LTM4608 LINER [Linear Technology], LTM4608 Datasheet - Page 14

• Manufacturer Part Number: LTM4608
• Description: Low VIN, 8A DC/DC ?ModuleTM with Tracking, Margining, and Frequency Synchronization
• Manufacturer: LINER [Linear Technology]

LTM4608
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 fi xed based on the output load. This method
of conversion creates large components of noise at the
14
APPLICATIONS INFORMATION
Figure 4. Normalized Input RMS Ripple Current vs Duty Factor for One to Six Modules (Phases)
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
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
1-PHASE
2-PHASE
3-PHASE
4-PHASE
6-PHASE
DUTY FACTOR (V
frequency of operation (fundamental) and multiples of the
operating frequency (harmonics).
To reduce this noise, the LTM4608 can run in spread
spectrum operation by tying the CLKIN pin to SV
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 fi xed
between 70% and 130% of the nominal frequency. This
has the benefi t of spreading the switching noise over a
range of frequencies, thus signifi cantly 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
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4608 F04
4608f
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