREV blanking time but the LTC3775 will only turn off BG after the blanking time. In applications where a low value inductor is used, the high di/dt of the inductor ripple current together with the parasitic series inductance of the bottom MOSFET, QB, and PCB trace inductance creates an opposing voltage to the voltage drop across the RDS(ON) of QB. This can cause IREV to trip early, before the inductor current reverses. The parasitic series inductance of the PCB trace can be minimized by connecting the SW pin closer to the drain of QB. INTVCC Regulator The LTC3775 features a P-channel low dropout linear regulator (LDO) that supplies power to the INTVCC pin from the VIN supply. INTVCC powers the gate drivers and much
of the LTC3775's internal circuitry. The LDO regulates the voltage at the INTVCC pin to 5.2V when VIN is greater than 6.5V. The INTVCC pin must be bypassed to ground with a low ESR (X5R or better) ceramic capcitor of at least 4.7F . Good bypassing is needed to supply the high transient currents required by the MOSFET gate drivers. An internal undervoltage lockout (UVLO) monitors the voltage on INTVCC to ensure that the LTC3775 has sufficient gate drive voltage. If the INTVCC voltage falls below the UVLO threshold of 3.1V, the gate drive outputs remain low. Thermal Considerations The LTC3775 is offered in a 3mm x 3mm QFN package (UD16) that has a thermal resistance RTH(JA) of 68C/W. The UD16 package has a lead pitch of 0.5mm. The regulator can supply up to 50mA of gate drive load current. The expected LDO load current can be calculated from the gate charge requirement of the external MOSFET: IINTVCC = (fSW) * (QG(QT) + QG(QB)) + 3.5mA where: 3.5mA is the quiescent current of LTC3775 QG(QT) is the total gate charge of the top MOSFET QG(QB) is the total gate charge of the bottom MOSFET fSW is the switching frequency
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LTC3775 APPLICATIONS INFORMATION
The value of QG should come from the plot of VGS vs QG in the Typical Performance Characteristics section of the MOSFET data sheet. The value listed in the electrical specifications may be measured at a higher VGS, such as 10V, whereas the value of interest is at the 5V INTVCC gate drive voltage. Care must be taken to ensure that the maximum junction temperature of the LTC3775 is never exceeded. The junction temperature can be estimated using the following equations: PDISS = VIN * IINTVCC TJ = TA + PDISS * RTH(JA) As an example of the required thermal analysis, consider a buck converter with a 24V input voltage and an output voltage of 3.3V at 15A. The switching frequency is 500kHz and the maximum ambient temperature is 70C. The power MOSFET used for this application is the Vishay Siliconix Si7884DP which has a typical RDS(ON) of 7.5m at VGS , = 4.5V and 5.5m at VGS = 10V. From the plot of VGS vs QG, the total gate charge at VGS = 5V is 18.5nC (the temperature coefficient of the gate charge is low). One power MOSFET is used for the top side and one for the bottom side. For the UD package: IINTVCC = 3.5mA + 2 * 18.5nC * 500kHz = 22mA PDISS = 24V * 22mA = 528mW TJ = 70C + 528mW * 68C/W = 105.9C In this example, the junction temperature rise is 35.9C. These equations demonstrate how the gate charge current typically dominates the quiescent current of the IC, and how the choice of the operating frequency and board heat sinking can have a significant effect on the thermal performance of the solution. To prevent the maximum junction temperature from being exceeded, the input supply current of the IC should be checked when operating in continuous mode (heavy load) at maximum VIN. A tradeoff between the operating frequency and the size of the power MOSFETs may need to be made in order to maintain a reliable junction temperature. Finally, it is important to verify the calculations by performing a thermal analysis of the final PCB using an infrared camera or thermal probe. Operation at Low Supply Voltage The LTC3775 has a minimum input voltage of 4.5V. The gate driver for the LTC3775 consists of a PMOS pull-up and an NMOS pull-down device, allowing the full INTVCC voltage to be applied to the gates during power MOSFET switching. Nonetheless, care should be taken to determine the minimum gate drive supply voltage (INTVCC) in order to choose the optimum power MOSFETs. Important parameters that can affect the minimum gate drive voltage are the minimum input voltage (VIN(MIN)), the LDO dropout voltage, the QG of the power MOSFETs, and the operating frequency. If the input voltage VIN is low enough for the INTVCC LDO to be in dropout, then the minimum gate drive supply voltage is: VINTVCC = VIN(MIN) - VDROPOUT The LDO dropout voltage is a function of the total gate drive current and the quiescent current of the IC (typically 3.5mA). A curve of dropout voltage versus output current for the LDO is shown in Figure 10. The temperature coefficient of the LDO dropout voltage is approximately 6000ppm/C. See the INTVCC Regulator and Thermal Considerations sections for information about calculating the total quiescent current.
