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LTM4601/LTM4601-1

4601f

APPLICATIO S I FOR ATIO

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

by adding an external resistor from fSET to ground which

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:

ΔVOUT(P-P) ≈ (ΔIL/(8 • f • m • COUT) + ESR • ΔIL), where f

Figure 4. Normalized Output Ripple Current vs Duty Cycle, Dlr = VOT/LI, Dlr = Each Phase’s Inductor Current

DUTY CYCLE (VO/VIN)

0.1

0.15 0.2 0.25

0.35

0.3

0.4 0.45

0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9

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

4601 F04

6-PHASE

4-PHASE

3-PHASE

2-PHASE

1-PHASE

Figure 3. Inductor Ripple Current vs Duty Cycle

DUTY CYCLE (VOUT/VIN)

4601 F03

2.5V OUTPUT

5V OUTPUT

1.8V OUTPUT

1.5V OUTPUT

1.2V OUTPUT

3.3V OUTPUT WITH

130k ADDED FROM

VOUT TO fSET

5V OUTPUT WITH

100k ADDED FROM

fSET TO GND

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