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LTC3857

3857fd

TYPICAL APPLICATION

FEATURES

APPLICATIONS

DESCRIPTION

Low I

, Dual, 2-Phase

Synchronous Step-Down

Controller

High Efficiency Dual 3.3V/8.5V Step-Down Converter

Low Operating I

: 50μA (One Channel On)

Wide Output Voltage Range: 0.8V ≤ V

OUT

≤ 24V

Wide V

Range: 4V to 38V (40V Abs Max)

SENSE

or DCR Current Sensing

Out-of-Phase Controllers Reduce Required Input

Capacitance and Power Supply Induced Noise

OPTI-LOOP

Compensation Minimizes C

OUT

Phase-Lockable Frequency (75kHz-850kHz)

Programmable Fixed Frequency (50kHz-900kHz)

Selectable Continuous, Pulse-Skipping or Low Ripple

Burst Mode

Operation at Light Loads

Selectable Current Limit

Very Low Dropout Operation: 99% Duty Cycle

Adjustable Output Voltage Soft-Start or Tracking

Power Good Output Voltage Monitors

Output Overvoltage Protection

Low Shutdown I

: <8µA

Internal LDO Powers Gate Drive from V

or EXTV

No Current Foldback During Start-up

5mm × 5mm QFN Package

Automotive Always-On Systems

Battery Operated Digital Devices

Distributed DC Power Systems

0.1µF

62.5k

3.3µH

680pF

150µF

4.7µF

22µF

50V

0.007Ω

20k

15k

OUT1

3.3V

150µF

0.1µF

193k

7.2µH

680pF

0.010Ω

20k

15k

OUT2

8.5V

3.5A

TG1 TG2

BOOST1 BOOST2

SW1 SW2

BG1 BG2

SGND

PGND

SENSE1

SENSE2

SENSE1

–

SENSE2

–

FB1

FB2

TH1

TH2

INTV

TRACK/SS1 TRACK/SS2

9V TO 38V

3857 TA01

0.1µF

LTC3857

Efficiency and Power Loss

vs Output Current

The LTC

3857 is a high performance dual step-down

switching regulator controller that drives all N-channel

synchronous power MOSFET stages. A constant frequency

current mode architecture allows a phase-lockable fre-

quency of up to 850kHz. Power loss and noise due to the

ESR of the input capacitor ESR are minimized by operating

the two controller output stages out of phase.

The 50A no-load quiescent current extends operating run

time in battery-powered systems. The LTC3857 features a

precision 0.8V reference and power good output indicators.

A wide 4V to 38V input supply range encompasses a wide

range of intermediate bus voltages and battery chemistries.

Independent TRACK/SS pins for each controller ramp the

output voltages during start-up. Current foldback limits

MOSFET heat dissipation during short-circuit conditions.

The PLLIN/MODE pin selects among Burst Mode opera-

tion, pulse-skipping mode, or continuous inductor current

mode at light loads.

For a leaded 28-lead SSOP package with a fixed current

limit and one PGOOD output, without phase modulation

or a clock output, see the LTC3857-1 data sheet.

OUTPUT CURRENT (A)

0.00001 0.0001

EFFICIENCY (%)

POWER LOSS (mW)

0.001 0.01 0.1 1 10

3857 TA01b

100

1000

0.1

10000

= 12V

OUT

= 3.3V

FIGURE 13 CIRCUIT

L, LT, LTC, LTM, Burst Mode, OPTI-LOOP, PolyPhase, µModule, Linear Technology and the

Linear logo are registered trademarks and No R

SENSE

and UltraFast are trademarks of Linear

Technology Corporation. All other trademarks are the property of their respective owners.

Protected by U.S. Patents, including 5481178, 5929620, 6177787, 6144194, 5408150,

6580258, 5705919, 6100678.