0 INTVCC DROPOUT VOLTAGE (V) TA = 25C
-0.2
-0.4
-0.6
-0.8
-1.0
0
10 30 40 20 INTVCC LOAD CURRENT (mA)
50
3775 F10
Figure 10. INTVCC LDO Dropout Voltage vs Current
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LTC3775 APPLICATIONS INFORMATION
After the calculations have been completed, it is important to measure the gate drive waveforms and the gate driver supply voltage (INTVCC to PGND) over all operating conditions (low VIN, nominal VIN and high VIN, as well as from light load to full load) to ensure adequate power MOSFET enhancement. Consult the power MOSFET data sheet to determine the actual RDS(ON) for the measured VGS, and verify your thermal calculations by measuring the component temperatures using an infrared camera or thermal probe. Operation at High Supply Voltage At high input voltages, the LTC3775's internal LDO can dissipate a significant amount of power, which could cause the maximum junction temperature to be exceeded. Conditions such as a high operating frequency, or the use of more than one power MOSFET in parallel, could push the junction temperature rise to high levels. To prevent the maximum junction temperature from being exceeded, the input supply current must be checked while operating in continuous conduction mode at maximum VIN. See the Thermal Considerations section for calculation of the maximum junction temperature. Low Duty Cycle Operation The LTC3775 uses a leading edge modulation architecture. Because the top MOSFET turns on when the PWM comparator trips, the top MOSFET minimum on-time is not dependent on the propagation delay of the PWM comparator; it is only limited by the internal delays of the gate drivers and the rise/fall time of the power MOSFET gate. This allows the LTC3775 to operate in very low duty cycle applications with a large step-down ratio. Figure 11 shows minimum on-time waveforms for forced continuous mode operation. If pulse-skipping mode is selected, the LTC3775 allows the controller to skip pulses at light load, thereby reducing switching losses and improving the efficiency. Figure 12 shows waveforms of the minimum on-time in pulse-skipping mode. If the TG on-time is less than the blanking time of the topside current limit comparator, CTLIM, the topside comparator never trips during normal operation. The blanking time
VIN = 28V 20ns/DIV VOUT = 0.6V LOAD = 1A MODE/SYNC = INTVCC SW FREQ = 1MHz
3775 F12
TG 10V/DIV
VSW 10V/DIV
VIN = 28V 20ns/DIV VOUT = 0.6V LOAD = 1A MODE/SYNC = 0V SW FREQ = 1MHz
3775 F11
Figure 11. Minimum On-Time Waveforms in Forced Continuous Mode
TG 10V/DIV
VSW 10V/DIV
Figure 12. Minimum On-Time Waveforms in Pulse-Skipping Mode
is 200ns for RDS(ON) sensing and 100ns when a sense resistor is used. For TG on-times smaller than the topside blanking times, the LTC3775 relies on the bottom current limit comparator, CBLIM, to monitor the inductor current. If CBLIM trips, the LTC3775 starts to skip pulses and at the same time pulls down the soft-start capacitor to limit the duty cycle. If VOUT drops sufficiently, the TG on-time can increase enough to turn on CTLIM and limit the peak inductor current. The minimum on-time of the application circuit can be calculated at maximum VIN: tON(MIN) = VOUT fSW * VIN(MAX )
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LTC3775 APPLICATIONS INFORMATION
High Duty Cycle Operation The maximum duty cycle is limited by the LTC3775 internal oscillator reset time, the propagation delay of the PWM comparator and the BOOST pin supply refresh rate. The minimum off-time is typically 300ns. The top MOSFET driver is biased from the floating bootstrap capacitor, CB, which normally recharges during each off cycle through an external diode when the top MOSFET turns off. If the input voltage, VIN, decreases to a voltage close to VOUT , the controller will enter dropout and attempt to turn on the top MOSFET continuously. To avoid depleting the charge on the bootstrap capacitor, CB, the LTC3775 has an internal counter that turns on the bottom MOSFET every eight cycles for 200ns to refresh the bootstrap capacitor. Figure 13 shows maximum duty cycle operation with the 200ns BOOST pin supply refresh. step-down VIN to VOUT ratios, another consideration is the minimum on-time of the LTC3775 (see the Minimum On-Time Considerations section). A final consideration for operating frequency is that in noise-sensitive communications systems, it is often desirable to keep the switching noise out of a sensitive frequency band. The LTC3775 uses a constant frequency architecture that can be programmed over a 250kHz to 1MHz range with a single resistor from the FREQ pin to ground, as shown in Figure 14. The nominal voltage on the FREQ pin is 1.22V, and the current that flows from this pin is used to charge and discharge an internal oscillator capacitor. The value of RSET for a given operating frequency can be chosen from Figure 14 or from the following equation: RSET (k) = 19500 f(kHz)
TG 5V/DIV OSCILLATOR RESET BOOST PIN SUPPLY REFRESH BG 5V/DIV
The oscillator can also be synchronized to an external clock applied to the MODE/SYNC pin with a frequency in the range of 20% of the programmed free-running frequency set by the FREQ pin. In this synchronized mode, pulse-skipping mode operation is disabled. The clock high level must exceed 1.5V for at least 25ns. The bottom MOSFET will turn-on following the rising edge of the external clock.
