SC2544 datasheet(12/24 Pages) SEMTECH

12 / 24 page

ã 2005 Semtech Corp.

www.semtech.com

SC2544

POWER MANAGEMENT

Req and Ceq are both functions of frequency. For

rigorous design, the equivalent ESR should be

evaluated at the ripple frequency for voltage ripple

calculation when both ceramic and electrolytic

capacitors are used. If R

1a = R1b = R1 and C1a = C1b = C1,

then R

eq and Ceq will be frequency-independent and

eq = 1/2 R1 and Ceq = 2C1.

where R

1a and C1a are the ESR and capacitance of

electrolytic capacitors, and R

1b and C1b are the ESR

and capacitance of the ceramic capacitors, respectively

(Figure 5).

Figure 5. Equivalent RC branch.

In Figure 6 the DC input voltage source has an internal

impedance R

in and the input capacitor Cin has an ESR

of R

esr.

MOSFET and input capacitor current

waveforms, ESR voltage ripple and input voltage ripple

are shown in Figure 7.

Input Capacitor (Cin)

The input supply to the converter usually comes from

a pre-regulator. Since the input supply is not ideal,

input capacitors are needed to filter the current pulses

at the switching frequency. A simple buck converter

is shown in Figure 6.

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Figure 6. A simple model for the converter input.

Figure 7. Typical waveforms at converter input.

It can be seen that the current in the input capacitor

pulses with high di/dt. Capacitors with low ESL should

be used. It is also important to place the input capacitor

close to the MOSFETs on the PC board to reduce trace

inductances around the pulse current loop.

The RMS value of the capacitor current is

approximately

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The power dissipated in the input capacitors is then

Cin = ICin

esr.

For reliable operation, the maximum power dissipation

in the capacitors should not result in more than 10oC

of temperature rise. Many manufacturers specify the

maximum allowable ripple current (ARMS) rating of

the capacitor at a given ripple frequency and ambient

temperature. The input capacitance should be high

enough to handle the ripple current. It is common

pratice that multiple capacitors are placed in parallel

to increase the ripple current handling capability.

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Applications Information (Cont.)

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