LTM4647

13

4647fb

For more information www.linear.com/LTM4647

APPLICATIONS INFORMATION

The LTM4647 device is an inherently current mode con-

trolled device, so parallel modules will have good current

sharing.Thiswillbalancethethermalsinthedesign.Tiethe

together to share the current evenly. Figures 25 and 28

show a schematic of the parallel design.

Table 2. PHASMD and CLKOUT Signal Relationship

PHASMD

GND

FLOAT

CLKOUT

90°

90°

120°

60°

180°

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).

Soft-Start And Output Voltage Tracking

The TRACK/SS pin provides a means to either soft-start

the regulator or track it to a different power supply. A ca-

pacitor on the TRACK/SS pin will program the ramp rate

of the output voltage. An internal 1.25µA current source

will charge up the external soft-start capacitor towards

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:

1.25µA

0.75 0.8

4647 F04

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

0

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

Figure 4. Normalized Input RMS Ripple Current vs Duty Cycle for One to Six µModule Regulators (Phases)