1000
3775 F13
FREQUENCY (kHz)
VIN = 4.5V 2s/DIV VOUT = 4.2V LOAD = 0A SW FREQ = 500kHz
900 800 700 600 500 400 300 200 0 10 20 30 40 50 60 70 80 90 100 RSET VALUE (k)
3775 F14
Figure 13. Maximum Duty Cycle Waveforms
EXTERNAL COMPONENTS SELECTION Operating Frequency The choice of operating frequency and inductor value is a tradeoff between efficiency and component size. Low frequency operation improves efficiency by reducing MOSFET switching losses and gate charge losses. However, lower frequency operation requires more inductance for a given amount of ripple current, resulting in a larger inductor size and higher cost. If the ripple current is allowed to increase, larger output capacitors may be required to maintain the same output ripple. For converters with high
Figure 14. Frequency Set Resistor (RSET) Value
Top MOSFET Driver Supply An external bootstrap capacitor, CB, connected to the BOOST pin supplies the gate drive voltage for the topside MOSFET. This capacitor is charged through diode DB from
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LTC3775 APPLICATIONS INFORMATION
INTVCC when the switch node is low. When the top MOSFET turns on, the switch node rises to VIN and the BOOST pin rises to approximately VIN + INTVCC. The boost capacitor needs to store at least 100 times the gate charge required by the top MOSFET. In most applications a 0.1F to 1F X5R or X7R dielectric capacitor is adequate. The reverse breakdown of the Schottky diode, DB, must be greater than VIN(MAX). Power MOSFET Selection The LTC3775 requires two external N-channel power MOSFETs, one for the top (main) switch and one for the bottom (synchronous) switch. Important parameters for the power MOSFETs are the threshold voltage V(GS)TH, breakdown voltage V(BR)DSS, maximum current IDS(MAX), on-resistance RDS(ON) and input capacitance. The gate drive voltage is set by the 5.2V INTVCC supply. Consequently, logic-level threshold MOSFETs must be used in LTC3775 applications. If the INTVCC voltage is expected to drop below 5V, then sub-logic level threshold MOSFETs should be considered. Pay close attention to the V(BR)DSS specification because most logic-level MOSFETs are limited to 30V or less. The MOSFETs selected should have a V(BR)DSS rating greater than the maximum input voltage and some margin should be added for transients and spikes. MOSFET input capacitance is a combination of several components but can be taken from the typical "gate charge" curve included on most data sheets (Figure 15). The curve is generated by forcing a constant input current into the gate of a common source, current source loaded stage and then plotting the gate voltage versus time. The initial slope is the effect of the gate-to-source and the gateto-drain capacitance. The flat portion of the curve is the result of the Miller multiplication effect of the drain-to-gate
VIN MILLER EFFECT a QIN CMILLER = (QB - QA)/VDS b VGS V
capacitance as the drain voltage drops. The upper sloping line is due to the drain-to-gate accumulation capacitance and the gate-to-source capacitance. The Miller charge (the increase in coulombs on the horizontal axis from a to b while the curve is flat) is specified for a given VDS drain voltage, but can be adjusted for different VDS voltages by multiplying by the ratio of the application VDS to the curve specified VDS values. To estimate the capacitance CMILLER, take the change in gate charge from points a and b on a manufacturer's data sheet and divide by the stated VDS voltage specified. CMILLER is the most important selection criteria for determining the transition loss term in the top MOSFET but is not directly specified on MOSFET data sheets. CRSS and COS are specified sometimes but definitions of these parameters are not included. When the controller is operating in continuous mode the duty cycles for the top and bottom MOSFETs are given by: Top Gate Duty Cycle = VOUT VIN
V -V Bottom Gate Duty Cycle = IN OUT VIN The power dissipation for the top and bottom MOSFETs at maximum output current are given by:
PTOP = VOUT 2 I T( TOP) RDS(ON)(MAX ) VIN OUT(MAX ) IOUT(MAX ) + VIN2 RDR CMILLER * 2
(
)(
)(
)
(
)(
)
1 1 + * fSW INTVCC - VTH(IL ) VTH(IL ) PBOT = VIN - VOUT IOUT(MAX )2 T(TOP) RDS(ON)(MAX ) VIN
(
)(
)(
)
VGS
+
+V DS -
3775 F15
where: RDR = Effective top driver resistance VTH(IL) = MOSFET data sheet specified typical gate threshold voltage at the specified drain current
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Figure 15. Gate Charge Characteristics
20
LTC3775 APPLICATIONS INFORMATION
CMILLER = Calulated Miller capacitance using the gate charge curve from the MOSFET data sheet fSW = Switching frequency Both MOSFETs have conduction losses (I2R) while the topside N-channel equation includes an additional term for transition losses, which peak at the highest input voltage. For VIN < 12V, the high current efficiency generally improves with larger MOSFETs, while for VIN > 12V, the transition losses rapidly increase to the point that the use of a higher RDS(ON) device with lower CMILLER actually provides higher efficiency. The bottom MOSFET losses are greatest at high input voltage when the top switch duty factor is low or during a short circuit when the bottom switch is on close to 100% of the period. Schottky Diode Selection An optional Schottky diode connected between the SW node (cathode) and the source of the bottom MOSFET (anode) conducts during the dead time between the conduction of the power MOSFET switches. It is intended to prevent the body diode of the bottom MOSFET from turning on and storing a charge during the dead time, which can cause a modest (about 1%) efficiency loss. The diode can be rated for about one half to one fifth of the full load current since it is on for only a fraction of the duty cycle. In order for the diode to be effective, the inductance between it and the bottom MOSFET must be as small as possible, mandating that these components be placed next to each other on the same layer of the PC board. Input Capacitor Selection The input bypass capacitor has three primary requirements: its ESR must be low to minimize the supply drop when the top MOSFETs turn on, its RMS current capability must be adequate to withstand the ripple current at the input, and its capacitance must be large enough to maintain the input voltage until the input supply can respond. Generally, a capacitor (particularly a non-ceramic type) that meets the first two parameters will have far more capacitance than is required to keep capacitance-based droop under control. The input capacitor's voltage rating should be at least 1.4 times the maximum input voltage. In continuous mode, the source current of the top N-channel MOSFET is approximately a square wave of duty cycle VOUT/ VIN. The maximum RMS capacitor current is given by: IRMS IOUT(MAX) VOUT ( VIN - VOUT ) VIN
This formula has a maximum at VIN = 2VOUT , where IRMS = IOUT/2. This simple worst-case condition is commonly used for design because even significant deviations do not offer much relief. Note that capacitor manufacturer's ripple current ratings are often based on only 2000 hours of life. This makes it advisable to further derate the capacitor or to choose a capacitor rated at a higher temperature than required. Several capacitors may also be paralleled to meet size or height requirements in the design. Always consult the manufacturer if there is any question. Medium voltage (20V to 35V) ceramic, tantalum, OS-CON and switcher-rated electrolytic capacitors can be used as input capacitors, but each has drawbacks: ceramics have high voltage coefficients of capacitance and may have audible piezoelectric effects; tantalums need to be surge-rated; OS-CONs suffer from higher inductance, larger case size and limited surface mount applicability; and electrolytics' higher ESR and dryout may require several to be used in parallel. Sanyo OS-CON SVP SVPD , series; Sanyo POSCAP TQC series or aluminum electrolytic capacitors from Panasonic WA series or Cornel Dublilier SPV series, in parallel with a couple of high performance ceramic capacitors, can be used as an effective means of achieving low ESR and high bulk capacitance. Output Capacitor Selection The selection of COUT is primarily determined by the ESR required to minimize voltage ripple and load step transients. The output ripple VOUT is approximately bounded by: 1 VOUT IL ESR + 8 * fSW * COUT where IL is the inductor ripple current.
