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LTC3409A 600mA Low VIN Buck Regulator in 3mm x 3mm DFN DESCRIPTION
The LTC(R)3409A is a high efficiency, monolithic synchronous buck regulator using a constant frequency, current mode architecture. The LTC3409A improves upon the LTC3409's light load regulation in Burst Mode operation. The output voltage is adjusted via an external resistor divider. Fixed switching frequencies of 1.7MHz and 2.6MHz are supported. Alternatively, an internal PLL will synchronize to an external clock in the frequency range of 1MHz to 3MHz. This range of switching frequencies allows the use of small surface mount inductors and capacitors, including ceramics. Supply current during Burst Mode operation is only 65A dropping to <1A in shutdown. The 1.6V to 5.5V input voltage range makes the LTC3409A ideally suited for single cell Li-Ion, Li-Metal and 2-cell alkaline, NiCd or NiMH battery-powered applications. 100% duty cycle capability provides low dropout operation, extending battery life in portable systems. Burst Mode operation can be user-enabled, increasing efficiency at light loads, further extending battery life. The internal synchronous switch increases efficiency and eliminates the need for an external Schottky diode. Internal soft-start offers controlled output voltage rise time at startup without the need for external components.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. Protected by U.S. Patents, including 5481178, 6580258, 6304066, 6127815, 6498466, 6611131.
FEATURES
n n n n n n n n n n n n n n n n n
1.6V to 5.5V Input Voltage Range 0.62V to 5.5V Output Voltage Range Internal Soft-Start Selectable 1.7MHz or 2.6MHz Constant Frequency Operation Internal Oscillator Can Be Synchronized to an External Clock, 1MHz to 3MHz Range High Efficiency: Up to 95% 65A Quiescent Current, Burst Mode(R) Operation 600mA Output Current (VIN = 1.8V, VOUT = 1.2V) 750mA Peak Inductor Current No Schottky Diode Required Low Dropout Operation: 100% Duty Cycle 0.612V Reference Voltage Stable with Ceramic Capacitors Shutdown Mode Draws <1A Supply Current Current Mode Operation for Excellent Line and Load Transient Response Overtemperature Protection Available in a Low Profile (0.75mm) 8-Lead (3mm x 3mm) DFN Package
APPLICATIONS
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Cellular Phones Digital Cameras MP3 Players
TYPICAL APPLICATION
High Efficiency Step-Down Converter
VIN 1.8V TO 5.5V 4.7F LTC3409A SW VIN RUN MODE SYNC *SUMIDA CDRH2D18/LD VFB GND 137k 2.2H* 10pF 267k 22F 2 VOUT 1.8V
Burst Mode Efficiency, 1.8VOUT
100 90 80 EFFICIENCY (%) 70 60 50 40 30
3409A TA01
1 2.5VIN, BURST 0.1 POWER LOSS (W) 3.6VIN, BURST 0.01 4.2VIN, BURST POWER LOST 3.6VIN, BURST
20 10 0 0 1 100 10 LOAD CURRENT (mA)
0.001
0.0001 1000
3409A TA01b
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LTC3409A ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW VFB GND VIN VIN 1 2 3 4 9 8 7 6 5 SYNC RUN SW MODE
Input Supply Voltage ................................... -0.3V to 6V RUN, VFB, MODE, SYNC Voltages . -0.3V to (VIN + 0.3V) SW Voltage ................................... -0.3V to (VIN + 0.3V) Operating Temperature Range (Note 2) ..-40C to 85C Junction Temperature (Note 3) ........................... 125C Storage Temperature Range.................. -65C to 125C
DD PACKAGE 8-LEAD (3mm 3mm) PLASTIC DFN TJMAX = 125C, JA = 43C/W EXPOSED PAD (PIN 9) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH LTC3409AEDD#PBF LTC3409AIDD#PBF TAPE AND REEL LTC3409AEDD#TRPBF LTC3409AIDD#TRPBF PART MARKING* LFGY LFGY PACKAGE DESCRIPTION 8-Lead (3mm x 3mm) Plastic DFN 8-Lead (3mm x 3mm) Plastic DFN TEMPERATURE RANGE -40C to 85C -40C to 85C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *Temperature grades are 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/
The denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25C. VIN = 2.2V unless otherwise specified.
