LTM4601 LINER [Linear Technology], LTM4601 Datasheet - Page 12

• Manufacturer Part Number: LTM4601
• Description: 12A DC/DC ?Modules with PLL, Output Tracking and Margining
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTM4601/LTM4601-1
Multiphase operation with multiple LTM4601 devices in
parallel will lower the effective output ripple current due to
the interleaving operation of the regulators. For example,
each LTM4601’s inductor current of a 12V to 2.5V multi-
phase design can be read from the Inductor Ripple Current
verses Duty Cycle graph (Figure 3). The large ripple current
at low duty cycle and high output voltage can be reduced
12
12
10
8
6
4
2
0
0
Figure 3. Inductor Ripple Current vs Duty Cycle
Figure 4. Normalized Output Ripple Current vs Duty Cycle, Dlr = V
20
DUTY CYCLE (V
U
40
OUT
1.00
0.95
0.90
0.85
0.80
0.75
0.70
0.65
0.60
0.55
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
U
/V
0
IN
0.1 0.15 0.2 0.25
60
)
4601 F03
W
80
2.5V OUTPUT
5V OUTPUT
1.8V OUTPUT
1.5V OUTPUT
1.2V OUTPUT
3.3V OUTPUT WITH
130k ADDED FROM
V
5V OUTPUT WITH
100k ADDED FROM
f
0.3
SET
OUT
TO GND
0.35
U
TO f
SET
0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9
DUTY CYCLE (V
by adding an external resistor from f
increases the frequency. If the duty cycle is DC = 2.5V/12V
= 0.21, the inductor ripple current for 2.5V output at 21%
duty cycle is ~6A in Figure 3.
Figure 4 provides a ratio of peak-to-peak output ripple cur-
rent to the inductor current as a function of duty cycle and
the number of paralleled phases. Pick the corresponding
duty cycle and the number of phases to arrive at the correct
output ripple current ratio value. If a 2-phase operation is
chosen at a duty cycle of 21%, then 0.6 is the ratio. This
0.6 ratio of output ripple current to inductor ripple of 6A
equals 3.6A of effective output ripple current. Refer to Ap-
plication Note 77 for a detailed explanation of output ripple
current reduction as a function of paralleled phases.
The output voltage ripple has two components that are
related to the amount of bulk capacitance and effective
series resistance (ESR) of the output bulk capacitance.
Therefore, the output voltage ripple can be calculated with
the known effective output ripple current. The equation:
ΔV
O
/V
OUT(P-P)
IN
)
O
T/L
≈ (ΔI
I
, Dlr = Each Phase’s Inductor Current
L
/(8 • f • m • C
1-PHASE
2-PHASE
3-PHASE
4-PHASE
6-PHASE
4601 F04
OUT
) + ESR • ΔI
SET
to ground which
L
), where f
4601f