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FEATURES
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LTC3608 18V, 8A Monolithic Synchronous Step-Down DC/DC Converter DESCRIPTION
The LTC(R)3608 is a high efficiency, monolithic synchronous step-down DC/DC converter that can deliver up to 8A output current from a 4V to 18V (20V maximum) input supply. It uses a valley current control architecture to deliver very low duty cycle operation at high frequency with excellent transient response. The operating frequency is selected by an external resistor and is compensated for variations in VIN and VOUT. The LTC3608 can be configured for discontinuous or forced continuous operation at light load. Forced continuous operation reduces noise and RF interference while discontinuous mode provides high efficiency by reducing switching losses at light loads. Fault protection is provided by internal foldback current limiting, an output overvoltage comparator and an optional short-circuit shutdown timer. Soft-start capability for supply sequencing is accomplished using an external timing capacitor. The regulator current limit is user programmable. A power good output voltage monitor indicates when the output is in regulation. The LTC3608 is available in a compact 7mm x 8mm QFN package.
8A Output Current Wide VIN Range = 4V to 18V Internal N-Channel MOSFETs True Current Mode Control Optimized for High Step-Down Ratios tON(MIN) 100nsec Extremely Fast Transient Response Stable with Ceramic COUT 1% 0.6V Voltage Reference Power Good Output Voltage Monitor Adjustable On-Time/Switching Frequency Adjustable Current Limit Programmable Soft-Start Output Overvoltage Protection Optional Short-Circuit Shutdown Timer Low Shutdown IQ: 15A Available in a 7mm x 8mm 52-Lead QFN Package
APPLICATIONS
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Point of Load Regulation Distributed Power Systems
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 5481178, 6100678, 6580258, 5847554, 6304066.
TYPICAL APPLICATION
High Efficiency Step-Down Converter
0.1F VOUT 187k VON RUN/SS 100pF LTC3608 0.8H SW 1500pF 11.3k ITH SGND BOOST INTVCC FCB VRNG PGOOD EXTVCC PGND VFB
3608 TA01a
Efficiency and Power Loss vs Load Current
100 10000 EFFICIENCY 1000 POWER LOSS (mW) VIN 4V TO 18V 95 90 EFFICIENCY (%) VOUT 2.5V 8A 85 80 75 70 65 60 VIN = 12V VOUT = 2.5V EXTVCC = 5V 0.1 1 LOAD CURRENT (A) POWER LOSS 10 100
ION VIN 10F x3
0.22F
100F x2
30.1k 4.7F
55 50 0.01 9.53k
1 10
3608 TA01b
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LTC3608 ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW 52 PVIN 51 PVIN 50 PVIN 49 PVIN 48 PVIN 47 SW 46 SW 45 SW 44 SW 43 SW 42 SW 41 SW
Input Supply Voltage (SVIN, PVIN, ION)....... 20V to -0.3V Boosted Topside Driver Supply Voltage (BOOST) ................................................ 26V to -0.3V SW Voltage ............................................... 20V to -5V INTVCC, EXTVCC, (BOOST - SW), RUN/SS, PGOOD Voltages ...................................... 7V to -0.3V FCB, VON, VRNG Voltages............ INTVCC + 0.3V to -0.3V ITH, VFB Voltages ....................................... 2.7V to -0.3V Operating Junction Temperature Range (Notes 2, 4)........................................ -40C to 125C Storage Temperature Range................... -55C to 125C
PVIN 1 PVIN 2 PVIN 3 PVIN 4 PVIN 5 PVIN 6 PVIN 7 SW 8 NC 9 SGND 10 BOOST 11 RUN/SS 12 VON 13 SGND 14 54 SGND 53 PVIN
40 PGND 39 PGND 38 PGND 55 SW 37 PGND 36 PGND 35 PGND 34 PGND 33 SW 32 INTVCC 31 INTVCC 30 SVIN 29 EXTVCC 28 NC 27 SGND
PGOOD 16
SGND 20
SGND 15
WKG PACKAGE 52-LEAD (7mm x 8mm) QFN MULTIPAD
TJMAX = 125C, JA = 29C/W
ORDER INFORMATION
LEAD FREE FINISH LTC3608EWKG#PBF LTC3608IWKG#PBF TAPE AND REEL LTC3608EWKG#TRPBF LTC3608IWKG#TRPBF PART MARKING* LTC3608WKG LTC3608WKG PACKAGE DESCRIPTION 52-Lead (7mm x 8mm) Plastic QFN 52-Lead (7mm x 8mm) Plastic QFN TEMPERATURE RANGE -40C to 125C -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. 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/
SGND 26
FCB 19
NC 21
NC 24
VRNG 17
ITH 18
ION 22
VFB 23
NC 25
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LTC3608 ELECTRICAL CHARACTERISTICS
SYMBOL Main Control Loop SVIN IQ Operating Input Voltage Range Input DC Supply Current Normal Shutdown Supply Current Feedback Reference Voltage Feedback Voltage Line Regulation Feedback Voltage Load Regulation Feedback Input Current Error Amplifier Transconductance Forced Continuous Threshold Forced Continuous Pin Current On-Time Minimum On-Time Minimum Off-Time Maximum Valley Current Maximum Reverse Valley Current Output Overvoltage Fault Threshold RUN Pin Start Threshold RUN Pin Latchoff Enable Threshold RUN Pin Latchoff Threshold Soft-Start Charge Current Soft-Start Discharge Current Undervoltage Lockout Undervoltage Lockout Release Top Switch On-Resistance Bottom Switch On-Resistance Internal VCC Voltage Internal VCC Load Regulation EXTVCC Switchover Voltage EXTVCC Switch Drop Voltage EXTVCC Switchover Hysteresis PGOOD Upper Threshold PGOOD Lower Threshold PGOOD Hysteresis PGOOD Low Voltage VFB Rising VFB Falling VFB Returning IPGOOD = 5mA 7 -7 6V < VIN < 18V, VEXTVCC = 4V ICC = 0mA to 20mA, VEXTVCC = 4V ICC = 20mA, VEXTVCC Rising ICC = 20mA, VEXTVCC = 5V
l l l
The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25C. VIN = 15V unless otherwise noted.
PARAMETER CONDITIONS MIN 4 900 15 ITH = 1.2V, -40C to 85C (Note 3) ITH = 1.2V, -40C to 125C (Note 3) VIN = 4V to 18V, ITH = 1.2V (Note 3) ITH = 0.5V to 1.9V (Note 3) VFB = 0.6V ITH = 1.2V (Note 3) VFCB = 0.6V ION = 60A, VON = 1.5V ION = 60A, VON = 0V ION = 180A, VON = 0V ION = 30A, VON = 1.5V VRNG = 0.5V, VFB = 0.56V, FCB = 0V VRNG = 0V, VFB = 0.56V, FCB = 0V VRNG = 0.5V, VFB = 0.64V, FCB = 0V VRNG = 0V, VFB = 0.64V, FCB = 0V
l l l l l
TYP
MAX 18 2000 30 0.606 0.610 -0.3 50 2 0.66 -2 340 100 500
UNITS V A A V V %/V % nA mS V A ns ns ns ns A A
VFB VFB(LINEREG) VFB(LOADREG) IFB gm(EA) VFCB IFCB tON tON(MIN) tOFF(MIN) IVALLEY(MAX) IVALLEY(MIN) VFB(OV) VRUN/SS(ON) VRUN/SS(LE) VRUN/SS(LT) IRUN/SS(C) IRUN/SS(D) VIN(UVLO) VIN(UVLOR) RDS(ON)
0.594 0.590
0.600 0.600 0.002 -0.05 -5
1.4 0.54 220
1.7 0.6 -1 280 110 60 320
5 8 3.5 5.5 7 0.8
11 16 5.5 7.5 10 1.5 4 3.5 7.5 9.5 13 2 4.5 4.2 -3 3 3.9 4 19 14 5.5 2 300
A A % V V V A A V V m m V % V mV mV
RUN/SS Pin Rising RUN/SS Pin Falling VRUN/SS = 0V VRUN/SS = 4.5V, VFB = 0V INTVCC Falling INTVCC Rising
l l
-0.5 0.8
-1.2 1.8 3.4 3.5 10 8
Internal VCC Regulator VINTVCC VLDO(LOADREG) VEXTVCC VEXTVCC VEXTVCC(HYS) PGOOD Output VFBH VFBL VFB(HYS) VPGL 10 -10 1 0.15 13 -13 2.5 0.4 % % % V
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4.7 4.5
5 -0.1 4.7 150 500
3
LTC3608 ELECTRICAL CHARACTERISTICS
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: TJ is calculated from the ambient temperature TA and power dissipation PD as follows: LTC3608: TJ = TA + (PD * 29C/W)(JA is simulated per JESD51-7 high effective thermal conductivity test board) JC = 1C/W (JC is simulated when heat sink is applied at the bottom of the package.) Note 3: The LTC3608 is tested in a feedback loop that adjusts VFB to achieve a specified error amplifier output voltage (ITH). The specification at 85C is not tested in production. This specification is assured by design, characterization, and correlation to testing at 125C. Note 4: The LTC3608E is guaranteed to meet performance specifications from 0C to 125C. Specifications over the -40C to 125C operating junction temperature range are assured by design, characterization and correlation with statistical process controls. The LTC3608I is guaranteed over the full -40C to 125C operating junction temperature range.