Summary of content (38 pages)

PAGE 1
LTC3857 Low IQ, Dual, 2-Phase Synchronous Step-Down Controller DESCRIPTION FEATURES n n n n n n n n n n n n n n n n n n The LTC®3857 is a high performance dual step-down switching regulator controller that drives all N-channel synchronous power MOSFET stages. A constant frequency current mode architecture allows a phase-lockable frequency of up to 850kHz. Power loss and noise due to the ESR of the input capacitor ESR are minimized by operating the two controller output stages out of phase.
PAGE 2
LTC3857 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) Input Supply Voltage (VIN) ......................... –0.3V to 40V Topside Driver Voltages BOOST1, BOOST2 ................................. –0.3V to 46V Switch Voltage (SW1, SW2) ........................ –5V to 40V PLLIN/MODE, (BOOST1-SW1), (BOOST2-SW2), INTVCC ........................................................ –0.3V to 6V RUN1, RUN2 ................................................ –0.
PAGE 3
LTC3857 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25°C (Note 2). VIN = 12V, VRUN1,2 = 5V, EXTVCC = 0V, unless otherwise noted. SYMBOL PARAMETER CONDITIONS VLOADREG Output Voltage Load Regulation (Note 4) Measured in Servo Loop, ∆ITH Voltage = 1.2V to 0.7V (Note 4) Measured in Servo Loop, ∆ITH Voltage = 1.2V to 2V gm1,2 Transconductance Amplifier gm (Note 4) ITH1, 2 = 1.
PAGE 4
LTC3857 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25°C (Note 2). VIN = 12V, VRUN1,2 = 5V, EXTVCC = 0V unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN 4.
PAGE 5
LTC3857 TYPICAL PERFORMANCE CHARACTERISTICS Efficiency and Power Loss vs Output Current 90 50 10 40 30 20 70 60 94 VIN = 12V 50 40 30 VOUT = 3.3V FIGURE 13 CIRCUIT 10 10 0 0.1 0.1 1 10 0.00001 0.0001 0.001 0.01 38 57 G01 OUTPUT CURRENT (A) BURST EFFICIENCY BURST LOSS PULSE-SKIPPING PULSE-SKIPPING EFFICIENCY LOSS CCM EFFICIENCY CCM LOSS 0 0.1 0.00001 0.0001 0.001 0.
PAGE 6
LTC3857 TYPICAL PERFORMANCE CHARACTERISTICS Total Input Supply Current vs Input Voltage EXTVCC Switchover and INTVCC Voltages vs Temperature 500 VOUT1 = 3.3V RUN2 = 0V FIGURE 13 CIRCUIT 5.8 350 300 250 500μA 200 300μA 150 100 NO LOAD 50 0 5 10 5.2 5.6 5.4 INTVCC 5.2 5.0 EXTVCC RISING 4.8 EXTVCC FALLING 4.6 25 20 15 30 INPUT VOLTAGE (V) 35 40 –20 55 30 5 80 TEMPERATURE (°C) –100 60 Burst Mode OPERATION 20 ILIM = GND ILIM = FLOAT ILIM = INTVCC FORCED CONTINUOUS MODE 0 0.2 0.4 0.
PAGE 7
LTC3857 TYPICAL PERFORMANCE CHARACTERISTICS TRACK/SS Pull-Up Current vs Temperature Regulated Feedback Voltage vs Temperature Shutdown (RUN) Threshold vs Temperature 1.40 800 REGULATED FEEDBACK VOLTAGE (mV) 1.10 1.05 RUN PIN VOLTAGE (V) TRACK/SS CURRENT (μA) 1.35 1.00 0.95 RUN RISING 1.30 1.25 RUN FALLING 1.20 1.15 0.90 –45 –20 80 5 30 55 TEMPERATURE (°C) 105 1.
PAGE 8
LTC3857 PIN FUNCTIONS SENSE1–, SENSE2– (Pin 1, Pin 9): The (–) Input to the Differential Current Comparators. When greater than INTVCC – 0.5V, the SENSE– pin supplies current to the current comparator. FREQ (Pin 2): The frequency control pin for the internal VCO. Connecting the pin to GND forces the VCO to a fixed low frequency of 350kHz. Connecting the pin to INTVCC forces the VCO to a fixed high frequency of 535kHz.
PAGE 9