SYMBOL VIN VFB PARAMETER Input Voltage Range Regulated Feedback Voltage (Note 4) TA = 25C (Note 4) 0C TA 85C (Note 4) -40C TA 85C VFB = 0.612V VOVL = VFBOVL - VFB (Note 6) 1.6V < VIN < 5.5V (Note 4) 1.9V VIN 3.6V, 0C TA 85C (Note 4) VFB = 0.5V or VOUT = 90% VOUT = 1.8V, VMODE = 0V, 1mA < ILOAD < 210mA, 0C TA 85C (Note 8)
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ELECTRICAL CHARACTERISTICS
CONDITIONS
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MIN 1.6 0.604 0.600 0.598 35
TYP 0.612 0.612 0.612 61 0.04 0.05
MAX 5.5 0.620 0.624 0.626 30 85 0.4 0.5 1.3 0.5 1.1 1 1.1 1
UNITS V V V V nA mV %/V % A % V A V A
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IVFB VOVL VFB IPK VLOADREG VRUN IRUN VMODE IMODE
Feedback Current VFBOVL Overvoltage Lockout Reference Voltage Line Regulation Peak Inductor Current Output Voltage Load Regulation RUN Threshold RUN Leakage Current MODE Threshold MODE Leakage Current
0.75
1 0.2
0.3 0.3
0.65 0.01 0.65 0.01
VRUN = 0V or = 2.2V
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VMODE = 0V or = 2.2V
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LTC3409A
ELECTRICAL CHARACTERISTICS
SYMBOL VSYNCTH ISYNC IS PARAMETER SYNC Threshold SYNC Leakage Current Input DC Bias Current Active Mode Sleep Mode Shutdown Nominal Oscillator Frequency SYNC Threshold Minimum SYNC Pin Frequency Maximum SYNC Pin Frequency Minimum SYNC Pulse Width Soft-Start Period SYNC Timeout RDS(ON) of P-channel FET RDS(ON) of N-channel FET SW Leakage RUN
The denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25C. VIN = 2.2V unless otherwise specified.
CONDITIONS
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MIN 0.3
TYP 0.65 0.01 350 65 0.1
MAX 1.1 1 475 120 1 2.2 3.2
UNITS V A A A A MHz MHz V MHz MHz ns ms s
VSYNC = 0V or = 2.2V (Note 5) VOUT = 90%, ILOAD = 0A VOUT = 103%, ILOAD = 0A VRUN = 0V, VIN = 5.5V SYNC = GND SYNC = VIN When SYNC Input is Toggling (Note 7)
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fOSC SYNC TH SYNC fMIN SYNC fMAX SYNC PW tSS SYNC tO RPFET RNFET ILSW
0.9 1.8
1.7 2.6 0.63 1 3 100 1 30 0.33 0.35 0.22 0.25 0.1
Delay from Removal of EXT CLK Until Fixed Frequency Operation Begins (Note 7) ISW = 100mA, Wafer Level ISW = 100mA, DD Package ISW = 100mA, Wafer Level ISW = 100mA, DD Package VRUN = 0V, VSW = 0V or 5V, VIN = 5V
3
A
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 LTC3409AE is guaranteed to meet performance specifications from 0C to 85C. Specifications over the -40C to 85C operating temperature range are assured by design, characterization and correlation with statistical process controls. The LTC3409AI is guaranteed to meet specified performance over the full -40C to 85C operating temperature range. Note 3: TJ is calculated from the ambient temperature TA and power dissipation PD according to the following formula: LTC3409A: TJ = TA + (PD)(43C/W)
This IC includes overtemperature protection that is intended to protect the device during momentary overload conditions. Overtemperature protection becomes active at a junction temperature greater than the maximum operating junction temperature. Continuous operation above the specified maximum operating junction temperature may impair device reliability. Note 4: The LTC3409A is tested in a proprietary test mode that connects VFB to the output of the error amplifier. Note 5: Dynamic supply current is higher due to the gate charge being delivered at the switching frequency. Note 6: VOVL is the amount VFB must exceed the regulated feedback voltage. Note 7: Determined by design, not production tested. Note 8: Guaranteed by measurement at the wafer level and design, characterization and correlation with statistical process controls.
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LTC3409A TYPICAL PERFORMANCE CHARACTERISTICS
(TA = 25C, from Typical Application on the front page except for the resistive divider resistor values) Pulse-Skipping Efficiency/Power Lost vs Load Current, VOUT = 1.8V
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0 1 VIN = 2.5V VIN = 4.2V 0.01 VIN = 3.6V EFFICIENCY VIN = 3.6V VIN = 4.2V EFFICIENCY (%) 0.1 VIN = 2.5V POWER LOST (W) 1 100 90 80 70 60 50 40 30 20 10 0.001 1000
3409A G01
Efficiency vs Input Voltage, VOUT = 1.2V, Burst Mode Operation
