LTM4612 Ver la hoja de datos (PDF) - Linear Technology

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Fabricante

LTM4612

Ultralow Noise 36VIN, 15VOUT, 5A, DC/DC µModule

Linear Technology 

LTM4612

APPLICATIONS INFORMATION

Output Capacitors

The LTM4612 is designed for low output voltage ripple.

The bulk output capacitors defined as COUT are chosen

with low enough effective series resistance (ESR) to meet

the output voltage ripple and transient requirements. COUT

can be low ESR tantalum capacitor, low ESR polymer

capacitor or ceramic capacitor. The typical capacitance is

150μF if all ceramic output capacitors are used. Additional

output filtering may be required by the system designer,

if further reduction of output ripple or dynamic transient

spike is required. Table 2 shows a matrix of different output

voltages and output capacitors to minimize the voltage

droop and overshoot during a 2A/μs transient. The table

optimizes total equivalent ESR and total bulk capacitance

to maximize transient performance.

Multiphase operation with multiple LTM4612 devices in

parallel will also lower the effective output ripple current

due to the phase interleaving operation. Refer to Figure 5

for the normalized output ripple current versus the duty

cycle. Figure 5 provides a ratio of peak-to-peak output

ripple current to the inductor ripple current as functions

of duty cycle and the number of paralleled phases. Pick

the corresponding duty cycle and the number of phases

to get the correct output ripple current value. For example,

each phase’s inductor ripple current DIr at zero duty cycle

is ~4.3A for a 36V to 12V design. The duty cycle is about

0.33. The 2-phase curve has a ratio of ~0.33 for a duty

cycle of 0.33. This 0.33 ratio of output ripple current to

the inductor ripple current DIr at 4.3A equals 1.4A of the

output ripple current (ΔIL).

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.

The equation is:

ΔVOUT(P−P)

≈

⎛

⎝⎜

8

•

f

ΔIL

• N • COUT

⎞

⎠⎟

+

ESR

•

ΔIL

Where f is the frequency and N is the number of paral-

leled phases.

1.00

0.95

1-PHASE

0.90

2-PHASE

3-PHASE

0.85

4-PHASE

6-PHASE

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

0

0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9

DUTY CYCLE (VO/VIN)

4612 F05

Figure 5. Normalized Output Ripple Current vs Duty Cycle, Dlr = VOT/LI

4612f

12

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