LTC3857 PIN FUNCTIONS TG1, TG2 (Pin 26, Pin 15): High Current Gate Drives for Top N-Channel MOSFETs. These are the outputs of floating drivers with a voltage swing equal to INTVCC – 0.5V superimposed on the switch node voltage SW. ITH1, ITH2 (Pin 30, Pin 12): Error Amplifier Outputs and Switching Regulator Compensation Points. Each associated channel’s current comparator trip point increases with this control voltage. PGOOD1, PGOOD2 (Pin 27, Pin 14): Open-Drain Logic Output.
PAGE 10
LTC3857 FUNCTIONAL DIAGRAM VIN INTVCC PGOOD1 27 0.88V PHASMD CLKOUT 3 4 DUPLICATE FOR SECOND CONTROLLER CHANNEL + – VFB1 DROP OUT DET + – + PGOOD2 14 0.72V 0.88V S Q R Q TOP BOT SWITCH LOGIC BOT – SHDN INTVCC + 0.72V TG 26, 15 CB D CIN BG 23, 18 COUT PGND – 20μA FREQ DB SW 25, 16 TOP ON VFB2 BOOST 24, 17 VOUT 21 VCO 2 CLK2 0.425V CLK1 + SLEEP RSENSE L – ICMP PFD + – CLP –+ +– IR – SENSE+ 32, 10 + 3mV SYNC DET PLLIN/MODE 2.7V 0.
PAGE 11
LTC3857 OPERATION (Refer to the Functional Diagram) Main Control Loop The LTC3857 uses a constant frequency, current mode step-down architecture with the two controller channels operating 180 degrees out of phase. During normal operation, each external top MOSFET is turned on when the clock for that channel sets the RS latch, and is turned off when the main current comparator, ICMP, resets the RS latch.
PAGE 12
LTC3857 OPERATION (Refer to the Functional Diagram) Light Load Current Operation (Burst Mode Operation, Pulse-Skipping or Forced Continuous Mode) (PLLIN/MODE Pin) The LTC3857 can be enabled to enter high efficiency Burst Mode operation, constant frequency pulse-skipping mode, or forced continuous conduction mode at low load currents. To select Burst Mode operation, tie the PLLIN/ MODE pin to ground. To select forced continuous operation, tie the PLLIN/MODE pin to INTVCC.
PAGE 13
LTC3857 OPERATION (Refer to the Functional Diagram) selects 535kHz. Placing a resistor between FREQ and SGND allows the frequency to be programmed between 50kHz and 900kHz, as shown in Figure 10. A phase-locked loop (PLL) is available on the LTC3857 to synchronize the internal oscillator to an external clock source that is connected to the PLLIN/MODE pin.
PAGE 14
LTC3857 OPERATION (Refer to the Functional Diagram) With 2-phase operation, the two channels of the dual switching regulator are operated 180 degrees out of phase. This effectively interleaves the current pulses drawn by the switches, greatly reducing the overlap time where they add together. The result is a significant reduction in total RMS input current, which in turn allows less expensive input capacitors to be used, reduces shielding requirements for EMI and improves real world operating efficiency.
PAGE 15
LTC3857 APPLICATIONS INFORMATION The Typical Application on the first page is a basic LTC3857 application circuit. LTC3857 can be configured to use either DCR (inductor resistance) sensing or low value resistor sensing. The choice between the two current sensing schemes is largely a design trade-off between cost, power consumption and accuracy. DCR sensing is becoming popular because it saves expensive current sensing resistors and is more power efficient, especially in high current applications.
PAGE 16
LTC3857 APPLICATIONS INFORMATION placed close to the switching node, to prevent noise from coupling into sensitive small-signal nodes. Low Value Resistor Current Sensing A typical sensing circuit using a discrete resistor is shown in Figure 4a. RSENSE is chosen based on the required output current. The current comparator has a maximum threshold VSENSE(MAX) determined by the ILIM setting.