TYPICAL PERFORMANCE CHARACTERISTICS
Transient Response
VOUT 200mV/DIV VOUT 200mV/DIV
Transient Response
Start-Up
RUN/SS 2V/DIV
IL 5A/DIV ILOAD 5A/DIV 20s/DIV LOAD STEP 0A TO 8A VIN = 12V VOUT = 2.5V FCB = 0V FIGURE 6 CIRCUIT
3608 G01
IL 5A/DIV ILOAD 5A/DIV 20s/DIV VIN = 12V VOUT = 2.5V FCB = INTVCC FIGURE 6 CIRCUIT
3610 G02
VOUT 1V/DIV IL 5A/DIV 40ms/DIV VIN = 12V VOUT = 2.5V RLOAD = 0.5 FIGURE 6 CIRCUIT
3608 G03
Efficiency vs Load Current
100 90 80 95 100
Efficiency vs Input Voltage
FCB = 5V FIGURE 6 CIRCUIT 650
Frequency vs Input Voltage
FCB = 0V FIGURE 6 CIRCUIT
ILOAD = 10A
600 FREQUENCY (kHz) EFFICIENCY (%)
EFFICIENCY (%)
70 60 50 40 30 20 10 VIN = 12V FREQ = 550kHz 0 0.1 1 0.01 LOAD CURRENT (A)
ILOAD = 10A
90
550
VOUT = 5V VOUT = 3.3V VOUT = 2.5V VOUT = 2.5V VOUT = 1.8V VOUT = 1.2V VOUT = 1V
10
3608 G04
ILOAD = 1A
85
500
ILOAD = 1A
450
80 5 10 15 INPUT VOLTAGE (V) 20
3608 G05
400 5 10 15 INPUT VOLTAGE (V) 20
3608 G06
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LTC3608 TYPICAL PERFORMANCE CHARACTERISTICS
Frequency vs Load Current
700 600 CONTINUOUS MODE FREQUENCY (kHz) 500 VOUT (%) 400 DISCONTINUOUS MODE 300 200 100 0 0 2 4 6 LOAD CURRENT (A) 8 10
3608 G07
Load Regulation
0.80 0.60 2.0 0.40 0.20 0 ITH VOLTAGE (V) 1.5 FIGURE 6 CIRCUIT 2.5
ITH Voltage vs Load Current
-0.20 -0.40 -0.60 -0.80 0 2 4 6 LOAD CURRENT (A) 8 10
3608 G08
1.0
CONTINUOUS MODE DISCONTINUOUS MODE
0.5
0
0
5 LOAD CURRENT (A)
10
3608 G09
Load Current vs ITH Voltage and VRNG
25 20 0.7V LOAD CURRENT (A) 15 10 5 0 -5 -10 0 0.5 1.0 1.5 2.0 ITH VOLTAGE (V) 2.5 3.0
3608 G10
On-Time vs ION Current
10000 1V VVON = 0V 1000
On-Time vs VON Voltage
ION = 30A
VRNG =
800 ON-TIME (ns) ON-TIME (ns) 1000
0.5V
600
400
100 200
10 1 10 ION CURRENT (A) 100
3608 G11
0
0
1 2 VON VOLTAGE (V)
3
3608 G12
On-Time vs Temperature
300 250 200 150 100 50 0 -50 -25 MAXIMUM VALLEY CURRENT LIMIT (A) IION = 30A VVON = 0V 25
Maximum Valley Current Limit vs VRNG Voltage
15 MAXIMUM VALLEY CURRENT LIMIT (A) 0.5 0.6 0.7 0.8 VRNG VOLTAGE (V) 0.9 1.0
3608 G14
Maximum Valley Current Limit vs RUN/SS Voltage
20
12
ON-TIME (ns)
9
15
6
10
3
0 50 25 75 TEMPERATURE (C)
100
125
5
3608 G13
0 1.65 1.90 2.15 2.40 2.65 2.90 3.15 3.40 RUN/SS VOLTAGE (V)
3608 G15
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LTC3608 TYPICAL PERFORMANCE CHARACTERISTICS
Maximum Valley Current Limit vs Temperature
20 MAXIMUM VALLEY CURRENT LIMIT (A) MAXIMUM VALLEY CURRENT (A) 18 16 14 12 10 8 6 4 -25 0 50 75 25 TEMPERATURE (C) 100 125
3608 G16
Input Voltage vs Maximum Valley Current
15 MAXIMUM VALLEY CURRENT LIMIT (A)
Maximum Valley Current Limit in Foldback
15
10
10
5
5
0 -50
0 4 8 12 16 INPUT VOLTAGE (V) 20
3608 G17
0
0.1
0.2
0.3 VFB (V)
0.4
0.5
0.6
3608 G18
Feedback Reference Voltage vs Temperature
0.62 FEEDBACK REFERENCE VOLTAGE (V) 2.0
Error Amplifier gm vs Temperature
0.61 gm (mS)
1.8
1.6
0.60
1.4
0.59 1.2
0.58 -50
-25
75 0 25 50 TEMPERATURE (C)
100
125
1.0 -50
-25
50 25 0 75 TEMPERATURE (C)
100
125
3608 G19
3608 G20
Input and Shutdown Currents vs Input Voltage
1400 1200 INPUT CURRENT (A) 1000 800 SHUTDOWN 600 15 400 200 0 0 5 15 10 INPUT VOLTAGE (V) 20
3608 G21
INTVCC Load Regulation
40 0.30 0.20 SHUTDOWN CURRENT (A) 0.10 INTVCC (%) 0 35 30 25 20
EXTVCC OPEN
-0.10 -0.20 -0.30 -0.40
EXTVCC = 5V
10 5 0
0
40 10 20 30 INTVCC LOAD CURRENT (mA)
50
3608 G22
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LTC3608 TYPICAL PERFORMANCE CHARACTERISTICS
IEXTVCC vs Frequency
20 18 EXTVCC SWITCH RESISTANCE () 500 600 700 800 FREQUENCY (kHz) 900 1000
3608 G23
EXTVCC Switch Resistance vs Temperature
10
VIN = 20V
16 14 IEXTVCC (mA) 12 10 8 6 4 2 0 400
8
6
4
2
0 -50
-25
0 50 75 25 TEMPERATURE (C)
100
125
3608 G24
FCB Pin Current vs Temperature
0 -0.25 FCB PIN CURRENT (A) -0.50 -0.75 -1.00 -1.25 -1.50 -50 -25 3
RUN/SS Pin Current vs Temperature
RUN/SS PIN CURRENT (A)
2 PULL-DOWN CURRENT 1
0
-1 PULL-UP CURRENT
0 50 25 75 TEMPERATURE (C)
100
125
-2 -50
-25
3608 G25
0 50 75 25 TEMPERATURE (C)
100
125
3608 G26
RUN/SS Pin Current vs Temperature
UNDERVOLTAGE LOCKOUT THRESHOLD (V) 5.0 4.0
Undervoltage Lockout Threshold vs Temperature
RUN/SS PIN CURRENT (A)
4.5 LATCHOFF ENABLE 4.0
3.5
3.0
3.5
LATCHOFF THRESHOLD
2.5
3.0 -50
-25
75 0 25 50 TEMPERATURE (C)
100
125
2.0 -50
-25
3608 G27
75 0 25 50 TEMPERATURE (C)
100
125
3608 G28
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LTC3608 PIN FUNCTIONS
PVIN (Pins 1, 2, 3, 4, 5, 6, 7, 48, 49, 50, 51, 52, 53): Main Input Supply. Decouple this pin to power PGND with the input capacitance CIN SW (Pins 8, 33, 41, 42, 43, 44, 45, 46, 47, 55): Switch Node Connection to the Inductor. The (-) terminal of the bootstrap capacitor CB also connects here. This pin swings from a diode voltage drop below ground up to VIN. SGND (Pins 10, 14, 15, 20, 26, 27, 54): Signal Ground. All small-signal components and compensation components should connect to this ground, which in turn connects to PGND at one point. BOOST (Pin 11): Boosted Floating Driver Supply. The (+) terminal of the bootstrap capacitor CB connects here. This pin swings from a diode voltage drop below INTVCC up to VIN + INTVCC. RUN/SS (Pin 12): Run Control and Soft-Start Input. A capacitor to ground at this pin sets the ramp time to full output current (approximately 3s/F) and the time delay for overcurrent latchoff (see Applications Information). Forcing this pin below 0.8V shuts down the device. VON (Pin 13): On-Time Voltage Input. Voltage trip point for the on-time comparator. Tying this pin to the output voltage or an external resistive divider from the output makes the on-time proportional to VOUT. The comparator input defaults to 0.7V when the pin is grounded and defaults to 2.4V when the pin is tied to INTVCC. Tie this pin to INTVCC in high VOUT applications to use a lower RON value. PGOOD (Pin 16): Power Good Output. Open drain logic output that is pulled to ground when the output voltage is not within 10% of the regulation point. VRNG (Pin 17): Current Limit Range Input. The voltage at this pin adjusts maximum valley current and can be set from 0.5V to 0.7V by a resistive divider from INTVCC. It defaults to 0.7V if the VRNG pin is tied to ground which results in a typical 19A current limit. ITH (Pin 18): Current Control Threshold and Error