POWER LOST 10 100 LOAD CURRENT (mA)
0 1.6
IOUT = 0.1mA IOUT = 1mA IOUT = 10mA
IOUT = 100mA IOUT = 600mA 5.5
3409A G02
2.6 3.6 4.6 INPUT VOLTAGE (VIN)
Efficiency vs Input Voltage VOUT = 1.2V, Pulse-Skipping
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 1.5 IOUT = 0.1mA 3.5 4.5 2.5 INPUT VOLTAGE (VIN) 5.5
3409A G03
Efficiency vs Load Current, VOUT = 2.5V
100 100 2.7VIN 4.2VIN EFFICIENCY (%) 3.6VIN 90 80 70 60 50 40 30 20 10 90
Efficiency vs Load Current, VOUT = 1.2V
2.5VIN 1.6VIN
IOUT = 100mA IOUT = 600mA EFFICIENCY (%)
80 70 60 50 40 30 20 10
3.1VIN
3.1VIN 2.5VIN 1.6VIN
IOUT = 10mA IOUT = 1mA
4.2VIN
2.7VIN
3.6VIN BURST PULSE-SKIPPING 1 10 100 LOAD CURRENT (mA) 1000
3409A G04
0 0.1
0 0.1
BURST PULSE-SKIPPING 1 10 100 LOAD CURRENT (mA) 1000
3409A G05
Reference Voltage vs Temperature
0.616 OSCILLATOR FREQUENCY (MHz) 0.615 REFERENCE VOLTAGE (V) 0.614 0.613 0.612 0.611 0.610 0.609 0.608 -50 -30 -10 10 30 50 70 TEMPERATURE (C) 90 110 125
3409A G06
Oscillator Frequency vs Temperature
2.7 2.6 2.5 2.4 2.3 2.2 2.1 2.0 1.9 1.8 1.7 1.6 1.5 1.4 1.3 1.2 -50 6 OSCILLATOR FREQUENCY SHIFT (%) VIN = 2.7V VIN = 1.6V VIN = 4.2V OSC 2.6MHz 4 2 0 -2 -4 -6 -8
Oscillator Frequency Shift vs Input Voltage
fLOW 1.7MHz
fHIGH 2.6MHz
VIN = 4.2V
OSC 1.7MHz
VIN = 1.6V
VIN = 2.7V
-25
50 25 75 0 TEMPERATURE (C)
100
125
-10 1.5
2.5
3.5 INPUT VOLTAGE (V)
4.5
5.5
3409A G08
3409A G07
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LTC3409A
TYPICAL PERFORMANCE CHARACTERISTICS
(TA = 25C, from Typical Application on the front page except for the resistive divider resistor values) Output Voltage vs Load Current VIN = 1.6V
1.210 1.205 OUTPUT VOLTAGE (V) 1.2VOUT, BURST 1.200 1.195 1.190 1.185 0.05 1.100 1 10 100 LOAD CURRENT (mA) 1000
3409A G09
RDS(ON) vs Input Voltage
0.45 0.40 0.35 0.30 RDS(0N) () 0.25 0.20 0.15 0.10 SYNCHRONOUS SWITCH MAIN SWITCH RDS(ON) () 0.55 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 1.5 2.5 4.5 3.5 INPUT VOLTAGE (V) 5.5
3409A G10
RDS(ON) vs Temperature
1.6VIN 4.2VIN 2.7VIN 1.6VIN 2.7VIN 4.2VIN MAIN SWITCH SYNCHRONOUS SWITCH 50 25 75 0 TEMPERATURE (C) 100 125
1.2VOUT, PULSE-SKIPPING
0
0.10 -50 -25
3409A G11
Dynamic Input Current vs Input Voltage
3500 3000 DYNAMIC INPUT CURRENT, PULSE-SKIPPING MODE (A) BURST 2500 2000 1500 1000 500 0 1.5 VFB = 1V 2.5 4.5 3.5 INPUT VOLTAGE (V) VOUT = 1.5V IOUT = 0 PULSE-SKIPPING 10 0 5.5
3409A G12
Dynamic Supply Current vs Temperature, VIN = 3.6V, VOUT = 1.5V, No Load
90 DYNAMIC SUPPLY CURRENT (A) 80 DYNAMIC SUPPLY CURRENT, Burst Mode OPERATION (A) 70 60 50 40 30 20 500 450 400 350 300 250 200 150 100 50 0 -50 -25 25 50 75 0 TEMPERATURE (C) 100 125 BURST PULSE-SKIPPING
VOUT = 1.5V IOUT = 0 VFB = 1V
3409A G13
Switch Leakage vs Temperature VIN = 5.5V
6000 5000 SWITCH LEAKAGE (nA) 4000 3000 MAIN SWITCH 2000 SYNCHRONOUS SWITCH 1000 0 -50 -25 SWITCH LEAKAGE (nA) 35 30 25 45 40
Switch Leakage vs Input Voltage
MAIN SWITCH 20 15 10 5 SYNCHRONOUS SWITCH 0 4 2 INPUT VOLTAGE (V) 6
3409A G15
50 25 75 0 TEMPERATURE (C)
100
125
0
3409A G14
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LTC3409A TYPICAL PERFORMANCE CHARACTERISTICS
(TA = 25C, from Typical Application on the front page except for the resistive divider resistor values) Start-Up from Shutdown, VIN = 3.6V, VOUT = 1.8V, Load = 1k
RUN 2V/DIV VOUT 100mV/DIV
Load Step 0mA to 600mA PulseSkipping, VIN = 3.6V, VOUT = 1.8V
VOUT 1V/DIV INDUCTOR CURRENT 200mA/DIV 200s/DIV
3409A G16
ILOAD 500mA/DIV INDUCTOR CURRENT 500mA/DIV 20s/DIV
3409A G17
Load Step 50mA to 600mA PulseSkipping, VIN = 3.6V, VOUT = 1.8V
VOUT 50mV/DIV VOUT 20mV/DIV
Burst Mode Operation, ILOAD = 35mA, VIN = 3.6V, VOUT = 1.8V
ILOAD 500mA/DIV
VSWITCH 2V/DIV
INDUCTOR CURRENT 500mA/DIV 20s/DIV
3409A G18
INDUCTOR CURRENT 500mA/DIV 4s/DIV
3409A G19
Load Step 10mA to 600mA, Burst Mode Operation, VIN = 3.6V, VOUT = 1.8V
VOUT 100mV/DIV VOUT 50mV/DIV
Load Step 50mA to 600mA, Burst Mode Operation, VIN = 3.6V, VOUT = 1.8V
ILOAD 500mA/DIV INDUCTOR CURRENT 500mA/DIV 20s/DIV
3409A G20
ILOAD 500mA/DIV
INDUCTOR CURRENT 500mA/DIV 20s/DIV
3409A G21
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LTC3409A
