LTM4636 datasheet(13/36 Pages) LINER

13 / 36 page

LTM4636

4636f

For more information www.linear.com/LTM4636

Multiphase Operation

For outputs that demand more than 40A of load current,

multipleLTM4636devicescanbeparalleledtoprovidemore

output current without increasing input and output ripple

voltage. The MODE_PLLIN pin allows the LTM4636 to be

synchronized to an external clock and the internal phase-

locked loop allows the LTM4636 to lock onto input clock

phase as well. The FREQ resistor is selected for normal

frequency, then the incoming clock can synchronize the

device over the specified range.

A multiphase power supply significantly reduces the

amount of ripple current in both the input and output ca-

pacitors. 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 is greater

than the number of phases used times the output voltage).

The output ripple amplitude is also reduced by the number

of phases used. See Application Note 77.

The LTM4636 device is an inherently current mode con-

trolled device, so parallel modules will have good current

sharing.Thiswillbalancethethermalsinthedesign.Tiethe

COMPA to COMPB and then tie the COMPA pins together,

tie VFB pins of each LTM4636 together to share the cur-

rent evenly. Figure 21 shows a schematic of the parallel

design. For external compensation and parallel operation

onlytieCOMPApinstogetherwithexternalcompensation.

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 2).

PLL, Frequency Adjustment and Synchronization

TheLTM4636switchingfrequencyissetbyaresistor(RFREQ)

from the FREQ pin to signal ground. A 20µA current

(IFREQ)flowingoutoftheFREQpinthroughRFREQdevelops

a voltage on the FREQ pin. RFREQ can be calculated as:

RFREQ =

FREQV

20µA

applicaTions inForMaTion

Figure 2. Normalized Input RMS Ripple Current vs Duty Cycle for One to Six µModule Regulators (Phases)

0.75 0.8

4636 F02

0.7

0.65

0.6

0.55

0.5

0.45

0.4

0.35

0.3

0.25

0.2

0.15

0.1

0.85 0.9

DUTY CYCLE (VOUT/VIN)

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

1 PHASE

2 PHASE

3 PHASE

4 PHASE

6 PHASE

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