Amplifier Compensation Point. The current comparator threshold increases with this control voltage. The voltage ranges from 0V to 2.4V with 0.8V corresponding to zero sense voltage (zero current). FCB (Pin 19): Forced Continuous Input. Tie this pin to ground to force continuous synchronous operation at low load, to INTVCC to enable discontinuous mode operation at low load or to a resistive divider from a secondary output when using a secondary winding. NC (Pins 9, 21, 24, 25, 28): No Connection. ION (Pin 22): On-Time Current Input. Tie a resistor from VIN to this pin to set the one-shot timer current and thereby set the switching frequency. VFB (Pin 23): Error Amplifier Feedback Input. This pin connects the error amplifier input to an external resistive divider from VOUT. EXTVCC (Pin 29): External VCC Input. When EXTVCC exceeds 4.7V, an internal switch connects this pin to INTVCC and shuts down the internal regulator so that controller and gate drive power is drawn from EXTVCC. Do not exceed 7V at this pin and ensure that EXTVCC < VIN. SVIN (Pin 30): Supply pin for internal PWM controller. INTVCC (Pins 31, 32): Internal 5V Regulator Output. The driver and control circuits are powered from this voltage. Decouple this pin to power ground with a minimum of 4.7F low ESR tantalum or ceramic capacitor. PGND (Pins 34, 35, 36, 37, 38, 39, 40): Power Ground. Connect this pin closely to the (-) terminal of CVCC and the (-) terminal of CIN.
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LTC3608 FUNCTIONAL DIAGRAM
RON VON 13 ION 22 FCB 19 4.7V 0.7V 2.4V 1A EXTVCC 29 30 PVIN SVIN
+
0.6V
-
0.6V REF 5V REG
1, 2, 3, 4, 5, 6, 7, 48, 49, 50, 51, 52, 53 CIN INTVCC 31, 32 11 BOOST
-
F VVON tON = (10pF) IION
+
R S Q FCNT ON M1 CB
+
ICMP
20k
+
IREV SWITCH LOGIC
SW 8, 33, 41, 42, 43, 44, 45, 46, 47, 55
DB
L1 VOUT
-
1.4V
-
SHDN
+
COUT M2 PGND 34, 35, 36, 37, 38, 39, 40 16 PGOOD CVCC
OV VRNG 17
x (0.5 TO 2)
0.7V
1 240k Q2 Q4 ITHB Q6
1V UV
+ -
R2 0.54V
23 Q3 Q1 OV
VFB R1 SGND
+ -
SS RUN SHDN 1.2A 27 NC 9, 21, 24, 25, 28 0.66V 10, 14, 15, 20, 26, 27, 54
+ -
0.8V
x3.3
EA
0.6V 18 ITH
+
- + - - +
6V
0.4V 12 RUN/SS CSS
3608 FD
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LTC3608 OPERATION
Main Control Loop The LTC3608 is a high efficiency monolithic synchronous, step-down DC/DC converter utilizing a constant on-time, current mode architecture. It operates from an input voltage range of 4V to 18V (20V maximum) and provides a regulated output voltage at up to 8A of output current. The internal synchronous power switch increases efficiency and eliminates the need for an external Schottky diode. In normal operation, the top MOSFET is turned on for a fixed interval determined by a one-shot timer OST. When the top MOSFET is turned off, the bottom MOSFET is turned on until the current comparator ICMP trips, restarting the one-shot timer and initiating the next cycle. Inductor current is determined by sensing the voltage between the PGND and SW pins using the bottom MOSFET on-resistance. The voltage on the ITH pin sets the comparator threshold corresponding to inductor valley current. The error amplifier EA adjusts this voltage by comparing the feedback signal VFB from the output voltage with an internal 0.6V reference. If the load current increases, it causes a drop in the feedback voltage relative to the reference. The ITH voltage then rises until the average inductor current again matches the load current. At light load, the inductor current can drop to zero and become negative. This is detected by current reversal comparator IREV which then shuts off M2 (see Functional Diagram), resulting in discontinuous operation. Both switches will remain off with the output capacitor supplying the load current until the ITH voltage rises above the zero current level (0.8V) to initiate another cycle. Discontinuous mode operation is disabled by comparator F when the FCB pin is brought below 0.6V, forcing continuous synchronous operation. The operating frequency is determined implicitly by the top MOSFET on-time and the duty cycle required to maintain regulation. The one-shot timer generates an on-time that is proportional to the ideal duty cycle, thus holding frequency approximately constant with changes in VIN. The nominal frequency can be adjusted with an external resistor RON. Overvoltage and undervoltage comparators OV and UV pull the PGOOD output low if the output feedback voltage exits a 10% window around the regulation point. Furthermore, in an overvoltage condition, M1 is turned off and M2 is turned on and held on until the overvoltage condition clears. Foldback current limiting is provided if the output is shorted to ground. As VFB drops, the buffered current threshold voltage ITHB is pulled down by clamp Q3 to a 1V level set by Q4 and Q6. This reduces the inductor valley current level to one sixth of its maximum value as VFB approaches 0V. Pulling the RUN/SS pin low forces the controller into its shutdown state, turning off both M1 and M2. Releasing the pin allows an internal 1.2A current source to charge up an external soft-start capacitor CSS. When this voltage reaches 1.5V, the controller turns on and begins switching, but with the ITH voltage clamped at approximately 0.6V below the RUN/SS voltage. As CSS continues to charge, the soft-start current limit is removed. INTVCC/EXTVCC Power Power for the top and bottom MOSFET drivers and most of the internal controller circuitry is derived from the INTVCC pin. The top MOSFET driver is powered from a floating bootstrap capacitor CB. This capacitor is recharged from INTVCC through an external Schottky diode DB when the top MOSFET is turned off. When the EXTVCC pin is grounded, an internal 5V low dropout regulator supplies the INTVCC power from VIN. If EXTVCC rises above 4.7V, the internal regulator is turned off, and an internal switch connects EXTVCC to INTVCC. This allows a high efficiency source connected to EXTVCC, such as an external 5V supply or a secondary output from the converter, to provide the INTVCC power. Voltages up to 7V can be applied to EXTVCC for additional gate drive. If the input voltage is low and INTVCC drops below 3.5V, undervoltage lockout circuitry prevents the power switches from turning on.