SYNC (Pin 8): External CLK Input/Fixed Switching Frequency Selection. Forcing this pin above 1.1V for greater than 30s selects 2.6MHz switching frequency. Forcing this pin below 0.3V for greater than 30s selects 1.7MHz switching frequency. External clock input, 1MHz to 3MHz frequency range. When the SYNC pin is clocked in this frequency range the SYNC threshold is nominally 0.63V. To allow for good noise immunity, SYNC signal should swing at least 0.3V below and above this nominal value (0.33V to 0.93V). Do not leave SYNC floating. Exposed Pad (Pin 9): The Exposed Pad is ground. It must be soldered to PCB ground to provide both electrical contact and optimum thermal performance.
PIN FUNCTIONS
VFB (Pin 1): Feedback Pin. Receives the feedback voltage from an external resistive divider across the output. GND (Pin 2): Ground Pin. VIN (Pins 3, 4): Main Supply Pins. Must be closely decoupled to GND, Pin 2 and Pin 9, with a 4.7F or greater ceramic capacitor. MODE (Pin 5): Mode Select Input. To select pulse-skipping mode, force this pin above 1.1V. Forcing this pin below 0.3V selects Burst Mode operation. Do not leave MODE floating. SW (Pin 6): Switch Node Connection to Inductor. This pin connects to the drains of the internal main and synchronous power MOSFET switches. RUN (Pin 7): Run Control Input. Forcing this pin above 1.1V enables the part. Forcing this pin below 0.3V shuts down the device. In shutdown, all functions are disabled drawing <1A supply current. Do not leave RUN floating.
FUNCTIONAL DIAGRAM
MODE 5 SLOPE COMP PLL OSC 3, 4 VIN VFB 1 0.612V
SYNC 8
0.65V
- + - + +
OVDET 0.675 SHUTDOWN EA 0.4V
- +
EN SLEEP
-
BURST Q Q SWITCHING LOGIC AND BLANKING CIRCUIT
ICOMP
+
5
S SOFTSTART VIN RUN 7 REFERENCE R
RS LATCH
ANTISHOOTTHRU
6 SW
OV
-
-
3409A FD
IRCMP
+
2, 9 GND
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LTC3409A OPERATION
Main Control Loop Burst Mode Operation The LTC3409A is capable of Burst Mode operation in which the internal power MOSFETs operate intermittently based on load demand. To enable Burst Mode operation, simply connect the MODE pin to GND. To disable Burst Mode operation and enable PWM pulse-skipping mode, connect the MODE pin to VIN or drive it with a logic high (VMODE >1.1V). In this mode, the efficiency is lower at light loads, but becomes comparable to Burst Mode operation when the output load exceeds 30mA. The advantage of pulse-skipping mode is lower output ripple and less interference to audio circuitry. When the converter is in Burst Mode operation, the minimum peak current of the inductor is set to approximately 200mA regardless of the output load. Each burst event can last from a few cycles at light loads to almost continuously cycling with short sleep intervals at moderate loads. In between these burst events, the power MOSFETs and any unneeded circuitry are turned off, reducing the quiescent current to 65A. In this sleep state, the load current is being supplied solely from the output capacitor. As the output voltage droops, the EA amplifier's output rises above the sleep threshold signaling the BURST comparator to trip and turn the top MOSFET on. This process repeats at a rate that is dependent on the load demand.
The LTC3409A uses a constant frequency, current mode stepdown architecture. Both the main (P-channel MOSFET) and synchronous (N-channel MOSFET) switches are internal. During normal operation, the internal top power MOSFET is turned on each cycle when the oscillator sets the RS latch, and turned off when the current comparator, ICOMP, resets the RS latch. The peak inductor current at which ICOMP resets the RS latch is controlled by the output of error amplifier EA. The VFB pin, described in the Pin Functions section, allows EA to receive an output feedback voltage from an external resistive divider. When the load current increases, it causes a slight decrease in the feedback voltage relative to the 0.612V reference, which in turn, causes the EA amplifier's output voltage to increase until the average inductor current matches the new load current. While the top MOSFET is off, the bottom MOSFET is turned on until either the inductor current starts to reverse, as indicated by the current reversal comparator IRCMP, or the beginning of the next clock cycle. Comparator OVDET guards against transient overshoots >10% by turning the main switch off and keeping it off until the transient has ended.
