Datasheet

LTM4608A
14
4608afd
applicaTions inForMaTion
The LTM4608A device is an inherently current mode con-
trolled device. Parallel modules will have very good current
sharing. This will balance the thermals on the design.
Tie the I
TH
pins of each LTM4608A together to share the
current evenly. To reduce ground potential noise, tie the
I
THM
pins of all LTM4608As together and then connect to
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 LTM4608A can run in spread
spectrum operation by tying the CLKIN pin to SV
IN
. In
spread spectrum operation, the LTM4608A’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
DUTY FACTOR (V
O
/V
IN
)
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
4608A F04
RMS INPUT RIPPLE CURRENT
DC LOAD CURRENT
6-PHASE
4-PHASE
3-PHASE
2-PHASE
1-PHASE
Figure 4. Normalized Input RMS Ripple Current vs Duty Factor for One to Six Phases