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LTC3608 APPLICATIONS INFORMATION
The basic LTC3608 application circuit is shown on the front page of this data sheet. External component selection is primarily determined by the maximum load current. The LTC3608 uses the on-resistance of the synchronous power MOSFET for determining the inductor current. The desired amount of ripple current and operating frequency also determines the inductor value. Finally, CIN is selected for its ability to handle the large RMS current into the converter and COUT is chosen with low enough ESR to meet the output voltage ripple and transient specification. VON and PGOOD The LTC3608 has an open-drain PGOOD output that indicates when the output voltage is within 10% of the regulation point. The LTC3608 also has a VON pin that allows the on-time to be adjusted. Tying the VON pin high results in lower values for RON which is useful in high VOUT applications. The VON pin also provides a means to adjust the on-time to maintain constant frequency operation in applications where VOUT changes and to correct minor frequency shifts with changes in load current. VRNG Pin and ILIMIT Adjust The VRNG pin is used to adjust the maximum inductor valley current, which in turn determines the maximum average output current that the LTC3608 can deliver. The maximum output current is given by: IOUT(MAX) = IVALLEY(MAX) + 1/2 IL, The IVALLEY(MAX) is shown in the figure "Maximum Valley Current Limit vs VRNG Voltage" in the Typical Performance Characteristics. An external resistor divider from INTVCC can be used to set the voltage on the VRNG pin from 0.5V to 1V, or it can be simply tied to ground force a default value equivalent to 0.7V. When setting current limit ensure that the junction temperature does not exceed the maximum rating of 125C. Do not float the VRNG pin. Operating Frequency The choice of operating frequency is a tradeoff between efficiency and component size. Low frequency operation improves efficiency by reducing MOSFET switching losses but requires larger inductance and/or capacitance in order to maintain low output ripple voltage. The operating frequency of LTC3608 applications is determined implicitly by the one-shot timer that controls the on-time tON of the top MOSFET switch. The on-time is set by the current into the ION pin and the voltage at the VON pin according to: tON = VVON (10pF) IION
Tying a resistor RON from VIN to the ION pin yields an on-time inversely proportional to VIN. The current out of the ION pin is V ION = IN RON For a step-down converter, this results in approximately constant frequency operation as the input supply varies: f= VVON VOUT [H ] RON (10pF) Z
To hold frequency constant during output voltage changes, tie the VON pin to VOUT or to a resistive divider from VOUT when VOUT > 2.4V. The VON pin has internal clamps that limit its input to the one-shot timer. If the pin is tied below 0.7V, the input to the one-shot is clamped at 0.7V. Similarly, if the pin is tied above 2.4V, the input is clamped at 2.4V. In high VOUT applications, tying VON to INTVCC so that the comparator input is 2.4V results in a lower value for RON. Figures 1a and 1b show how RON relates to switching frequency for several common output voltages.
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LTC3608 APPLICATIONS INFORMATION
1000
VOUT = 3.3V VOUT = 1.5V VOUT = 2.5V
100 100 1000 RON (k) 10000
3608 F01a
load current increases. By lengthening the on-time slightly as current increases, constant frequency operation can be maintained. This is accomplished with a resistive divider from the ITH pin to the VON pin and VOUT. The values required will depend on the parasitic resistances in the specific application. A good starting point is to feed about 25% of the voltage change at the ITH pin to the VON pin as shown in Figure 2a. Place capacitance on the VON pin to filter out the ITH variations at the switching frequency. The resistor load on ITH reduces the DC gain of the error amp and degrades load regulation, which can be avoided by using the PNP emitter follower of Figure 2b. Minimum Off-time and Dropout Operation The minimum off-time tOFF(MIN) is the smallest amount of time that the LTC3608 is capable of turning on the bottom MOSFET, tripping the current comparator and turning the MOSFET back off. This time is generally about 320ns. The minimum off-time limit imposes a maximum duty cycle of tON/(tON + tOFF(MIN)). If the maximum duty cycle is reached, due to a dropping input voltage for example, then the output will drop out of regulation. The minimum input voltage to avoid dropout is:
Figure 1a. Switching Frequency vs RON (VON = 0V)
SWITCHING FREQUENCY (kHz)
SWITCHING FREQUENCY (kHz) 1000
VOUT = 12V VOUT = 5V VOUT = 3.3V
100 100 1000 RON (k) 10000
3608 F01b
VIN(MIN) = VOUT
tON + tOFF(MIN) tON
Figure 1b. Switching Frequency vs RON (VON = INTVCC)
A plot of Maximum Duty Cycle vs Frequency is shown in Figure 3. Setting the Output Voltage The LTC3608 develops a 0.6V reference voltage between the feedback pin, VFB, and the signal ground as shown in Figure 6. The output voltage is set by a resistive divider according to the following formula: VOUT = 0.6V 1+ R2 R1
Because the voltage at the ION pin is about 0.7V, the current into this pin is not exactly inversely proportional to VIN, especially in applications with lower input voltages. To correct for this error, an additional resistor RON2 connected from the ION pin to the 5V INTVCC supply will further stabilize the frequency. RON2 = 5V R 0.7V ON
Changes in the load current magnitude will also cause frequency shift. Parasitic resistance in the MOSFET switches and inductor reduce the effective voltage across the inductance, resulting in increased duty cycle as the
To improve the frequency response, a feed forward capacitor C1 may also be used. Great care should be taken to route the VFB line away from noise sources, such as the inductor or the SW line.
