LTM4613 datasheet(14/30 Pages) LINER

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LTM4649

4649fc

For more information www.linear.com/LTM4649

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

Minimum on-time tON is the smallest time duration that

the LTM4649 is capable of turning on the top MOSFET.

It is determined by internal timing delays and the gate

charge required to turn on the top MOSFET. Low duty

cycle applications may approach this minimum on-time

limit and care should be taken to ensure that:

VOUT

VIN •FREQ

> tON(MIN)

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 output ripple and current will increase. The mini-

mum on-time can be increased by lowering the switching

frequency. A good rule of thumb is to assume a 110ns

minimum on-time.

Soft-Start

The TRACK/SS pin of the master can be controlled by a

capacitorplacedfromthemasterregulatorTRACK/SSpinto

ground. A 1.2µA current source will charge the TRACK/SS

pin up to the reference voltage and then proceed up to

DUTY CYCLE (VO/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

4649 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

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