LTM4608
14
4608fd
applicaTions inForMaTion
The LTM4608 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 LTM4608 together to share the current
evenly. To reduce ground potential noise, tie the I
THM
pins
of all LTM4608s 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 output 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
spectrum operation by tying the CLKIN pin to SV
IN
.
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
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
4608 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 Modules (Phases)