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LTC3409A
Slope Compensation Slope compensation provides stability in constant frequency architectures by preventing subharmonic oscillations at high duty cycles. It is accomplished internally by adding a compensating ramp to the inductor current signal at duty cycles in excess of 40%. User Controlled Switching Frequency The internal oscillator of the LTC3409A can be synchronized to a user-supplied external clock applied to the SYNC pin. Alternately, when this pin is held at a fixed high or low level for more than 30s, the internal oscillator will revert to fixed-frequency operation; where the frequency may be selected as 1.7MHz (SYNC Low) or 2.6MHz (SYNC High). Internal Soft-Start At start-up when the RUN pin is brought high, the internal reference is linearly ramped from 0V to 0.612V in 1ms. The regulated feedback voltage will follow this ramp resulting in the output voltage ramping from 0% to 100% in 1ms. The current in the inductor during soft-start will be defined by the combination of the current needed to charge the output capacitance and the current provided to the load as the output voltage ramps up. The start-up waveform, shown in the Typical Performance Characteristics, shows the output voltage start-up from 0V to 1.8V with a 1k load and VIN = 3.6V. The 1k load results in an output of 1.8mA at 1.8V.
OPERATION
Burst Mode Operation Near Dropout With a light load the part will transition from Burst Mode operation to 100% duty cycle as (VIN to VOUT) approaches ILOAD * (RMAINSWITCH + RINDUCTOR). When (VIN to VOUT) results in near 100% duty cycle the inductor up slope will be quite shallow compared to the inductor down slope, with low peak currents. The LTC3409A is a micropower part and the speed of the comparator controlling the synchronous switch may allow the inductor current to reverse in a low (VIN to VOUT) situation, resulting in CCM operation. Transition from CCM back to Burst Mode operation will occur when (VIN to VOUT) is sufficient for the average inductor current to exceed the load current. This occurs when the average positive current in the inductor exceeds the average reverse current caused by synchronous switch propagation delay. The CCM to Burst Mode operation re-entry transition point will be a function of VIN, VOUT, ILOAD, COUT and the inductor used in the application. Short-Circuit Protection When the output is shorted to ground the LTC3409A limits the synchronous switch current to 1.5A. If this limit is exceeded, the top power MOSFET is inhibited from turning on until the current in the synchronous switch falls below 1.5A. Dropout Operation As the input supply voltage decreases to a value approaching the output voltage, the duty cycle increases toward the maximum on-time. Further reduction of the supply voltage forces the main switch to remain on for more than one cycle until it reaches 100% duty cycle. The output voltage will then be determined by the input voltage minus the voltage drop across the P-channel MOSFET and the inductor.
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LTC3409A APPLICATIONS INFORMATION
The basic LTC3409A application circuit is shown on the first page of this data sheet. External component selection is driven by the load requirement and begins with the selection of L followed by CIN and COUT.
Table 1. Representative Surface Mount Inductors
PART NUMBER & MANUFACTURER Sumida CDRH2D18/HP Sumida CDRH2D18/LD Sumida CDRH2D11 Murata LQH32C_33 TDK VLF3010AT TDK VLF3012AT Wurth WE-TPC T/TH 74402800 Wurth 74402900 VALUE MAX DCR (H) (m) 1.7 2.2 3.3 1.5 2.2 1.0 2.2 1.5 2.2 1.5 2.2 1.0 2.2 2.2 3.3 44 41 54 68 98 60 97 78 120 68 100 85 155 110 135 MAX DC CURRENT (A) 1.85 0.85 0.75 0.90 0.78 1.00 0.79 1.20 1.00 1.20 1.00 1.50 1.00 1.15 0.95 SIZE W x L x H (mm3) 3.2 x 3.2 x 2.0 3.2 x 3.2 x 2.0 3.2 x 3.2 x 1.2 2.5 x 3.2 x 2.0 2.6 x 2.8 x 1.0 2.6 x 2.8 x 1.2 2.8 x 2.8 x 1.1 2.8 x 2.8 x 1.35
Inductor Selection For most applications, the value of the inductor will fall in the range of 1H to 10H. Its value is chosen based on the desired ripple current. Large value inductors lower ripple current and small value inductors result in higher ripple currents. Higher VIN or VOUT also increases the ripple current as shown in Equation 1. A reasonable starting point for setting ripple current is IL = 240mA (40% of 600mA). V 1 IL = VOUT 1- OUT f *L VIN (1)
The DC current rating of the inductor should be at least equal to the maximum load current plus half the ripple current to prevent core saturation. Thus, a 720mA rated inductor should be enough for most applications (600mA + 120mA). For better efficiency, choose a low DC resistance inductor. The inductor value also has an effect on Burst Mode operation. The transition to low current operation begins when the inductor current peaks fall to approximately 200mA. Lower inductor values (higher IL) will cause this to occur at lower load currents, which can cause a dip in efficiency in the upper range of low current operation. In Burst Mode operation, lower inductance values will cause the burst frequency to increase. Inductor Core Selection Different core materials and shapes will change the size/current and price/current relationship of an inductor. Toroid or shielded pot cores in ferrite or permalloy materials are small and don't radiate much energy, but generally cost more than powdered iron core inductors with similar electrical characteristics. The choice of which style inductor to use often depends more on the price vs size requirements and any radiated field/EMI requirements than on what the LTC3409A requires to operate. Table 1 shows some typical surface mount inductors that work well in LTC3409A applications.
