LTM4657 datasheet(13/28 Pages) AD

13 / 28 page

LTM4657

Rev. A

For more information www.analog.com

APPLICATIONS INFORMATION

Input RMS Ripple Current Cancellation

Application Note 77 provides a detailed explanation of mul-

tiphase operation. The input RMS ripple current cancella-

tion mathematical derivations are presented, and a graph

is displayed representing the RMS ripple current reduc-

tion as a function of the number of interleaved phases.

Figure 4 shows this graph.

Soft-Start And Output Voltage Tracking

The TRACK/SS pin provides a means to either soft start

the regulator or track it to a different power supply. A

capacitor on the TRACK/SS pin will program the ramp

rate of the output voltage. An internal 6µA current source

will charge up the external soft-start capacitor towards

INTVCC voltage. When the TRACK/SS voltage is below

0.5V, it will take over the internal 0.5V reference voltage

to control the output voltage. The total soft-start time can

be calculated as:

tSS =0.5•

CSS

6µA

where CSS is the capacitance on the TRACK/SS pin. Cur-

rent foldback and forced continuous mode are disabled

during the soft-start process.

Output voltage tracking can also be programmed exter-

nally using the TRACK/SS pin. The output can be tracked

up and down with another regulator. Figure 5 and Figure 6

show an example waveform and schematic of ratiometric

tracking where the slave regulator’s output slew rate is

proportional to the master’s.

Figure 5. Output Ratiometric Tracking Waveform

Figure 6. Example Schematic of Ratiometric Output Voltage Tracking

TIME

SLAVE OUTPUT

MASTER OUTPUT

4657 F05

FREQ

VIN

RUN

INTVCC

MODE

TRACK/SS

PGOOD

VOUT

VOSNS+

COMPa

COMPb

GND

VOSNS–

RFB(MA)

30.1k

LTM4657

22µF

25V

VIN

3.1V TO 20V

VOUT(MA)

1.5V

100µF

6.3V

×2

RTR(BOT)

30.1k

RFB(SL)

60.4k

RTR(TOP)

60.4k

FREQ

VIN

RUN

INTVCC

MODE

TRACK/SS

PGOOD

VOUT

COMPa

COMPb

GND

VOSNS–

4657 F06

LTM4657

22µF

25V

VOUT(SL)

1.0V

100µF

6.3V

×2

VOSNS+

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