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LTC3608 APPLICATIONS INFORMATION
RVON1 30k VOUT RVON2 100k CVON 0.01F RC ITH CC VON LTC3608
ripple. Highest efficiency operation is obtained at low frequency with small ripple current. However, achieving this requires a large inductor. There is a tradeoff between component size, efficiency and operating frequency. A reasonable starting point is to choose a ripple current that is about 40% of IOUT(MAX). The largest ripple current occurs at the highest VIN. To guarantee that ripple current does not exceed a specified maximum, the inductance should be chosen according to: L= VOUT f IL(MAX) 1 VOUT VIN(MAX)
(2a)
RVON1 3k VOUT INTVCC 10k Q1 2N5087 RVON2 10k CVON 0.01F RC ITH CC
3608 F02
VON LTC3608
(2b) Figure 2. Correcting Frequency Shift with Load Current Changes
2.0
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. A variety of inductors designed for high current, low voltage applications are available from manufacturers such as Sumida, Panasonic, Coiltronics, Coilcraft and Toko. CIN and COUT Selection
SWITCHING FREQUENCY (MHz)
1.5 DROPOUT REGION 1.0
The input capacitance CIN is required to filter the square wave current at the drain of the top MOSFET. Use a low ESR capacitor sized to handle the maximum RMS current. IRMS IOUT(MAX) VOUT VIN VIN -1 VOUT
0.5
0 0 0.25 0.50 0.75 DUTY CYCLE (VOUT/VIN) 1.0
3608 F03
Figure 3. Maximum Switching Frequency vs Duty Cycle
Inductor Selection Given the desired input and output voltages, the inductor value and operating frequency determine the ripple current: IL = VOUT fL 1 VOUT VIN
This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT(MAX)/2. This simple worst-case condition is commonly used for design because even significant deviations do not offer much relief. Note that ripple current ratings from capacitor manufacturers are often based on only 2000 hours of life which makes it advisable to derate the capacitor. 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: VOUT IL ESR + 1 8fCOUT
Lower ripple current reduces core losses in the inductor, ESR losses in the output capacitors and output voltage
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LTC3608 APPLICATIONS INFORMATION
Since IL increases with input voltage, the output ripple 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. Multiple capacitors placed in parallel may be needed to meet the ESR and RMS current handling requirements. Dry tantalum, special polymer, aluminum electrolytic and ceramic capacitors are all available in surface mount packages. Special polymer capacitors offer very low ESR but have lower capacitance density than other types. Tantalum capacitors have the highest capacitance density but it is important to only use types that have been surge tested for use in switching power supplies. Aluminum electrolytic capacitors have significantly higher ESR, but can be used in cost-sensitive applications providing that consideration is given to ripple current ratings and long term reliability. Ceramic capacitors have excellent low ESR characteristics but can have a high voltage coefficient and audible piezoelectric effects. The high Q of ceramic capacitors with trace inductance can also lead to significant ringing. When used as input capacitors, care must be taken to ensure that ringing from inrush currents and switching does not pose an overvoltage hazard to the power switches and controller. To dampen input voltage transients, add a small 5F to 50F aluminum electrolytic capacitor with an ESR in the range of 0.5 to 2. High performance through-hole capacitors may also be used, but an additional ceramic capacitor in parallel is recommended to reduce the effect of their lead inductance. Top MOSFET Driver Supply (CB, DB) 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 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 about 100 times the gate charge required by the top MOSFET. In most applications an 0.1F to 0.47F X5R , or X7R dielectric capacitor is adequate. Discontinuous Mode Operation and FCB Pin The FCB pin determines whether the bottom MOSFET remains on when current reverses in the inductor. Tying this pin above its 0.6V threshold enables discontinuous operation where the bottom MOSFET turns off when inductor current reverses. The load current at which current reverses and discontinuous operation begins depends on the amplitude of the inductor ripple current and will vary with changes in VIN. Tying the FCB pin below the 0.6V threshold forces continuous synchronous operation, allowing current to reverse at light loads and maintaining high frequency operation. In addition to providing a logic input to force continuous operation, the FCB pin provides a means to maintain a flyback winding output when the primary is operating in discontinuous mode. The secondary output VOUT2 is normally set as shown in Figure 4 by the turns ratio N of the transformer. However, if the controller goes into discontinuous mode and halts switching due to a light primary load current, then VOUT2 will droop. An external resistor divider from VOUT2 to the FCB pin sets a minimum voltage VOUT2(MIN) below which continuous operation is forced until VOUT2 has risen above its minimum: VOUT2(MIN) = 0.6V 1+ R4 R3
Fault Conditions: Current Limit and Foldback The LTC3608 has a current mode controller which inherently limits the cycle-by-cycle inductor current not only in steady state operation but also in transient. To further limit current in the event of a short circuit to ground, the LTC3608 includes foldback current limiting. If the output falls by more than 25%, then the maximum sense voltage is progressively lowered to about one sixth of its full value. INTVCC Regulator and EXTVCC Connection An internal P-channel low dropout regulator produces the 5V supply that powers the drivers and internal circuitry
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LTC3608 APPLICATIONS INFORMATION
IN4148 VOUT2 CSEC 1F VOUT1 COUT GND SW 40 39 38 37 36 35 34 33 32 31 30 29 28 27 PGND PGND PGND PGND PGND PGND PGND NC INTVCC INTVCC EXTVCC SGND SW SVIN
+
*
41 T1 1:N 42 43 44 45 46 47 VIN 48 49 50 51
*
SW SW SW SW SW SW SW PVIN PVIN PVIN PVIN RUN/SS PVIN BOOST SGND PVIN PVIN PVIN PVIN PVIN PVIN PVIN SW NC LTC3608
SGND NC NC VFB ION NC SGND FCB ITH VRNG PGOOD SGND SGND VON
26 25 24 23 22 21 20 19 18 17 16 15 OPTIONAL EXTVCC CONNECTION 5V < VOUT2 < 7V R3 R4
+
+
CIN
= PGND = SGND
52
1
2
3
4
5
6
7
8 SW
9 10 11 12 13 14
3608 F04
SGND
Figure 4. Secondary Output Loop and EXTVCC Connection
within the LTC3608. The INTVCC pin can supply up to 50mA RMS and must be bypassed to ground with a minimum of 4.7F tantalum or ceramic capacitor. Good bypassing is necessary to supply the high transient currents required by the MOSFET gate drivers. The EXTVCC pin can be used to provide MOSFET gate drive and control power from the output or another external source during normal operation. Whenever the EXTVCC pin is above 4.7V the internal 5V regulator is shut off and an internal 50mA P-channel switch connects the EXTVCC pin to INTVCC. INTVCC power is supplied from EXTVCC until this pin drops below 4.5V. Do not apply more than 7V to the EXTVCC pin and ensure that EXTVCC VIN. The following list summarizes the possible connections for EXTVCC: 1. EXTVCC grounded. INTVCC is always powered from the internal 5V regulator. 2. EXTVCC connected to an external supply. A high efficiency supply compatible with the MOSFET gate drive requirements (typically 5V) can improve overall efficiency.
3. EXTVCC connected to an output derived boost network. The low voltage output can be boosted using a charge pump or flyback winding to greater than 4.7V. The system will start-up using the internal linear regulator until the boosted output supply is available. Soft-Start and Latchoff with the RUN/SS Pin The RUN/SS pin provides a means to shut down the LTC3608 as well as a timer for soft-start and overcurrent latchoff. Pulling the RUN/SS pin below 0.8V puts the LTC3608 into a low quiescent current shutdown (IQ < 30A). Releasing the pin allows an internal 1.2A current source to charge up the external timing capacitor CSS. If RUN/SS has been pulled all the way to ground, there is a delay before starting of about: tDELAY = 1.5V C = (1.3s/F) CSS 1.2A SS
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LTC3608 APPLICATIONS INFORMATION
When the voltage on RUN/SS reaches 1.5V, the LTC3608 begins operating with a clamp on ITH of approximately 0.9V. As the RUN/SS voltage rises to 3V, the clamp on ITH is raised until its full 2.4V range is available. This takes an additional 1.3s/F during which the load current is folded , back until the output reaches 75% of its final value. After the controller has been started and given adequate time to charge up the output capacitor, CSS is used as a short-circuit timer. After the RUN/SS pin charges above 4V, if the output voltage falls below 75% of its regulated value, then a short-circuit fault is assumed. A 1.8A current then begins discharging CSS. If the fault condition persists until the RUN/SS pin drops to 3.5V, then the controller turns off both power MOSFETs, shutting down the converter permanently. The RUN/SS pin must be actively pulled down to ground in order to restart operation. The overcurrent protection timer requires that the soft-start timing capacitor CSS be made large enough to guarantee that the output is in regulation by the time CSS has reached the 4V threshold. In general, this will depend upon the size of the output capacitance, output voltage and load current characteristic. A minimum soft-start capacitor can be estimated from: CSS > COUT VOUT RSENSE (10 -4 [F/V s]) Generally 0.1F is more than sufficient. Overcurrent latchoff operation is not always needed or desired. Load current is already limited during a short-circuit by the current foldback circuitry and latchoff operation can prove annoying during troubleshooting. The feature can be overridden by adding a pull-up current greater than 5A to the RUN/SS pin. The additional current prevents the discharge of CSS during a fault and also shortens the soft-start period. Using a resistor to VIN as shown in Figure 5a is simple, but slightly increases shutdown current. Connecting a resistor to INTVCC as shown in Figure 5b eliminates the additional shutdown current, but requires a diode to isolate CSS. Any pull-up network must be able to pull RUN/SS above the 4.2V maximum threshold of the latchoff circuit and overcome the 4A maximum discharge current.