CIN and COUT Selection In continuous mode, the source current of the top MOSFET is a square wave of duty cycle VOUT/VIN. To prevent large voltage transients, a low ESR input capacitor sized for the maximum RMS current must be used. The maximum RMS capacitor current is given by: VOUT ( VIN - VOUT ) CIN Required IRMS IOUT(MAX) VIN
1/ 2
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 the capacitor manufacturer's ripple current ratings are often based on 2000 hours of life. This makes it advisable to further derate the capacitor, or choose a capacitor rated at a higher temperature than required. Always consult the manufacturer if there is any question. The selection of COUT is driven by the required effective series resistance (ESR). Typically, once the ESR requirement for COUT has been met, the RMS current rating generally far exceeds the IRIPPLE(P-P) requirement.
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LTC3409A
Output Voltage Programming The output voltage is set by a resistive divider according to Equation 2: R1 VOUT = 0.612V 1+ R2 (2)
APPLICATIONS INFORMATION
The output ripple VOUT is determined by: 1 VOUT = IL ESR + 8 * f * COUT where f = operating frequency, COUT = output capacitance and IL = ripple current in the inductor. For a fixed output voltage, the output ripple is highest at maximum input voltage since IL increases with input voltage. Aluminum electrolytic and dry tantalum capacitors are both available in surface mount configurations. In the case of tantalum, it is critical that the capacitors are surge tested for use in switching power supplies. An excellent choice is the AVX TPS series of surface mount tantalum. These are specially constructed and tested for low ESR so they give the lowest ESR for a given volume. Other capacitor types include Sanyo POSCAP Kemet T510 and T495 series, and , Sprague 593D and 595D series. Consult the manufacturer for other specific recommendations. Using Ceramic Input and Output Capacitors Higher value, lower cost ceramic capacitors are now available in smaller case sizes. Their high ripple current, high voltage rating and low ESR make them ideal for switching regulator applications. Because the LTC3409A's control loop does not depend on the output capacitor's ESR for stable operation, ceramic capacitors can be used to achieve very low output ripple and small circuit size. However, care must be taken when these capacitors are used at the input and the output. When a ceramic capacitor is used at the input and the power is supplied by a wall adapter through long wires, a load step at the output can induce ringing at the input, VIN. At best, this ringing can couple to the output and be mistaken as loop instability. At worst, a sudden inrush of current through the long wires can potentially cause a voltage spike at VIN, large enough to damage the part. When choosing the input and output ceramic capacitors, choose the X5R or X7R dielectric formulations. These dielectrics have the best temperature and voltage characteristics of all the ceramics for a given value and size.
The external resistive divider is connected to the output, allowing remote voltage sensing as shown in Figure 1.
VOUT R1 VFB LTC3409A GND
3409A F01
R2
Figure 1
Efficiency Considerations The 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. Efficiency can be expressed as: Efficiency = 100% - (L1 + L2 + L3 + ...) where L1, L2, etc. are the individual losses as a percentage of input power. Although all dissipative elements in the circuit produce losses, two main sources usually account for most of the losses in LTC3409A circuits: VIN quiescent current and I2R losses. The VIN quiescent current loss dominates the efficiency loss at very low load currents whereas the I2R loss dominates the efficiency loss at medium to high load currents. In a typical efficiency plot, the efficiency curve at very low load currents can be misleading since the actual power lost is of no consequence as illustrated in Figure 2.
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LTC3409A APPLICATIONS INFORMATION
1.0000 VOUT = 1.8V 0.1000 POWER LOSS (W) 4.2VIN 3.6VIN 0.0100 2.5VIN 2.5VIN 0.0010 4.2VIN 3.6VIN 0.0001 0.1 1 BURST PULSE SKIP 1000
3409A F02
Other losses including CIN and COUT ESR dissipative losses, and inductor core losses, generally account for less than 2% total additional loss. Thermal Considerations In most applications the LTC3409A does not dissipate much heat due to its high efficiency. But, in applications where the LTC3409A is running at high ambient temperature with low supply voltage and high duty cycles, such as in dropout, the heat dissipated may exceed the maximum junction temperature of the part. If the junction temperature reaches approximately 150C, both power switches will be turned off and the SW node will become high impedance. To avoid the LTC3409A from exceeding the maximum junction temperature, the user will need to do a thermal analysis. The goal of the thermal analysis is to determine whether the operating conditions exceed the maximum junction temperature of the part. The temperature rise is given by: TR = (PD)(JA) where PD is the power dissipated by the regulator and JA is the thermal resistance from the junction of the die to the ambient temperature. The junction temperature, TJ, is given by: TJ = TA + TR where TA is the ambient temperature. As an example, consider the LTC3409A in dropout at an input voltage of 1.6V, a load current of 600mA and an ambient temperature of 75C. From the typical performance graph of switch resistance, the RDS(ON) of the P-channel switch at 75C is approximately 0.48. Therefore, power dissipated by the part is: PD = ILOAD2 * RDS(ON) = 172.8mW For the DD8 package, the JA is 43C/W. Thus, the junction temperature of the regulator is: TJ = 75C + (0.1728)(43) = 82.4C which is well below the maximum junction temperature of 125C.
