Datasheet

LTM4644/LTM4644-1
12
Rev. F
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APPLICATIONS INFORMATION
Multichannel Parallel Operation
For loads that demand more than 4A of output current,
the LTM4644 multiple regulator channels can be easily
paralleled to provide more output current without increas
-
ing input
and output voltage ripples. The LTM4644 has
preset built-in phase shift between each two of the four
regulator channels which is suitable to employ a 2+2, 3+1
or 4 channels parallel operation. Table 2 gives the phase
difference between regulator channels.
Table 2. Phase Difference Between Regulator Channels
CHANNEL CH1 CH2 CH3 CH4
Phase Difference 180° 90° 180°
Figure 2 shows a 2+2 and a 4-channels parallel concept
schematic for clock phasing.
Figure 2. 2+2 and 4 Channels Parallel Concept Schematic
4644 F02
FB1
TRACK/SS1
COMP1
RUN1
V
OUT1
CH1
(0°)
FB2
TRACK/SS2
COMP2
RUN2
V
OUT2
LTM4644
8A 8A
CH2
(180°)
180°
FB3
TRACK/SS3
COMP3
RUN3
V
OUT3
CH3
(0°)
FB4
TRACK/SS4
COMP4
RUN4
V
OUT4
CH4
(180°)
180°
FB1
TRACK/SS1
COMP1
RUN1
V
OUT1
CH1
(0°)
FB2
TRACK/SS2
COMP2
RUN2
V
OUT2
LTM4644
16A
CH2
(180°)
180°
FB3
TRACK/SS3
COMP3
RUN3
V
OUT3
CH3
(270°)
FB4
TRACK/SS4
COMP4
RUN4
V
OUT4
CH4
(90°)
180°90°
A multiphase power supply significantly reduces the
amount of ripple current in both the input and output ca-
pacitors. 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 is greater
than the number of phases used times the output voltage).
The output ripple amplitude is also reduced by the number
of phases used when all of the outputs are tied together
to achieve a single high output current design.
The LTM4644 device is an inherently current mode con-
trolled device, so parallel modules will have very good
current sharing. This will balance the thermals on the
design. Please tie the RUN, TRACK/SS, FB and COMP
pins of each paralleling channel together. Figure 35 and
Figure 36 shows an example of parallel operation and pin
connection.
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 3 shows this graph.
Soft-Start and Output Voltage Tracking
The TRACK/SS pin
provides a
means to either soft-start
of each regulator channel or track it to a different power
supply. A capacitor on the TRACK/SS pin will program the
ramp rate of the output voltage. An internal 2.5µA current
source will charge up the external soft-start capacitor
towards the INTV
CC
voltage. When the TRACK/SS voltage
is below 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:
t
SS
= 0.6 •
C
SS
2.5µA
where C
SS
is the capacitance on the TRACK/SS pin. Cur-
rent foldback and forced continuous mode are disabled
during the soft-start process.
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