LTM4644MPY datasheet(13/36 Pages) LINER

13 / 36 page

LTM4644/LTM4644-1

4644fe

For more information www.linear.com/LTM4644

APPLICATIONS INFORMATION

Figure 3. Normalized RMS Ripple Current for Single Phase or Polyphase Applications

DUTY CYCLE (VOUT/VIN)

0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9

0.60

0.55

0.50

0.45

0.40

0.35

0.30

0.25

0.20

0.15

0.10

0.05

4644 F03

1-PHASE

2-PHASE

4-PHASE

Outputvoltagetrackingcanalsobeprogrammedexternally

using the TRACK/SS pin of each regulator channel. The

output can be tracked up and down with another regula-

tor. Figure 4 and Figure 5 show an example waveform

and schematic of a ratiometric tracking where the slave

regulator’s (VOUT2, VOUT3 and VOUT4) output slew rate is

proportional to the master’s (VOUT1).

Since the slave regulator’s TRACK/SS is connected to

the master’s output through a RTR(TOP)/RTR(BOT) resistor

divider and its voltage used to regulate the slave output

voltage when TRACK/SS voltage is below 0.6V, the slave

outputvoltageandthemasteroutputvoltageshouldsatisfy

the following equation during the start-up.

VOUT(SL) •

RFB(SL)

RFB(SL) +60.4k

= VOUT(MA) •

RTR(BOT)

RTR(TOP) +RTR(BOT)

Where the 60.4k is the integrated top feedback resistor

and the RFB(SL) is the external bottom feedback resistor

of the LTM4644. The RTR(TOP)/RTR(BOT) is the resistor

divider on the TRACK/SS pin of the slave regulator, as

shown in Figure 5.

Following the upper equation, the master’s output slew

rate (MR) and the slave’s output slew rate (SR) in volts/

time is determined by:

RFB(SL)

RFB(SL) +60.4k

RTR(BOT)

RTR(TOP) +RTR(BOT)

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