PAGE 17
LTC3857 APPLICATIONS INFORMATION The equivalent resistance R1|| R2 is scaled to the room temperature inductance and maximum DCR: R1||R2= L (DCR at 20°C) •C1 The sense resistor values are: R1= R1||R2 R1•RD ; R2= RD 1–RD The maximum power loss in R1 is related to duty cycle, and will occur in continuous mode at the maximum input voltage: P LOSS R1= ( VIN(MAX) –VOUT ) •VOUT R1 Accepting larger values of ∆IL allows the use of low inductances, but results in higher output voltage ripple and greater core lo
PAGE 18
LTC3857 APPLICATIONS INFORMATION The peak-to-peak drive levels are set by the INTVCC voltage. This voltage is typically 5.1V during start-up (see EXTVCC Pin Connection). Consequently, logic-level threshold MOSFETs must be used in most applications. The only exception is if low input voltage is expected (VIN < 4V); then, sub-logic level threshold MOSFETs (VGS(TH) < 3V) should be used.
PAGE 19
LTC3857 APPLICATIONS INFORMATION maximum RMS capacitor current requirement. Increasing the output current drawn from the other controller will actually decrease the input RMS ripple current from its maximum value. The out-of-phase technique typically reduces the input capacitor’s RMS ripple current by a factor of 30% to 70% when compared to a single phase power supply solution. In continuous mode, the source current of the top MOSFET is a square wave of duty cycle (VOUT)/(VIN).
PAGE 20
LTC3857 APPLICATIONS INFORMATION Tracking and Soft-Start (TRACK/SS Pins) Soft-start is enabled by simply connecting a capacitor from the TRACK/SS pin to ground, as shown in Figure 6. An internal 1μA current source charges the capacitor, providing a linear ramping voltage at the TRACK/SS pin. The LTC3857 will regulate the VFB pin (and hence VOUT) according to the voltage on the TRACK/SS pin, allowing VOUT to rise smoothly from 0V to its final regulated value.
PAGE 21
LTC3857 APPLICATIONS INFORMATION INTVCC Regulators The LTC3857 features two separate internal P-channel low dropout linear regulators (LDO) that supply power at the INTVCC pin from either the VIN supply pin or the EXTVCC pin depending on the connection of the EXTVCC pin. INTVCC powers the gate drivers and much of the LTC3857’s internal circuitry. The VIN LDO and the EXTVCC LDO regulate INTVCC to 5.1V. Each of these can supply a peak current of 50mA and must be bypassed to ground with a minimum of 4.
PAGE 22
LTC3857 APPLICATIONS INFORMATION Fault Conditions: Current Limit and Current Foldback CIN BAT85 VIN BAT85 MTOP VN2222LL TG1 1/2 LTC3857 EXTVCC L BAT85 RSENSE VOUT SW MBOT BG1 PGND COUT 3857 F09 Figure 9. Capacitive Charge Pump for EXTVCC The LTC3857 includes current foldback to help limit load current when the output is shorted to ground.
PAGE 23
LTC3857 APPLICATIONS INFORMATION Phase-Locked Loop and Frequency Synchronization 1000 900 The LTC3857 has an internal phase-locked loop (PLL) comprised of a phase frequency detector, a lowpass filter, and a voltage-controlled oscillator (VCO). This allows the turn-on of the top MOSFET of controller 1 to be locked to the rising edge of an external clock signal applied to the PLLIN/MODE pin. The turn-on of controller 2’s top MOSFET is thus 180 degrees out of phase with the external clock.
PAGE 24