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21
LTC3775 APPLICATIONS INFORMATION
IL may be calculated using the equation: IL = VOUT VOUT 1- L * fSW VIN current. Lower ripple current reduces core losses in the inductor, ESR losses in the output capacitors and output voltage ripple. Highest efficiency is obtained at low frequency with small ripple current. However, achieving high efficiency requires a large inductor and generates higher output voltage excursion during load transients. There is a tradeoff between component size, efficiency and operating frequency. Given a specified limit for ripple current, the inductor value can be obtained using the following equation: L= VOUT V * 1 - OUT fSW * IL(MAX) VIN(MAX)
Since IL increases with input voltage, the output ripple voltage is highest at maximum input voltage. Typically, once the ESR requirement is satisfied, the capacitance is adequate for filtering and has the necessary RMS current rating. Manufacturers such as Sanyo, Panasonic and Cornell Dublilier should be considered for high performance through-hole capacitors. The OS-CON semiconductor electrolyte capacitor available from Sanyo has a good (ESR)(size) product. An additional ceramic capacitor in parallel with OS-CON capacitors is recommended to offset the effect of lead inductance. In surface mount applications, multiple capacitors may have to be connected in parallel to meet the ESR or transient current handling requirements of the application. Aluminum electrolytic and dry tantalum capacitors are both available in surface mount configurations. New special polymer surface mount capacitors offer very low ESR also but have much lower capacitive density per unit volume. In the case of tantalum, it is critical that the capacitors are surge tested for use in switching power supplies. Several excellent output capacitor choices are the Sanyo POSCAP TPD, POSCAP TPB, AVX TPS, AVX TPSV, the Kemet T510 series of surface mount tantalums,Kemet AO-CAPs or the Panasonic SP series of surface mount special polymer capacitors available in case heights ranging from 2mm to 4mm. Other capacitor types include Nichicon PL series and Sprague 595D series. Consult the manufacturer for other specific recommendations. Inductor Selection The inductor in a typical LTC3775 application circuit is chosen based on the required ripple current, its size and its saturation current rating. The inductor should not be allowed to saturate below the hard current limit threshold. The inductor value sets the ripple current, which is commonly chosen at around 40% of the anticipated full load
Once the value for L is known, the type of inductor must be selected. High efficiency converters generally cannot afford the core loss found in low cost powdered iron cores, forcing the use of more expensive ferrite, molypermalloy or Kool M(R) cores. A variety of inductors designed for high current, low voltage applications are available from manufacturers such as Sumida, Panasonic, Coiltronics, Coilcraft and Toko. See the Curret Limit Programming section for calculation of the inductor saturation current. Current Limit Programming If current sensing is implemented with a sense resistor, the topside current limit can be programmed by setting RILIMT as follows: RILIMT = CF * RSENSE * where: RSENSE = Sense resistor value IO(MAX) = Maximum output current IL = Inductor ripple current (refer to the Output Capacitor Selection section). ILIMT(MIN) = ILIMT pin minimum pull-down current of 90A CF = Correction factor to provide safety margin and account for RSENSE tolerance; use a value of CF = 1.2 is reasonable.
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IO(MAX ) + 0.5 * IL ILIMIT(MIN)
22
LTC3775 APPLICATIONS INFORMATION
If topside MOSFET RDS(ON) sensing is used, the RILIMT value is calculated from the following equation: RILIMIT = T * RDS(ON)(QT )(MAX ) * IO(MAX ) + 0.5 * IL ILIMIT(MIN) ILIMT(MAX) = ILIMT pin maximum pull-down current of 110A ILIMB(MAX) = ILIMB pin maximum pull-up current of 11A RDS(ON)(QT)(MIN) and RDS(ON)(QB)(MIN) are the power MOSFET minimum on-resistances. MOSFET data sheets typically specify nominal and maximum values for RDS(ON), but not a minimum. A reasonable assumption is that the minimum RDS(ON) is the same percentage below the typical value as the maximum lies above it. Consult the MOSFET manufacturer for further guidelines. The saturation current rating for the inductor should be determined at the maximum input voltage, maximum output current and the maximum expected core temperature. The saturation current ratings for most commercially available inductors drop at high temperature. To verify safe operation, it is a good idea to characterize the inductor's core/winding temperature under the following conditions: 1) worst-case operating conditions, 2) maximum allowable ambient temperature and 3) with the power supply mounted in the final enclosure. Thermal characterization can be done by placing a thermocouple in intimate contact with the winding/core structure, or by burying the thermocouple within the windings themselves.
