LTM4608

14

4608f

APPLICATIONS INFORMATION

the SGND at only one point. Figure 19 shows a schematic

of the parallel design. The FB pins of the parallel module

are tied together. With parallel operation, input and out-

put capacitors may be reduced in part according to the

operating duty cycle.

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

reduction as a function of the number of interleaved phases.

Figure 4 shows this graph.

Spread Spectrum Operation

Switching regulators can be particularly troublesome where

electromagnetic interference (EMI) is concerned.

Switching regulators operate on a cycle-by-cycle basis to

transfer power to an output. In most cases, the frequency

of operation is fixed based on the output load. This method

of conversion creates large components of noise at the

frequency of operation (fundamental) and multiples of the

operating frequency (harmonics).

To reduce this noise, the LTM4608 can run in spread

In spread spectrum operation, the LTM4608’s internal

oscillator is designed to produce a clock pulse whose

period is random on a cycle-by-cycle basis but fixed

between 70% and 130% of the nominal frequency. This

has the benefit of spreading the switching noise over a

range of frequencies, thus significantly reducing the peak

noise. Spread spectrum operation is disabled if CLKIN is

tied to ground or if it’s driven by an external frequency

synchronization signal. A capacitor value of 0.01μF must

be placed from the PLLLPF pin to ground to control the

slew rate of the spread spectrum frequency change.

Output Voltage Tracking

Output voltage tracking can be programmed externally

using the TRACK pin. The output can be tracked up and

down with another regulator. The master regulator’s output

is divided down with an external resistor divider that is the

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

4608 F04

6-PHASE

4-PHASE

3-PHASE

2-PHASE

1-PHASE

Figure 4. Normalized Input RMS Ripple Current vs Duty Factor for One to Six Modules (Phases)