INTVCC RSS* RUN/SS RSS* D2* RUN/SS VIN 3.3V OR 5V D1
2N7002 CSS
CSS
3608 F05
*OPTIONAL TO OVERRIDE OVERCURRENT LATCHOFF
(5a)
(5b)
Figure 5. RUN/SS Pin Interfacing with Latchoff Defeated
Efficiency Considerations The percent efficiency of a switching regulator is equal to the output power divided by the input power times 100%. It is often useful to analyze individual losses to determine what is limiting the efficiency and which change would produce the most improvement. Although all dissipative elements in the circuit produce losses, four main sources account for most of the losses in LTC3608 circuits: 1. DC I2R losses. These arise from the resistance of the internal resistance of the MOSFETs, inductor and PC board traces and cause the efficiency to drop at high output currents. In continuous mode the average output current flows through L, but is chopped between the top and bottom MOSFETs. The DC I2R loss for one MOSFET can simply be determined by [RDS(ON) + RL] * IO. 2. Transition loss. This loss arises from the brief amount of time the top MOSFET spends in the saturated region during switch node transitions. It depends upon the input voltage, load current, driver strength and MOSFET capacitance, among other factors. The loss is significant at input voltages above 20V and can be estimated from: Transition Loss (1.7A-1) VIN2 IOUT CRSS f 3. INTVCC current. This is the sum of the MOSFET driver and control currents. This loss can be reduced by supplying INTVCC current through the EXTVCC pin from a high efficiency source, such as an output derived boost network or alternate supply if available.
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LTC3608 APPLICATIONS INFORMATION
4. CIN loss. The input capacitor has the difficult job of filtering the large RMS input current to the regulator. It must have a very low ESR to minimize the AC I2R loss and sufficient capacitance to prevent the RMS current from causing additional upstream losses in fuses or batteries. Other losses, including COUT ESR loss, Schottky diode D1 conduction loss during dead time and inductor core loss generally account for less than 2% additional loss. When making adjustments to improve efficiency, the input current is the best indicator of changes in efficiency. If you make a change and the input current decreases, then the efficiency has increased. If there is no change in input current, then there is no change in efficiency. Checking Transient Response The regulator loop response can be checked by looking at the load transient response. Switching regulators take several cycles to respond to a step in load current. When a load step occurs, VOUT immediately 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 a feedback error signal used by the regulator to return VOUT to its steady-state value. During this recovery time, VOUT can be monitored for overshoot or ringing that would indicate a stability problem. The ITH pin external components shown in Figure 6 will provide adequate compensation for most applications. For a detailed explanation of switching control loop theory see Application Note 76. Design Example As a design example, take a supply with the following specifications: VIN = 5V to 20V (12V nominal), VOUT = 2.5V 5%, IOUT = 8A, f = 550kHz. First, calculate the timing resistor with VON = VOUT: 2.5V RON = 187k (550kHz )(10pF )(2.4V) and choose the inductor for about 40% ripple current at the maximum VIN: 2.5V 2.5V L= 1 = 1.24H (550kHz )(0.4)(8A ) 20V Selecting a standard value of 1.2H results in a maximum ripple current of: 2.5V 2.5V IL = 1- = 3A (550kHz )(1.2H) 12V Next, set up VRNG voltage and check the ILIMIT. Tying VRNG to 0.5V will set the typical current limit to 11A, and tying VRNG to GND will result in a typical current around 16A. CIN is chosen for an RMS current rating of about 5A at 85C. The output capacitors are chosen for a low ESR of 0.002 to minimize output voltage changes due to inductor ripple current and load steps. The ripple voltage will be only: VOUT(RIPPLE) = IL(MAX) (ESR) = (3A) (0.002) = 6mV However, a 0A to 8A load step will cause an output change of up to: VOUT(STEP) = ILOAD (ESR) = (8A) (0.002) = 16mV An optional 22F ceramic output capacitor is included to minimize the effect of ESL in the output ripple. The complete circuit is shown in Figure 6. PC Board Layout Checklist When laying out a PC board follow one of the two suggested approaches. The simple PC board layout requires a dedicated ground plane layer. Also, for higher currents, a multilayer board is recommended to help with heat sinking of power components. * The ground plane layer should not have any traces and it should be as close as possible to the layer with the LTC3608. * Place CIN and COUT all in one compact area, close to the LTC3608. It may help to have some components on the bottom side of the board. * Keep small-signal components close to the LTC3608. * Ground connections (including LTC3608 SGND and PGND) should be made through immediate vias to the ground plane. Use several larger vias for power components.
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LTC3608 APPLICATIONS INFORMATION
INTVCC CF RF1 0.47F 1 25V SW SGND 40 39 38 37 36 35 34 33 32 31 30 29 28 27 PGND PGND PGND PGND PGND PGND PGND INTVCC INTVCC EXTVCC SGND SW SVIN NC VIN EXTVCC C4 0.01F CVCC 4.7F 6.3V PGND
VOUT 2.5V AT 8A
41 C5 22F 6.3V COUT1 100F x2
SW SW SW SW SW SW SW PVIN PVIN PVIN PVIN RUN/SS PVIN BOOST SGND PVIN PVIN PVIN PVIN PVIN PVIN PVIN SW NC LTC3608
SGND NC NC VFB ION NC SGND FCB ITH VRNG PGOOD SGND SGND VON
26 25 24 23 22 21 20 19 18 17 16 15 R3 0 CON 0.01F R1 9.5k 1% C1 (OPTIONAL) RON 187k 1% R2 30.1k 1%
(OPTIONAL) C2 VOUT VIN (OPTIONAL) R5 11.3k CC1 1500pF
+
L1 0.8H
42 43 44 45 46 47
(OPTIONAL) GND
VIN 5V TO 18V CIN 10F 35V 3x C6 10F 35V
VIN GND
48 49 50 51 52
+
(OPTIONAL)
RPG1 100k
C3
CC2 100pF
1 CIN: TAIYO YUDEN GMK325BJ106MM-B COUT: TDKC2012X5ROJ226M L1: CDEP85NP-R80MC-50 C5: MURATA GRM31CR60J226KE19
2
3
4
5
6
7
8 SW
9 10 11 12 13 14
INTVCC
(OPTIONAL) RVON 0 SGND VOUT
INTVCC DB CMDSH-3 SW
CB1 0.22F CSS 0.1F
RSS1 510k VIN (OPTIONAL)
0.1F
3608 F06
KEEP POWER GROUND AND SIGNAL GROUND SEPARATE. CONNECT AT ONE POINT. = PGND = SGND
Figure 6. Design Example: 5V to 18V Input to 2.5V/8A at 550kHz
* Use a compact plane for the switch node (SW) to improve cooling of the MOSFETs and to keep EMI down. * Use planes for VIN and VOUT to maintain good voltage filtering and to keep power losses low. * Flood all unused areas on all layers with copper. Flooding with copper reduces the temperature rise of power components. Connect these copper areas to any DC net (VIN, VOUT, GND or to any other DC rail in your system). When laying out a printed circuit board without a ground plane, use the following checklist to ensure proper operation of the controller. These items are also illustrated in Figure 7.
