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LTM4644/LTM4644-1

4644fe

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APPLICATIONS INFORMATION

Multichannel Parallel Operation

For loads that demand more than 4A of output current,

the LTM4644 multiple regulator channels can be easily

paralleled to provide more output current without increas-

ing input and output voltage ripples. The LTM4644 has

preset built-in phase shift between each two of the four

regulator channels which is suitable to employ a 2+2, 3+1

or 4 channels parallel operation. Table 2 gives the phase

difference between regulator channels.

Table 2. Phase Difference Between Regulator Channels

CHANNEL

CH1

CH2

CH3

CH4

Phase Difference

180°

90°

180°

Figure 2 shows a 2+2 and a 4-channels parallel concept

schematic for clock phasing.

Figure 2. 2+2 and 4 Channels Parallel Concept Schematic

4644 F02

VOUT1

CH1

(0°)

VOUT2

LTM4644

CH2

(180°)

180°

VOUT3

CH3

(0°)

VOUT4

CH4

(180°)

180°

VOUT1

CH1

(0°)

VOUT2

LTM4644

16A

CH2

(180°)

180°

VOUT3

CH3

(270°)

VOUT4

CH4

(90°)

180°

90°

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 when all of the outputs are tied together

to achieve a single high output current design.

The LTM4644 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 channel together. Figure 35 and

Figure 36 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

of each regulator channel 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 2.5µA current

source will charge up the external soft-start capacitor

towards the 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

2.5µA

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

rent foldback and forced continuous mode are disabled

during the soft-start process.

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