LTM4623 datasheet(13/28 Pages) LINER

13 / 28 page

LTM4623

4623fc

For more information www.linear.com/LTM4623

applicaTions inForMaTion

The LTM4623 device is an inherently current mode con-

trolled device, so parallel modules will have very good

current sharing. This will balance the thermals on the

design. Please tie the RUN, TRACK/SS, FB and COMP pins

of each paralleling module together. Figure 26 shows an

example of parallel operation and pin connection.

Input RMS Ripple Current Cancellation

Application Note 77 provides a detailed explanation of

multiphase operation. The input RMS ripple current can-

cellation mathematical derivations are presented, and a

graph is displayed representing the RMS ripple current

reductionasafunctionofthenumberofinterleavedphases.

Figure 3 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 2µA current source

will charge up the external soft-start capacitor towards

INTVCC voltage. When the TRACK/SS voltage is below

0.6V, it will take over the internal 0.6V reference voltage

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

be calculated as:

tSS = 0.6 •

CSS

2µA

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

rent foldback and forced continuous mode are disabled

during the soft-start process.

Outputvoltagetrackingcanalsobeprogrammedexternally

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

and down with another regulator. Figure 4 and Figure 5

show an example waveform and schematic of ratiometric

tracking where the slave regulator’s output slew rate is

proportional to the master’s.

Figure 4. Output Ratiometric Tracking Waveform

Figure 5. Example Schematic of Ratiometric Output Voltage Tracking

TIME

SLAVE OUTPUT

MASTER OUTPUT

4623 F04

FREQ

CLKIN

CLKOUT

VIN

SVIN

RUN

INTVCC

MODE

TRACK/SS

PGOOD

VOUT

COMP

GND

SGND

RFB(MA)

40.2k

LTM4623

10µF

16V

VIN

4V TO 15V

VOUT(MA)

1.5V

47µF

6.3V

CSS

RTR(BOT)

40.2k

RFB(SL)

60.4k

RTR(TOP)

60.4k

FREQ

VIN

SVIN

RUN

INTVCC

MODE

TRACK/SS

PGOOD

VOUT

COMP

GND

SGND

4623 F05

LTM4623

10µF

16V

VOUT(SL)

1.2V

47µF

6.3V

CLKIN

CLKOUT

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