* Segregate the signal and power grounds. All small signal components should return to the SGND pin at one point, which is then tied to the PGND pin. * Connect the input capacitor(s) CIN close to the IC. This capacitor carries the MOSFET AC current. * Keep the high dV/dT SW, BOOST and TG nodes away from sensitive small-signal nodes. * Connect the INTVCC decoupling capacitor CVCC closely to the INTVCC and PGND pins. * Connect the top driver boost capacitor CB closely to the BOOST and SW pins. * Connect the VIN pin decoupling capacitor CF closely to the VIN and PGND pins.
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LTC3608 APPLICATIONS INFORMATION
CVCC SW 40 39 38 37 36 35 34 33 32 31 30 29 28 27 PGND PGND PGND PGND PGND PGND PGND INTVCC INTVCC EXTVCC SGND SW SVIN NC
41 COUT 42 43 44 45 VOUT 46 47 48 49 50 CIN 51 52
SW SW SW SW SW SW SW PVIN PVIN PVIN PVIN RUN/SS PVIN BOOST SGND PVIN PVIN PVIN PVIN PVIN PVIN PVIN SW NC LTC3608
SGND NC NC VFB ION NC SGND FCB ITH VRNG PGOOD SGND SGND VON
26 25 24 23 22 21 20 19 18 17 16 15 CC2 RC CC1 R1 R2 RON
VIN
1
2
3
4
5
6
7
8
9 10 11 12 13 14 CB CSS
DB
3608 F07
Figure 7. LTC3608 Layout Diagram
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LTC3608 TYPICAL APPLICATIONS
3.6V Input to 1.5V/8A at 750kHz
INTVCC CF 0.47F 25V SW SGND 40 39 38 37 36 35 34 33 32 31 30 29 28 27 PGND PGND PGND PGND PGND PGND PGND INTVCC INTVCC EXTVCC SGND SW SVIN NC VIN2 = 5V EXTVCC C4 0.01F
CVCC 4.7F 6.3V PGND
VOUT 1.5V AT 8A
41 C5 22F 6.3V COUT1 100F x2
SW SW SW SW SW SW SW PVIN PVIN PVIN PVIN RUN/SS PVIN BOOST SGND PVIN PVIN PVIN PVIN PVIN PVIN PVIN SW NC LTC3608
SGND NC NC VFB ION NC SGND FCB ITH VRNG PGOOD SGND SGND VON
26 25 24 23 22 21 20 19 18 17 16 15 11k RPG1 100k INTVCC 39.2k INTVCC CON 0.01F R1 20.5k 1% C1 (OPTIONAL) RON 113k 1% R2 30.1k 1%
(OPTIONAL) C2 VOUT VIN (OPTIONAL) R5 6.19k CC1 3300pF
+
L1 0.2H
42 43 44 45 46 47
(OPTIONAL) GND
VIN 3.6V CIN 10F 3x C6 10F 10V
VIN GND
48 49 50 51 52
+
(OPTIONAL)
CC2 100pF
1 CIN: TAIYO YUDEN TMK432BJ106MM COUT1: TDKC4532X5ROJ107M L1: CDEP85NP-R20MC-50 C5: TAIYO YUDEN JMK316BJ226ML-T
2
3
4
5
6
7
8
9 10 11 12 13 14 SGND CB1 0.22F RSS1 510k CSS 0.1F VIN (OPTIONAL)
3608 TA02
VOUT VOUT 0.1F
INTVCC
KEEP POWER GROUND AND SIGNAL GROUND SEPARATE. CONNECT AT ONE POINT. = PGND = SGND
Transient Response
100 95 90 EFFICIENCY (%) IL 5A/DIV VOUT 200mV/DIV 85 80 75 70 65 200mV LOAD STEP 1A-8A VIN = 3.6V VOUT = 1.5V FCB = 0V
3608 TA02a
Efficiency Curve
DCM CCM
60 55 50 100 VIN = 3.6V FREQ = 750kHz 1000 1000 LOAD CURRENT (A) 10000
3608 TA02b
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LTC3608 TYPICAL APPLICATIONS
5V to 18V Input to 1.2V/8A at 550kHz
INTVCC CF 0.47F 25V SW SGND 40 39 38 37 36 35 34 33 32 31 30 29 28 27 PGND PGND PGND PGND PGND PGND PGND INTVCC INTVCC EXTVCC SGND SVIN SW NC RF1 1 VIN2 EXTVCC C4 0.01F
CVCC 4.7F 6.3V PGND
VOUT 1.2V AT 8A
41 C5 22F 6.3V COUT1 100F x2
SW SW SW SW SW SW SW PVIN PVIN PVIN PVIN RUN/SS PVIN BOOST SGND PVIN PVIN PVIN PVIN PVIN PVIN PVIN SW NC LTC3608
SGND NC NC VFB ION NC SGND FCB ITH VRNG PGOOD SGND SGND VON
26 25 24 23 22 21 20 19 18 17 16 15 RPG1 100k INTVCC CON 0.01F R1 30k 1% C1 (OPTIONAL) RON 187k 1% R2 30.1k 1%
(OPTIONAL) C2 VOUT VIN (OPTIONAL) R5 7.68k CC1 1500pF
+
L1 0.5H
42 43 44 45 46 47
(OPTIONAL) GND
VIN 5V TO 18V CIN 10F 25V 3x C6 10F 35V
VIN GND
48 49 50 51 52
+
(OPTIONAL)
CC2 100pF
1 C5: TAIYO YUDEN JMK316BJ226ML-T CIN: TAIYO YUDEN TMK432BJ106MM COUT1: TDKC4532X5R107M L1: CDEP85NP-R50MC-125
2
3
4
5
6
7
8
9 10 11 12 13 14 SGND CB1 0.22F VOUT RSS1 510k CSS 0.1F VIN (OPTIONAL)
3608 TA03
RVON VOUT 0.1F (OPTIONAL) CVON
INTVCC DB CMDSH-3
KEEP POWER GROUND AND SIGNAL GROUND SEPARATE. CONNECT AT ONE POINT. = PGND = SGND
Transient Response
90
Efficiency vs Load Current
VIN = 12V 85 FREQ = 550kHz
EFFICIENCY (%)
IL 5A/DIV VOUT 200mV/DIV
80 75 70 DCM 65 60 200mV LOAD STEP 1A-8A VIN = 12V VOUT = 1.2V FCB = 0V
3608 TA03a
OCM
55 50 100 1000 1000 LOAD CURRENT (A) 10000
3608 TA03b
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LTC3608 TYPICAL APPLICATIONS
5V to 18V Input to 1.8V/8A All Ceramic 1MHz
INTVCC RF1 1 VIN CF 0.1F 25V SW SGND 40 39 38 37 36 35 34 33 32 31 30 29 28 27 PGND PGND PGND PGND PGND PGND PGND INTVCC INTVCC EXTVCC SGND SW SVIN NC EXTVCC C4 0.01F
CVCC 4.7F 6.3V PGND
VOUT 1.8V AT 8A
41 C5 22F 6.3V COUT1 100F x2
SW SW SW SW SW SW SW PVIN PVIN PVIN PVIN RUN/SS PVIN BOOST SGND PVIN PVIN PVIN PVIN PVIN PVIN PVIN SW NC LTC3608
SGND NC NC VFB ION NC SGND FCB ITH VRNG PGOOD SGND SGND VON
26 25 24 23 22 21 20 19 18 17 16 15 RPG1 100k INTVCC CON 0.01F R1 10k 1% C1 (OPTIONAL) RON 102k 1% R2 30.1k 1%
(OPTIONAL) C2 VOUT VIN (OPTIONAL) R5 5.76k CC1 1500pF
+
L1 0.47H
42 43 44 45 46 47
(OPTIONAL) GND
VIN 5V TO 18V CIN 10F 25V 3x
VIN
48 49 50 51 52
CC2 100pF
1 COUT: TDKC3225XROJ107M L1: VISHAY IHLP2525-R47 C5: TAIYO YUDEN JMK316BJ226ML-T
2