2.0
T NORMALIZED ON-RESISTANCE
RDS(ON)(QT)(MAX) is the maximum MOSFET on-resistance typically specified at 25C. The T term is a normalization factor (unity at 25C) accounting for the significant variation in on-resistance with temperature, typically about 0.5%/C as shown in Figure 16. For a maximum junction temperature of 100C, using a value T = 1.4 is reasonable. The bottom side current limit can be programmed by setting RILIMB as follows: RILIMB = 5 * T * RDS(ON)(QB)(MAX ) * IO(MAX ) + 0.5 * IL ILIMB(MIN)
where ILIMB(MIN) = ILIMB pin minimum pull-up current of 9A. The resulting values of RILIMT and RILIMB should be checked in an actual circuit to ensure that the current limit kicks in as expected. Circuits that use MOSFETs with low value RDS(ON) for current sensing should be checked carefully. The PCB trace resistance and parasitic inductance can significantly change the actual current limit threshold. Care should be taken to shorten the PCB trace at the SENSE, SW and PGND connections. The current limit setting also determines the worst-case peak current flowing in the inductor during an overload condition. The inductor saturation current rating needs to be higher than the worst-case peak inductor current: IL(SAT) or IL(SAT) or IL(SAT ) ILIMT(MAX ) * RILIMT RDSON(QT )(MIN) ILIMT(MAX ) * RILIMT RSENSE(MIN)
1.5
1.0
0.5
0 -50
50 100 0 JUNCTION TEMPERATURE (C)
150
3775 F16
Figure 16. Typical MOSFET RDS(ON) vs Temperature
(0.2 * ILIMB(MAX) ) * RILIMB
RDSON(QB)(MIN)
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23
LTC3775 APPLICATIONS INFORMATION
MODE/SYNC Pin The MODE/SYNC pin is a dual function pin that can be used to program the operating mode or to synchronize the switching frequency to an external clock. Pulseskipping mode is enabled when the MODE/SYNC pin is above 1.2V. The mode is forced continuous when the pin is below 1.2V. If this pin is left floating, an internal 50k pull-down resistor defaults the selection to forced continuous mode. During power-up, the LTC3775 overrides this mode selection and operates in pulse-skipping mode to prevent the discharge of a pre-biased output capacitor. The internal LTC3775 oscillator can be synchronized to an external clock with a signal greater than 1.5V . A low-to-high transition on the MODE/SYNC pin resets the oscillator sawtooth waveform (high) and forces TG low (see Figure 17).The external oscillator frequency must be within 20% of the frequency programmed by the RSET resistor, or else the part will revert to free-running mode. The internal oscillator locks to the external clock after the second clock transition is received. When external synchronization is detected, the LTC3775 will operate in forced continuous mode. PC Board Layout Checklist When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC3775. Check the following in your layout: 1. Keep the signal and power grounds separate. The signal ground consists of the LTC3775 SGND pin and the (-) terminal of VOUT . The power ground consists of the optional Schottky diode anode, the source of the bottom
EXTERNAL CLOCK AT MODE/SYNC PIN
side MOSFET, and the (-) terminal of the input capacitor. Connect the signal ground to the (-) terminal of the output capacitor. Also, try to connect the (-) terminal of the output capacitor as close as possible to the (-) terminals of the input capacitor. 2. The high di/dt loop formed by the top N-channel MOSFET, the bottom MOSFET and the CIN capacitor should have short leads and PC trace lengths to minimize high frequency noise and voltage stress from inductive ringing. 3. Connect the drain of the topside MOSFET directly to the (+) plate of CIN, and connect the source of the bottom side MOSFET directly to the (-) terminal of CIN. This capacitor provides the AC current to the MOSFETs. 4. Place the ceramic CINTVCC decoupling capacitor immediately next to the IC, between INTVCC and SGND. Likewise, the CB capacitor should also be next to the IC between BOOST and SW. 5. Place the small-signal components away from high frequency switching nodes (BOOST, SW, TG and BG). 6. For optimum load regulation and true remote sensing, the top of the output resistor divider should connect independently to the top of the output capacitor (Kelvin connection), staying away from any high dV/dt traces. Place the divider resistors near the LTC3775 in order to keep the high impedance FB node short. 7. For applications with multiple switching power converters connected to the same input supply, make sure that the input filter capacitor for the LTC3775 is not shared with other converters. AC input current from another converter could cause substantial input voltage
PWM RAMP
TG
3775 F17
Figure 17. External Synchronization
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24
LTC3775 APPLICATIONS INFORMATION
ripple, and this could interfere with the operation of the LTC3775. A few inches of PC trace or wire (L 100nH) between CIN of the LTC3775 and the actual source VIN should be sufficient to prevent input noise interference problems. 8. The top current limit programming resistor, RILIMT , should be placed close to the LTC3775 and the other end of RILIMT should run parallel to the SENSE trace to the Kelvin sense connection underneath the sense resistor. 9. The bottom current limit programming resistor, RILIMB, should be placed close to the LTC3775 and the other end of RILIMB should connect to SGND. 10. The SW pin should be connected to the drain of the bottom MOSFET. Checking Transient Response For all new LTC3775 PCB circuits, transient tests need to be performed to verify the proper feedback loop operation. The regulator loop response can be checked by looking at the load current transient response. Switching regulators take several cycles to respond to a step in DC (resistive) load current. When a load step occurs, VOUT shifts by an amount equal to ILOAD * (ESR), where ESR is the effective series resistance of COUT . ILOAD also begins to charge or discharge COUT generating the feedback error signal that forces the regulator to adapt to the current change and return VOUT to its steady-state value. During this recovery time, VOUT can be monitored for excessive overshoot or ringing which would indicate a stability problem. Measuring transient response presents a challenge in two respects: obtaining an accurate measurement and generating a suitable transient for testing the circuit. Output measurements should be taken with a scope probe directly across the output capacitor. Proper high frequency probing techniques should be used. Do not use the 6" ground lead that comes with the probe! Use an adapter that fits on the tip of the probe and has a short ground clip to ensure that inductance in the ground path doesn't cause a bigger spike than the transient signal being measured. The typical probe tip ground shield is spaced just right to span the leads of a typical output capacitor. In general, it is best to take this measurement with the 20MHz bandwidth limit on the oscilloscope turned on to limit high frequency noise. Note that microprocessor manufacturers typically specify ripple 20MHz, as energy above 20MHz is generally radiated (and not conducted) and does not affect the load even if it appears at the output capacitor. Now that we know how to measure the signal, we need to have something to measure. The ideal situation is to use the actual load for the test, switching it on and off while watching the output. If this isn't convenient, a current step generator is needed. This generator needs to be able to turn on and off in nanoseconds to simulate a typical switching logic load, so stray inductance and long clip leads between the LTC3775 and the transient generator must be minimized. Figure 18 shows an example of a simple transient generator. Be sure to use a noninductive resistor as the load element. Many power resistors use an inductive spiral pattern and are not suitable for use here. A simple solution is to take ten 1/4W film resistors and wire them in parallel to get the desired value. This gives a noninductive resistive load which can dissipate 2.5W continuously or 250W if pulsed with a 1% duty cycle, enough for most LTC3775 circuits. Solder the MOSFET and the resistor(s) as close to the output of the LTC3775 circuit as possible and set up the signal generator to pulse at a 100Hz rate with a 1% duty cycle. This pulses the LTC3775 with 100s transients 10ms apart, adequate for viewing the entire transient recovery time for both positive and negative transitions while keeping the load resistor cool.