LTC3857 APPLICATIONS INFORMATION If the duty cycle falls below what can be accommodated by the minimum on-time, the controller will begin to skip cycles. The output voltage will continue to be regulated, but the ripple voltage and current will increase. out of INTVCC that is typically much larger than the control circuit current. In continuous mode, IGATECHG = f(QT + QB), where QT and QB are the gate charges of the topside and bottom side MOSFETs. The minimum on-time for the LTC3857 is approximately 95ns.
PAGE 25
LTC3857 APPLICATIONS INFORMATION Other hidden losses such as copper trace and internal battery resistances can account for an additional 5% to 10% efficiency degradation in portable systems. It is very important to include these system level losses during the design phase. The internal battery and fuse resistance losses can be minimized by making sure that CIN has adequate charge storage and very low ESR at the switching frequency.
PAGE 26
LTC3857 APPLICATIONS INFORMATION Design Example As a design example for one channel, assume VIN = 12V (nominal), VIN = 22V (max), VOUT = 3.3V, IMAX = 5A, VSENSE(MAX) = 75mV and f = 350kHz. The inductance value is chosen first based on a 30% ripple current assumption. The highest value of ripple current occurs at the maximum input voltage. Tie the FREQ pin to GND, generating 350kHz operation. The minimum inductance for 30% ripple current is: ⎛ V VOUT ⎞ ΔIL(NOM)= OUT ⎜ 1– ⎟ ƒ•L ⎝ VIN(NOM) ⎠ A 4.
PAGE 27
LTC3857 APPLICATIONS INFORMATION PC Board Layout Checklist When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the IC. These items are also illustrated graphically in the layout diagram of Figure 11. Figure 12 illustrates the current waveforms present in the various branches of the 2-phase synchronous regulators operating in the continuous mode. Check the following in your layout: 1.
PAGE 28
LTC3857 APPLICATIONS INFORMATION SW1 L1 D1 RSENSE1 VOUT1 COUT1 RL1 VIN RIN CIN SW2 BOLD LINES INDICATE HIGH SWITCHING CURRENT. KEEP LINES TO A MINIMUM LENGTH. D2 L2 RSENSE2 VOUT2 COUT2 RL2 3857 F12 Figure 12. Branch Current Waveforms 4. Are the SENSE– and SENSE+ leads routed together with minimum PC trace spacing? The filter capacitor between SENSE+ and SENSE– should be as close as possible to the IC. Ensure accurate current sensing with Kelvin connections at the SENSE resistor. 5.
PAGE 29
LTC3857 APPLICATIONS INFORMATION PC Board Layout Debugging Start with one controller on at a time. It is helpful to use a DC-50MHz current probe to monitor the current in the inductor while testing the circuit. Monitor the output switching node (SW pin) to synchronize the oscilloscope to the internal oscillator and probe the actual output voltage as well. Check for proper performance over the operating voltage and current range expected in the application.
PAGE 30
LTC3857 APPLICATIONS INFORMATION RB1 215k INTVCC CF1 15pF LTC3857 SENSE1+ PGOOD2 C1 1nF RA1 68.1k SENSE1– VFB1 CITH1A 150pF 100k 100k PGOOD1 MBOT1 BG1 SW1 ITH1 RSENSE1 6mΩ CB1 0.47μF BOOST1 RITH1 15k CITH1 820pF COUT1 150μF VOUT1 3.3V 5A MTOP1 TG1 D1 CSS1 0.1μF VIN TRACK/SS1 INTVCC ILIM PHSMD CLKOUT PGND PLLIN/MODE SGND TG2 EXTVCC RUN1 BOOST2 RUN2 FREQ CIN 22μF TRACK/SS2 SW2 ITH2 BG2 VIN 9V TO 38V CINT 4.7μF D2 MTOP2 CB2 0.47μF L2 7.2μH CSS2 0.1μF CITH2 680pF L1 3.
PAGE 31
