SC2441ITSTRT datasheet(16/35 Pages) SEMTECH

16 / 35 page

 2005 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC2441

)

(

)

(

)

(

)

(

)

(

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

7).

C1a

R1a

C1b

R1b

Ceq

Req

Figure 7. Equivalent RC branch.

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 =

1 and C1a = C1b = C1, then Req and Ceq will be frequency-

independent and

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

Input Capacitor (C

in)

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 8.

Figure 8. Buck converter input model

In Figure 8 the DC input voltage source has an internal

impedance R

in and the input capacitor Cin has an ESR

denoted as R

esr. MOSFET and input capacitor current

waveforms, ESR voltage ripple and input voltage ripple

are shown in Figure 9.

Figure 9. Typical waveforms at the input of a buck

converter.

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

MOSFET’s on the PC board to reduce trace inductances

around the pulse current loop.

The RMS value of the capacitor current is approximately

)

(

)

)(

[(

Cin

−

The power losses at 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. For higher power applications,

multiple capacitors are placed in parallel to increase the

ripple current handling capability.

Sometimes meeting tight input voltage ripple specifications

may require the use of larger input capacitance. At full

load, the peak-to-peak input voltage ripple due to the ESR

Applications Information (Cont.)

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