LTC3775 RLOAD PULSE GENERATOR 0V TO 10V 100Hz, 1% DUTY CYCLE 50
VOUT
IRFZ44 OR EQUIVALENT 10k
3775 F18
LOCATE CLOSE TO THE OUTPUT
Figure 18. Transient Load Generator
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25
LTC3775 APPLICATIONS INFORMATION
Design Example As a design example, take a supply with the following specifications: VIN = 5V to 26V (12V nominal), VOUT = 1.2V 5%, IOUT(MAX) = 15A, f = 500kHz. First, verify the minimum on-time which occurs at maximum VIN: tON(MIN) = 1.2V = 92.3ns (26V )(500kHz ) on for a short time, a RENESAS RJK0305DPB (RDS(ON) = , 13m (max), CMILLER = QGD/10V = 150pF VGS(TH) = 2.5V, JA = 40C/W) is sufficient. Check its power dissipation at current limit with = 100C = 1.4: P TOP = 1.2V (15A )2 * 1.4 * 13m 26 V 2 15A + ( 26 V ) (2.5)(150pF ) 2
(
)
The minimum on-time is lower than the top current limit comparator blanking time of 100ns with sense resistor sensing. The controller will rely on the bottom MOSFET RDS(ON) sensing at high VIN. Next, verify the maximum duty cycle which occurs at minimum VIN: 1.2V Maximum Duty Cycle = = 24% 5V This is below the LTC3775 maximum duty cycle of 90%. Next, calculate RSET to give the 500kHz operating frequency: RSET = 19500 = 39k 500
1 1 5.2 - 2.5 + 2.5 500kHz = 0.19 W + 0.73W = 0.92W And double check the assumed TJ in the MOSFET: TJ = 70C + (0.92W)(40C/W) = 107C The junction temperatures will be significantly less at nominal current, but this analysis shows that careful attention to heat sinking will be necessary. A RENESAS RJK0301DPB (RDS(ON) = 4m (max), JA = 40C/W) is chosen for the synchronous MOSFET. PBOT = 26 V - 1.2V (15A )2 * 1.4 * 4m = 1.26W 26 V
(
)
And double check the assumed TJ in the MOSFET: TJ = 70C + (1.26W)(40C/W) = 120C Next, the INTVCC LDO current is calculated: IINTVCC = (500kHz)(8nC + 32nC) + 3.5mA = 23.5mA And double check the TJ in the LTC3775: TJ = 70C + (23.5mA)(26V)(68C/W) = 112C Next, set the current limit resistors with a sense resistor of 3m. RILIMT = 1.2 * 3m * 15A + 0.5 * 6.4A = 728 90A 15A + 0.5 * 6.4A = 56.62k RILIMB = 5 * 1.4 * 4m * 9A
Next, choose the inductor value for about 40% ripple current at maximum VIN: L= 1.2V 1.2 1- = 0.38H (500kHz )(0.4)(15A ) 26
Select 0.36H which is the nearest standard value. The resulting maximum ripple current is: IL = 1.2V 1.2V 1- = 6.4A (500kHz )(0.36H) 26V
Next, choose the top and bottom MOSFET switch. Since the drain of each MOSFET will see the full supply voltage 26V (max) plus any ringing, choose a 30V MOSFET to provide a margin of safety. Because the top MOSFET is
Use the next higher standard values of 732 and 57.6k.
