LTM4601AV datasheet(12/28 Pages) LINER

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LTM4601A/LTM4601A-1

4601afb

Multiphase operation with multiple LTM4601A devices

in parallel will lower the effective output ripple current

due to the interleaving operation of the regulators. For

example, each LTM4601A’s inductor current of a 12V to

2.5V multiphase design can be read from the Inductor

Ripple Current vs 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

Figure 3. Inductor Ripple Current vs Duty Cycle

APPLICATIONS INFORMATION

DUTY CYCLE (VOUT/VIN)

4601A 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

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

4601A F04

6-PHASE

4-PHASE

3-PHASE

2-PHASE

1-PHASE

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