LTM4648

13

4648f

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applicaTions inForMaTion

A multiphase power supply could significantly reduce

the amount of ripple current in both the input and output

capacitors. The RMS input ripple current is reduced by,

and the effective ripple frequency is multiplied by, the

number of phases used (assuming that the input voltage

isgreaterthanthenumberofphasesusedtimestheoutput

voltage). The output ripple amplitude is also reduced by

the number of phases used.

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

(see Figure 4).

Minimum On-Time

the LTM4648 is capable of turning on the top MOSFET.

It is determined by internal timing delays, and the gate

charge required turning on the top MOSFET. Low duty

cycle applications may approach this minimum on-time

limit and care should be taken to ensure that:

If the duty cycle falls below what can be accommodated

by the minimum on-time, the controller will begin to skip

cycles.Theoutputvoltagewillcontinuetoberegulated,but

the output ripple and current will increase. The minimum

on-time can be increased by lowering the switching fre-

quency. A good rule of thumb is to use an 110ns on-time.

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

0

4648 F04

1-PHASE

2-PHASE

3-PHASE

4-PHASE

6-PHASE

Figure 4. Input RMS Current Ratios to DC Load Current as a Function of Duty Cycle