LTM4636
13
4636f
Multiphase Operation
For outputs that demand more than 40A of load current,
multiple LTM4636 devices can be paralleled to provide more
output current without increasing input and output ripple
voltage. The MODE_PLLIN pin allows the LTM4636 to be
synchronized to an external clock and the internal phase-
locked loop allows the LTM4636 to lock onto input clock
phase as well. The FREQ resistor is selected for normal
frequency, then the incoming clock can synchronize the
device over the specified range.
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. See Application Note 77.
The LTM4636 device is an inherently current mode con-
trolled device, so parallel modules will have good current
sharing. This will balance the thermals in the design. Tie the
COMPA to COMPB and then tie the COMPA pins together,
tie V
FB
pins of each LTM4636 together to share the cur-
rent evenly. Figure 21 shows a schematic of the parallel
design. For external compensation and parallel operation
only tie COMP A pins together with external compensation.
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
(see Figure 2).
PLL, Frequency Adjustment and Synchronization
The LTM4636 switching frequency is set by a resistor (R
FREQ
)
from the FREQ pin to signal ground. A 20µA current
(I
FREQ
) flowing out of the FREQ pin through R
FREQ
develops
a voltage on the FREQ pin. R
FREQ
can be calculated as:
R
FREQ
=
FREQV
20µA
applicaTions inForMaTion
Figure 2. Normalized Input RMS Ripple Current vs Duty Cycle for One to Six µModule Regulators (Phases)
0.75 0.8
4636 F02
0.7
0.65
0.6
0.55
0.5
0.45
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.85 0.9
DUTY CYCLE (V
OUT
/V
IN
)
0
DC LOAD CURRENT
RMS INPUT RIPPLE CURRENT
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.55
0.60
1 PHASE
2 PHASE
3 PHASE
4 PHASE
6 PHASE