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26
LTC3775 APPLICATIONS INFORMATION
The worst-case peak inductor current based on a sense resistor tolerance of 1% is IL(SAT) 110A * 732 = 27.1A 2.97m (470F/5m each) are used to minimize output voltage changes due to inductor current ripple and load steps. The ripple voltage will be: 0.005 1 VOUT(RIPPLE) = 6.4A * + 2 8 * 500kHz * 470F * 2 = 17.7mV However, a 0A to 15A load step will cause an output voltage change of at least: VOUT(STEP) = (15A)(0.0025) = 37.5mV
The input RMS current is highest at VIN(MIN) = 5V and IOUT(MAX) = 15A: IRMS 15A 1.2V ( 5V - 1.2V ) 5V = 6.4A
CIN is chosen for an RMS current rating of >6.4A at 85C. For the output capacitor, two low ESR OS-CON capacitors
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27
LTC3775 TYPICAL APPLICATIONS
5V to 26V Input, 1.2V/15A Output at 500kHz
CF 220pF VIN 5V TO 26V CIN1 330F 35V
DB RILIMT 732 ILIMT RILIMB 57.6k CVCC 4.7F CSS 0.01F RSET 39.2k ILIMB SENSE CB 0.1F BOOST
RSENSE 0.003 VIN TG QT
+
INTVCC
LTC3775 SS FREQ C2 330pF FB SW
L1 0.36H
+
BG PGND MODE/SYNC QB
COUT 470F 2.5V 2
VOUT 1.2V 15A
RA 10k
RB 10k R2 C1 4.7k 3.9nF
COMP RUN/SHDN SGND
3775 TA02
COUT : SANYO 2R5TPD470M5 DB: CMDSH4E L1: IHLP-4040DZ-ER-R36-M11 QB: RJK0301DPB-00-J0 QT: RJK0305DPB-00-J0
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28
LTC3775 TYPICAL APPLICATIONS
8V to 36V Input, 2.5V/10A Output at 500kHz
VIN 8V TO 36V
R4 43.2k R5 10k RILIMB 133k
CB 0.1F V BOOST IN DB CVCC 4.7F CSS 0.01F RSET 39.2k ILIMB SENSE ILIMT
RILIMT 464 R SENSE 0.003 CF 220pF
+
CIN1 330F 35V
INTVCC LTC3775 SS FREQ
TG
QT L1 1.2H VOUT 2.5V 10A
SW
+
BG QB RUN/SHDN FB PGND MODE/SYNC COMP SGND
COUT 330F 4V 3
C3 1500pF R3 390 RA 10k RB 3.16k
C2 330pF
R2 C1 15k 2200pF
3775 TA03
COUT : SANYO 4TPD330M DB: CMDSH4E L1: TOKO FDA1254-1R2M QB,QT: INFINEON BSZ097N04LS
Efficiency and Power Loss vs Load Current
100 VIN = 12V 90 VOUT = 2.5V CONTINUOUS MODE 80 SW FREQ = 500kHz 70 60 50 40 30 20 10 0 0.01 0 0.1 1 LOAD CURRENT (A) 10
3775 TA03b
Load Step
5 VOUT(AC) 100mV/DIV POWER LOSS (W)
Start-Up
4
VOUT 1V/DIV
EFFICIENCY (%)
3 EFFICIENCY
IL 5A/DIV IL 10A/DIV
SWITCHOVER FROM PULSESKIPPING TO CONTINUOUS MODE
2
POWER LOSS
1
VSS 1V/DIV
VIN = 12V 50s/DIV VOUT = 2.5V LOAD = 0A TO 10A TO 0A MODE = 0V SW FREQ = 500kHz
3775 TA03c
VIN = 12V 2ms/DIV VOUT = 2.5V CSS = 0.01F MODE = 0V SW FREQ = 500kHz
3775 TA03d
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29
LTC3775 TYPICAL APPLICATIONS
24V Input, 12V/5A Output at 500kHz
VIN 24V CIN1 330F 35V
R4 69.8k R5 10k
DB RILIMT 1.24k ILIMT RILIMB 56.2k CVCC 4.7F CSS 0.01F RSET 39.2k ILIMB SENSE CB 0.1F BOOST
+
VIN
TG
QT
INTVCC
LTC3775 SS FREQ SW
L1 4.7H
+
BG QB RUN/SHDN FB PGND MODE/SYNC COMP SGND
COUT 68F 16V 2
VOUT 12V 5A
C3 330pF R3 2.05k RA 191k RB 10k
C2 330pF
R2 C1 7.68k 3.3nF
3775 TA04
COUT : SANYO 16TQC68M DB: CMDSH4E L1: IHLP-4040DZ-ER-4R7-M11 QB, QT: RJK0305DPB-00-JO
Efficiency and Power Loss vs Load Current
100 VIN = 24V 90 VOUT = 12V 80 CONTINUOUS MODE SW FREQ = 500kHz 70 60 50 40 30 20 10 0 0.01 1 0.1 LOAD CURRENT (A) 0 10
3775 TA04b
Load Step
2.5
Start-Up
2.0 POWER LOSS (W)
VOUT(AC) 200mV/DIV
VOUT 5V/DIV
EFFICIENCY (%)
1.5 EFFICIENCY 1.0
IL 5A/DIV
SWITCHOVER FROM PULSESKIPPING TO CONTINUOUS MODE
POWER LOSS
0.5
IL 5A/DIV
VSS 1V/DIV
VIN = 24V 50s/DIV VOUT = 12V LOAD = 0A TO 5A TO 0A MODE = 0V SW FREQ = 500kHz
3775 TA04c
VIN = 24V 2ms/DIV VOUT = 12V CSS = 0.01F MODE = 0V SW FREQ = 500kHz
3775 TA04d
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30
LTC3775 PACKAGE DESCRIPTION
UD Package 16-Lead Plastic QFN (3mm x 3mm)
(Reference LTC DWG # 05-08-1691)
0.70 0.05
3.50
0.05 2.10
1.45 0.05 0.05 (4 SIDES)
PACKAGE OUTLINE 0.25 0.05 0.50 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 0.75 0.05 BOTTOM VIEW--EXPOSED PAD R = 0.115 TYP 15 16 0.40 1 1.45 0.10 (4-SIDES) 2 0.10 PIN 1 NOTCH R = 0.20 TYP OR 0.25 45 CHAMFER
3.00 0.10 (4 SIDES) PIN 1 TOP MARK (NOTE 6)
(UD16) QFN 0904
0.200 REF 0.00 - 0.05 NOTE: 1. DRAWING CONFORMS TO JEDEC PACKAGE OUTLINE MO-220 VARIATION (WEED-2) 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE