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100 10 LOAD CURRENT (mA)
Figure 2. Power Loss
1. The VIN quiescent current is due to two components: the DC bias current as given in the Electrical Characteristics and the internal main switch and synchronous switch gate charge currents. The gate charge current results from switching the gate capacitance of the internal power MOSFET switches. Each time the gate is switched from high to low to high again, a packet of charge, dQ, moves from VIN to ground. The resulting dQ/dt is the current out of VIN that is typically larger than the DC bias current. In continuous mode, IGATECHG = f(QT + QB) where QT and QB are the gate charges of the internal top and bottom switches. Both the DC bias and gate charge losses are proportional to VIN and thus their effects will be more pronounced at higher supply voltages. 2. I2R losses are calculated from the resistances of the internal switches, RSW, and external inductor RL. In continuous mode, the average output current flowing through inductor L is "chopped" between the main switch and the synchronous switch. Thus, the series resistance looking into the SW pin is a function of both top and bottom MOSFET RDS(ON) and the duty cycle (DC) as follows: RSW = (RDS(ON)TOP)(DC) + (RDS(ON)BOT)(1 - DC) The RDS(ON) for both the top and bottom MOSFETs can be obtained from the Typical Performance Characteristics. Thus, to obtain I2R losses, simply add RSW to RL and multiply the result by the square of the average output current.
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LTC3409A
2. Are the COUT and L1 closely connected? The (-) plate of COUT returns current to GND and the (-) plate of CIN. 3. The resistor divider, R1 and R2, must be connected between the (+) plate of COUT and a ground sense line terminated near GND (Exposed Pad). The feedback signals VFB should be routed away from noisy components and traces, such as the SW line (Pins 6), and its trace should be minimized. 4. The SW trace should be kept as small as possible. Keep sensitive components away from the SW pins. The input capacitor CIN and the resistors R1 and R2 should be routed away from the SW traces and the inductors. 5. A ground plane is preferred, but if not available, keep the signal and power grounds segregated with small signal components returning to the GND pin at one point. They should not share the high current path of CIN or COUT. 6. Flood all unused areas on all layers with copper. Flooding with copper will reduce the temperature rise of power components. These copper areas should be connected to VIN or GND (preferably).
VIN
APPLICATIONS INFORMATION
Note that at higher supply voltages, the junction temperature is lower due to reduced switch resistance (RDS(ON)). 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, which generates a feedback error signal. The regulator loop then acts 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. For a detailed explanation of switching control loop theory, see Application Note 76. A second, more severe transient is caused by switching in loads with large (>1F) supply bypass capacitors. The discharged bypass capacitors are effectively put in parallel with COUT, causing a rapid drop in VOUT. No regulator can deliver enough current to prevent this problem if the load switch resistance is low and it is driven quickly. The only solution is to limit the rise time of the switch drive so that the load rise time is limited to approximately (25 * CLOAD). Thus, a 10F capacitor charging to 3.3V would require a 250s rise time, limiting the charging current to about 130mA. Board Layout Considerations When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC3409A. These items are also illustrated graphically in the layout diagram of Figure 3. Check the following in your layout. 1. Does the capacitor CIN connect to the power VIN (Pins 3, 4) and GND (Exposed Pad) as close as possible? This capacitor provides the AC current to the internal power MOSFETs and their drivers.
CIN VIN RUN VFB VIN SYNC MODE SW SGND GND C1 L1 VOUT
LTC3409A
COUT R2 R1
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Figure 3
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LTC3409A APPLICATIONS INFORMATION
Design Example As a design example, assume the LTC3409A is used in a 2-alkaline cell battery-powered application. The VIN will be operating from a maximum of 3.2V down to about 1.8V. The load current requirement is a maximum of 600mA but most of the time it will be in standby mode, requiring only 2mA. Efficiency at both low and high load currents is important, so the minimum frequency setting of 1.7MHz is chosen. Output voltage is 1.5V. With this information we can calculate L using Equation 3: V 1 L= VOUT 1- OUT f * IL VIN (3) For best efficiency choose a 750mA or greater inductor with less than 0.3 series resistance. CIN will require an RMS current rating of at least 0.3A ILOAD(MAX) /2 at temperature. For the feedback resistors, choose R2 = 137k. R1 is then calculated to be 200k from Equation 2. Figure 4 shows the complete circuit along with its efficiency curve. Table 2 below gives 1% resistor values for selected output voltages.
