Datasheet

LTM4637
12
4637fc
For more information www.linear.com/LTM4637
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 LTM4637 switching frequency is set by a resistor (R
fSET
)
from the f
SET
pin to signal ground. A 10µA current (I
FREQ
)
flowing out of the f
SET
pin through R
fSET
develops a volt-
age on f
SET
. R
fSET
can be calculated as:
R
fSET
=
FREQ
500kHz / V
+ 0.2V






1
10µA
The relationship of f
SET
voltage to switching frequency is
shown in Figure 3. For low output voltages from 0.6V to
1.2V, 250kHz operation is an optimal frequency for the
best power conversion efficiency while maintaining the
applicaTions inForMaTion
Figure 3. Relationship Between Switching
Frequency and Voltage at the f
SET
Pin
Figure 2. Normalized Input RMS Ripple Current vs Duty Cycle for One to Six µModule Regulators (Phases)
0.75
0.8
4637 F02
0.70.650.60.550.50.450.40.350.30.250.20.150.1
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
f
SET
PIN VOLTAGE (V)
0
SWITCHING FREQUENCY (kHz)
0.5 1 1.5 2
4637 F03
2.5
0
100
300
400
500
900
800
700
200
600
inductor current to about 30% to 40% of maximum load
current. For output voltages from 1.5V to 1.8V, 350kHz is
optimal. For output voltages from 2.5V to 5V, 500kHz is
optimal. See efficiency graphs for optimal frequency set
point. Limit 5V output to 15A.
The LTM4637 can be synchronized from 250kHz to
800kHz with an input clock that has a high level above