0.25
0.05
0.50 BSC
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Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
31
LTC3775 RELATED PARTS
PART NUMBER DESCRIPTION LTC3854 LTC3851A/ LTC3851A-1 LTC3878 LTC3879 LTC3703-5 LTC3703 LTC3850/ LTC3850-1/ LTC3850-2 LTC3853 LTM(R)4600HV LTM4601AHV LTC3601 LTC3603 LTC3605 LTC3608 LTC3609 LTC3610 LTC3611 LTC3824 LTC3857/ LTC3857-1 LTC3868/ LTC3868-1 LT(R)3845 Small Footprint Wide VIN Range Synchronous Step-Down DC/DC Controller No RSENSETM Wide VIN Range Synchronous Step-Down DC/DC Controller No RSENSE Constant On-Time Synchronous Step-Down DC/DC Controller No RSENSE Constant On-Time Synchronous Step-Down DC/DC Controller High Voltage Synchronous Step-Down DC/DC Voltage Mode Controller High Voltage Synchronous Step-Down DC/DC Voltage Mode Controller Dual 2-Phase, High Efficiency Synchronous Step-Down DC/DC Controllers, RSENSE or DCR Current Sensing and Tracking Triple Output, Multiphase Synchronous Step-Down DC/DC Controller, RSENSE or DCR Current Sensing and Tracking 10A DC/DC Module Complete Power Supply 12A DC/DC Module Complete Power Supply 1.5A, 4MHz, Monolithic Synchronous Step-Down DC/DC Converter 2.5A, 3MHz, Monolithic Synchronous Step-Down DC/DC Converter 5A, 4MHz, Monolithic Synchronous Step-Down DC/DC Converter 8A, 1MHz, Monolithic Synchronous Step-Down DC/DC Converter 6A, 1MHz, Monolithic Synchronous Step-Down DC/DC Converter 12A, 1MHz, Monolithic Synchronous Step-Down DC/DC Converter 10A, 1MHz, Monolithic Synchronous Step-Down DC/DC Converter Low IQ, High Voltage DC/DC Controller, 100% Duty Cycle Low IQ, Dual Output 2-Phase Synchronous Step-Down DC/DC Controller with 99% Duty Cycle Low IQ, Dual Output 2-Phase Synchronous Step-Down DC/DC Controller with 99% Duty Cycle Low IQ , High Voltage Synchronous Step-Down DC/DC Controller COMMENTS Fixed 400kHz Operating Frequency, 4.5V VIN 38V, 0.8V VOUT 5.25V, 2mm x 3mm QFN-12 Phase-Lockable Fixed Operating Frequency 250kHz to 750kHz, 4V VIN 38V, 0.8V VOUT 5.25V, MSOP-16E, 3mm x 3mm QFN-16, SSOP-16 Very Fast Transient Response, tON(MIN) = 43ns, 4V VIN 38V, 0.8V VOUT 0.9VIN, SSOP-16 Very Fast Transient Response, tON(MIN) = 43ns, 4V VIN 38V, 0.6V VOUT 0.9VIN, MSOP-16E, 3mm x 3mm QFN-16 4.1V VIN 60V, 0.8V VOUT 0.9VIN, Logic-Level Gate Driver, SSOP-16, TSSOP-28 9.3V VIN 100V, 0.8V VOUT 0.9VIN, 9.3V to 15V Gate Driver, SSOP-16, TSSOP-28 Phase-Lockable Fixed Operating Frequency 250kHz to 780kHz, 4V VIN 30V, 0.8V VOUT 5.25V Phase-Lockable Fixed Operating Frequency 250kHz to 750kHz, 4V VIN 24V, VOUT Up to 13.5V High Efficiency, Compact Size, UltraFastTM Transient Response, 4.5V VIN 28V, 0.8V VOUT 5V, 15mm x 15mm x 2.8mm High Efficiency, Compact Size, UltraFast Transient Response, 4.5V VIN 28V, 0.8V VOUT 5V, 15mm x 15mm x 2.8mm High Efficiency, Phase Lockable IQ = 300A, 4V VIN 15V, VOUT(MIN) = 0.6V, 3mm x 3mm QFN-16, MSOP-16E High Efficiency, Phase Lockable IQ = 75A, 4.5V VIN 15V, VOUT(MIN) = 0.6V, 4mm x 4mm QFN-20 High Efficiency, Adjustable Frequency 800kHz to 4MHz, 4V VIN 15V, VOUT(MIN) = 0.6V, 4mm x 4mm QFN-24 High Efficiency, Adjustable Constant On-Time, 4V VIN 18V, VOUT(MIN) = 0.6V, 7mm x 8mm QFN-52 High Efficiency, Adjustable Constant On-Time, 4V VIN 32V, VOUT(MIN) = 0.6V, 7mm x 8mm QFN-52 High Efficiency, Adjustable Constant On-Time, 4V VIN 24V, VOUT(MIN) = 0.6V, 9mm x 9mm QFN-64 High Efficiency, Adjustable Constant On-Time, 4V VIN 32V, VOUT(MIN) = 0.6V, 9mm x 9mm QFN-64 Selectable Fixed Operating Frequency, 4V VIN 60V, 0.8V VOUT VIN, IQ = 40A, MSOP-10E Phase-Lockable Fixed Operating Frequency 50kHz to 900kHz, 4V VIN 38V, 0.8V VOUT 24V, IQ = 50A, Phase-Lockable Fixed Operating Frequency 50kHz to 900kHz, 4V VIN 24V, 0.8V VOUT 14V, IQ = 170A, Adjustable Fixed Operating Frequency 100kHz to 500kHz, 4V VIN 60V, 1.23V VOUT 36V, IQ = 30A, TSSOP-16
No RSENSE and UltraFast are trademarks of Linear Technology Corporation.
3775f
32 Linear Technology Corporation
(408) 432-1900 FAX: (408) 434-0507
LT 1009 * PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2009

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