LTC3857 TYPICAL APPLICATIONS High Efficiency Dual 2.5V/3.3V Step-Down Converter RB1 143k INTVCC CF1 22pF C1 1nF RA1 68.1k LTC3857 SENSE1+ PGOOD2 SENSE1– VFB1 CITH1A 100pF 100k 100k PGOOD1 MBOT1 BG1 SW1 ITH1 RSENSE1 6mΩ CB1 0.47μF BOOST1 RITH1 22k CITH1 820pF COUT1 150μF VOUT1 2.5V 5A MTOP1 TG1 D1 CSS1 0.01μF VIN TRACK/SS1 INTVCC ILIM PHSMD CLKOUT PGND PLLIN/MODE SGND TG2 EXTVCC RUN1 BOOST2 RUN2 FREQ CIN 22μF TRACK/SS2 SW2 ITH2 BG2 VIN 4.5V TO 38V CINT 4.7μF D2 MTOP2 CB2 0.
PAGE 32
LTC3857 TYPICAL APPLICATIONS High Efficiency Dual 12V/5V Step-Down Converter RB1 475k CF1 33pF RA1 34k C1 1nF 100k INTVCC SENSE1+ PGOOD2 SENSE1– PGOOD1 100k VFB1 MBOT1 BG1 CITH1A 100pF SW1 RFREQ 60k D1 LTC3857 VIN TRACK/SS1 INTVCC ILIM PHSMD CLKOUT PGND PLLIN/MODE SGND TG2 EXTVCC RUN1 BOOST2 RUN2 FREQ CIN 22μF TRACK/SS2 SW2 ITH2 BG2 VIN 12.5V TO 38V CINT 4.7μF D2 MTOP2 CB2 0.47μF L2 4.3μH CSS2 0.
PAGE 33
LTC3857 TYPICAL APPLICATIONS High Efficiency Dual 24V/5V Step-Down Converter RB1 487k CF1 18pF RA1 16.9k C1 1nF 100k SENSE1+ PGOOD2 SENSE1– PGOOD1 INTVCC 100k MBOT1 BG1 VFB1 CITH1A 100pF VOUT1 24V 1A SW1 CITH1 680pF D1 LTC3857 VIN TRACK/SS1 INTVCC ILIM PHSMD CLKOUT PGND PLLIN/MODE SGND TG2 EXTVCC RUN1 BOOST2 RUN2 FREQ RFREQ 60k MTOP1 TG1 ITH1 CSS1 0.01μF RSENSE1 25mΩ CB1 0.47μF BOOST1 RITH1 46k CIN 22μF CINT 4.
PAGE 34
LTC3857 TYPICAL APPLICATIONS High Efficiency Dual 1V/1.2V Step-Down Converter RB1 28.7k CF1 56pF RA1 115k C1 1nF 100k INTVCC SENSE1+ PGOOD2 SENSE1– PGOOD1 100k VFB1 L1 MBOT1 0.47μH BG1 CITH1A 200pF SW1 CITH1 1000pF CSS1 0.01μF RFREQ 60k MTOP1 TG1 ITH1 RSENSE1 3.5mΩ CB1 0.47μF BOOST1 RITH1 3.93k LTC3857 VIN TRACK/SS1 INTVCC ILIM PHSMD CLKOUT PGND PLLIN/MODE SGND TG2 EXTVCC RUN1 BOOST2 RUN2 FREQ CIN 22μF TRACK/SS2 SW2 ITH2 BG2 VIN 12V CINT 4.7μF D2 MTOP2 CB2 0.47μF L2 0.
PAGE 35
LTC3857 TYPICAL APPLICATIONS High Efficiency Dual 1V/1.2V Step-Down Converter with Inductor DCR Current Sensing RB1 28.7k CF1 56pF RA1 115k RS1 1.18k C1 0.1μF SENSE1+ SENSE1– INTVCC 100k PGOOD1 CITH1A 200pF L1 0.47μH MBOT1 BG1 VFB1 SW1 CB1 0.47μF BOOST1 RITH1 3.93k CITH1 1000pF VOUT1 1V 8A D1 LTC3857 CSS1 0.01μF MTOP1 TG1 ITH1 COUT1 220μF ×2 VIN CIN 22μF TRACK/SS1 INTVCC VIN 12V CINT 4.7μF PGND PLLIN/MODE SGND TG2 EXTVCC RUN1 BOOST2 RUN2 FREQ RFREQ 65k D2 MTOP2 CB2 0.47μF L2 0.
PAGE 36
LTC3857 PACKAGE DESCRIPTION Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. UH Package 32-Lead Plastic QFN (5mm × 5mm) (Reference LTC DWG # 05-08-1693 Rev D) 0.70 p0.05 5.50 p0.05 4.10 p0.05 3.45 p 0.05 3.50 REF (4 SIDES) 3.45 p 0.05 PACKAGE OUTLINE 0.25 p 0.05 0.50 BSC RECOMMENDED SOLDER PAD LAYOUT APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 5.00 p 0.10 (4 SIDES) BOTTOM VIEW—EXPOSED PAD 0.75 p 0.05 R = 0.05 TYP 0.00 – 0.05 PIN 1 NOTCH R = 0.
PAGE 37
LTC3857 REVISION HISTORY REV DATE DESCRIPTION PAGE NUMBER A 12/09 Change to Absolute Maximum Ratings Change to Order Information Change to Electrical Characteristics Change to Typical Performance Characteristics Change to Pin Functions Text Changes to Operations Section Text Changes to Applications Information Section C 08/10 11/10 8, 9 11, 12, 13 21, 22, 23, 24, 26 23 27 Change to Electrical Characteristics 38 2, 3, 4 Change to Graph G07 5 Added Typical Application to back page and updated R
PAGE 38
LTC3857 TYPICAL APPLICATION High Efficiency 2-Phase 12V/150W Step-Down Converter RB1 698k CF1 10pF RA1 49.9k C1 1nF 100k SENSE1+ PGOOD2 SENSE1– PGOOD1 INTVCC 100k VFB1 CITH1A 68pF L1 6μH MBOT1 BG1 SW1 CITH1 3300pF CSS1 0.1μF LTC3857 SS1 ITH1 ITH2 VFB1 VFB2 CIN 10μF 50V CINT 4.7μF 10μF 16V 10μF 50V VIN 19V TO 28V D2 MTOP2 CB2 0.