3
4
5
6
7
8
9 10 11 12 13 14 SGND CB1 0.22F VOUT VOUT RSS1 510k CSS 0.1F VIN (OPTIONAL)
3608 TA04
INTVCC DB CMDSH-3
0.1F
KEEP POWER GROUND AND SIGNAL GROUND SEPARATE. CONNECT AT ONE POINT. = PGND = SGND
Transient Response
90
Efficiency vs Load Current
DCM 80 EFFICIENCY (%) CCM
IL 5A/DIV VOUT 200mV/DIV
70 60 50 40 30 100 VIN = 12V 1000 LOAD CURRENT (mA) 10000
3608 TA04b
200mV LOAD STEP 1A-5A VIN = 12V VOUT = 1.8V FCB = 0V
3608 TA04a
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LTC3608 PACKAGE DESCRIPTION
WKG Package 52-Lead QFN Multipad (7mm x 8mm)
(Reference LTC DWG # 05-08-1768 Rev O)
SEATING PLANE 7.00 BSC PAD 1 CORNER A B 0.00 - 0.05 41 40 2.625 REF 2.90 REF 0.50 BSC
52 1
PIN 1 ID
4
7
bbb M C A B
3.40 REF 3.90 0.10 2.925 0.10
2.025 0.10
3.20 0.10 3.40 REF
8.00 BSC
33 32
8 9 10
1.00 REF
NX b
aaa C 2x
4.275 0.10
2.25 0.10
27 0.580 0.10 26
aaa C 2x
14 0.40 0.10 19 1.775 REF 15 0.25 0.05 1.35 0.10
TOP VIEW // ccc C 7.50 0.05
0.90 0.10
9
NX 0.08 C
8
BOTTOM VIEW (BOTTOM METALLIZATION DETAILS)
MLP52 QFN REV O 0807
2.90 REF 0.50 BSC
2.625 REF NOTE: 1. DIMENSIONING AND TOLERANCING CONFORM TO ASME Y14.5M-1994 2. ALL DIMENSIONS ARE IN MILLIMETERS, ANGLES ARE IN DEGREES () 3. N IS THE TOTAL NUMBER OF TERMINALS
PIN 1
4
3.20 0.10 3.40 REF 2.025 0.10 3.40 REF 2.925 0.10 3.90 0.10
THE LOCATION OF THE TERMINAL #1 IDENTIFIER AND TERMINAL NUMBERING CONVENTION CONFORMS TO JEDEC PUBLICATION 95 SPP-002
5. ND AND NE REFER TO THE NUMBER OF TERMINALS ON EACH D AND E SIDE RESPECTIVELY 6. NJR REFER TO NON JEDEC REGISTERED
7
1.00 REF 8.50 0.05 4.275 0.10
DIMENSION b APPLIES TO METALLIZED TERMINAL AND IS MEASURED BETWEEN 0.20mm AND 0.30mm FROM THE TERMINAL TIP. IF THE TERMINAL HAS THE OPTIONAL RADIUS ON THE OTHER END OF THE TERMINAL, THE DIMENSION b SHOULD NOT BE MEASURED IN THAT RADIUS AREA. COPLANARITY APPLIES TO THE TERMINALS AND ALL OTHER SURFACE METALLIZATION DRAWING SHOWN ARE FOR ILLUSTRATION ONLY SYMBOL TOLERANCE 0.15 aaa 0.10 bbb 0.10 ccc
8 9
2.25 0.10
0.40 0.10 0.25 0.05 1.775 REF 1.35 0.10
PACKAGE OUTLINE
RECOMMENDED SOLDER PAD LAYOUT TOP VIEW
3608fa
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.
23
LTC3608 TYPICAL APPLICATION
14V to 18V Input to 12V/5A at 500kHz
CVCC 4.7F 6.3V , PGND INTVCC RF1 CF 0.1F 1 25V VIN2 EXTVCC C4 0.01F SGND SW INTVCC INTVCC SVIN EXTVCC SW PGND PGND PGND PGND PGND PGND PGND
40 39 38 37 36 35 34 33 32 31 30 29 28 27 NC SGND
VOUT 12V AT 5A
41 C5 22F 25V COUT1 180F 16V
SW SW SW SW SW SW SW PVIN PVIN PVIN PVIN RUN/SS PVIN BOOST SGND PVIN PVIN PVIN PVIN PVIN PVIN PVIN SW NC LTC3608
SGND NC NC VFB ION NC SGND FCB ITH VRNG PGOOD SGND SGND VON
26 25 24 23 22 21 20 19 18 17 16 15 10k RPG1 100k 90.9k INTVCC CON 0.01F R1 3.16k 1% C1 (OPTIONAL) RON 1M 1% R2 60.4k 1%
(OPTIONAL) C2 VOUT VIN (OPTIONAL) R5 24.9k CC1 3300pF
+
L1 4.3H
42 43 44 45 46 47
(OPTIONAL) GND
VIN 14V TO 18V CIN 10F 25V 3x C6 10F 35V
VIN GND
48 49 50 51 52
+
(OPTIONAL)
CC2 100pF INTVCC SGND
CIN: TAIYO YUDEN TMK432BJ106MM COUT: SANYO 16SVP180MX L1: CDEP85NP-4R3MC-88 KEEP POWER GROUND AND SIGNAL GROUND SEPARATE. CONNECT AT ONE POINT. = PGND = SGND
1
2
3
4
5
6
7
8
9 10 11 12 13 14 CB1 0.22F RSS1 510k CSS 0.1F RUN/SS VIN (OPTIONAL)
3608 TA05
INTVCC (OPTIONAL) CVON
INTVCC DB CMDSH-3
Transient Response
Efficiency Curve
100 95 90 EFFICIENCY (%) DCM
IL 5A/DIV VOUT 200mV/DIV
85 80 75 70 65 CCM
200mV LOAD STEP 1A-8A VIN = 18V VOUT = 12V FCB = 0V
3608 TA05a
60 55 50 100 VIN = 18V FREQ = 500kHz 1000 1000 LOAD CURRENT (A) 10000
3608 TA05b
RELATED PARTS
PART NUMBER LTC1778 LTC3414 LTC3418 LTC3610 LTM4600HV LTM4601HV LTM4603HV DESCRIPTION No RSENSE Current Mode Synchronous Step-Down Controller 4A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 8A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 12A Current Mode Monolithic Synchronous Step-Down Converter 10A Complete Switch Mode Power Supply 12A Complete Switch Mode Power Supply 6A Complete Switch Mode Power Supply COMMENTS Up to 97% Efficiency, VIN: 4V to 36V, 0.8V VOUT (0.9)(VIN), IOUT Up to 20A 95% Efficiency, VIN: 2.25V to 5.5V, VOUT(MIN) = 0.8V, IQ = 64A, ISD: <1A, TSSOP20E Package 95% Efficiency, VIN: 2.25V to 5.5V, VOUT(MIN) = 0.8V, Thermally Enhanced 38-Lead QFN Package Up to 24V Input (28V Maximum). Current Mode Extremely Fast Transient Response 92% Efficiency, VIN: 4.5V to 28V, VOUT: 0.6V, True Current Mode Control, Ultrafast Transient Response 92% Efficiency, VIN: 4.5V to 28V, VOUT: 0.6V, True Current Mode Control, Ultrafast Transient Response 93% Efficiency, VIN: 4.5V to 28V, with PLL, Output Tracking and Margining with Ultrafast Transient Response
3608fa LT 0808 REV A * PRINTED IN USA
24 Linear Technology Corporation
(408) 432-1900
1630 McCarthy Blvd., Milpitas, CA 95035-7417
FAX: (408) 434-0507 www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2008

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