VOUT 0.85V 1.2V 1.5V 1.8V R1 53.6k 137k 200k 267k R2 137k 143k 137k 137k
Substituting VOUT = 1.5V, VIN = 3.2V, IL = 240mA and f = 1.7MHz in Equation 2 gives: L= 1 1.5V 1.5V 1- 2.2H 3.2V 1.7MHz * 240mA
Burst Mode Efficiency, 1.5VOUT
VIN 1.8V TO 3.2V CIN 4.7F
100
R2 137k
90
1.8VIN 3.2VIN
LTC3409A VFB GND VIN VIN R1 200k SYNC RUN SW MODE
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80 EFFICIENCY (%)
L1 2.2H COUT 22F 2
70 60 50 40 30 20 2.5VIN
VOUT 1.5V 0.6A
L1: SUMIDA CDRH2D18/LD C1 10pF
10 0 0.1 1 10 100 LOAD CURRENT (mA) 1000
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Figure 4
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LTC3409A
DD Package 8-Lead Plastic DFN (3mm x 3mm)
(Reference LTC DWG # 05-08-1698)
PACKAGE DESCRIPTION
0.675 0.05
3.5 0.05 1.65 0.05 2.15 0.05 (2 SIDES) PACKAGE OUTLINE 0.25 0.05
0.50 BSC 2.38 0.05 (2 SIDES) R = 0.115 TYP 5 0.38 0.10 8
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
3.00 0.10 (4 SIDES) PIN 1 TOP MARK (NOTE 6)
1.65 0.10 (2 SIDES)
(DD) DFN 1203
0.200 REF
0.75 0.05
4 0.25 0.05 2.38 0.10 (2 SIDES)
1 0.50 BSC
0.00 - 0.05
BOTTOM VIEW--EXPOSED PAD NOTE: 1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-1) 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 TOP AND BOTTOM OF PACKAGE
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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.
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LTC3409A RELATED PARTS
PART NUMBER LTC3409 DESCRIPTION 600mA (IOUT) Low VIN Step-Down DC/DC Converter 250mA Low VIN Step-Down DC/DC Converter Dual 1.5A/1A 4MHz Step-Down DC/DC Regulator 300mA, 2.25MHz, Synchronous Step-Down Regulator in SC-70 600mA (IOUT), 550kHz, Synchronous Step-Down DC/DC Converter 1.20A (IOUT), 550kHz, Synchronous Step-Down DC/DC Converter 100mA, Low Voltage VLDOTM 100mA, Low Voltage VLDO 600mA (IOUT), 1.4MHz, Synchronous Step-Down DC/DC Converter 300mA (IOUT), 1.5MHz, Synchronous Step-Down DC/DC Converter 600mA (IOUT), 1.5MHz, Synchronous Step-Down DC/DC Converter Dual, 600mA/800mA (IOUT), 1.5MHz/2.25MHz, Synchronous Step-Down DC/DC Converter 1.25A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter COMMENTS 95% Efficiency, VIN: 1.6V to 5.5V, VOUT(MIN) = 0.61V, IQ = 65A, ISD < 1A, 8-Lead DFN Package 93% Efficiency, VIN: 1.6V to 5.5V, VOUT(MIN) = 0.61V, IQ = 50A, ISD < 1A, 6-Lead DFN Package 95% Efficiency, Low Ripple, VIN: 2.25V to 5.5V, VOUT(MIN) = 0.8V, DFN and TSSOP Packages 96% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 26A 96% Efficiency, VIN: 2.7V to 6V, VOUT(MIN) = 0.8V, IQ = 10A, ISD <1A, MS8 Package 95% Efficiency, VIN: 2.7V to 10V, VOUT(MIN) = 0.8V, IQ = 15A, ISD <1A, 16-Lead TSSOP VIN: 0.9V to 10V, VOUT(MIN) = 0.20V, Dropout Voltage = 0.15V, IQ = 120A, ISD <3A, VOUT = ADJ, DFN/MS8 Packages VIN: 0.9V to 5.5V, VOUT(MIN) = 0.40V, Dropout Voltage = 0.05V, IQ = 54A, ISD <1A, VOUT = ADJ, DFN Package 96% Efficiency, VIN: 2.7V to 6V, VOUT(MIN) = 0.8V, IQ = 10A, ISD <1A, MS8 Package 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 20A, ISD <1A, ThinSOTTM Package 96% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 20A, ISD <1A, ThinSOT Package 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 40A, ISD <1A, 10-Lead MSE Package 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 60A, ISD <1A, 10-Lead MS Package
LTC3549 LTC3417A-2 LTC3410 LTC1878 LTC1879 LT3020 LTC3025 LTC3404 LTC3405A LTC3406A/ LTC3406AB LTC3407A/ LTC3407A-2 LTC3411A
VLDO and ThinSOT are trademarks of Linear Technology Corporation.
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16 Linear Technology Corporation
(408) 432-1900
LT 0509 * PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
FAX: (408) 434-0507 www.linear.com
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