SC2441ITSTRT View Datasheet(PDF) - Semtech Corporation

Part Name

Description

MFG CO.

SC2441ITSTRT

Very Low Input Voltage 2-Phase Synchronous Step-down Controllers with Step-up Converter

Semtech Corporation 

SC2441

POWER MANAGEMENT

Applications Information (Cont.)

Req (ω)

:=

R1aR1b (R1a

(R1a

+ R1b )ω2C1a2C1b2 + (R1bC1b2 + R1aC1a2 )

+ R1b )2 ω2C1a2C1b2 + (C1a + C1b )2

where R1a and C1a are the ESR and capacitance of

electrolytic capacitors, and R1b and C1b are the ESR and

capacitance of the ceramic capacitors respectively (Figure

7).

In Figure 8 the DC input voltage source has an internal

impedance Rin and the input capacitor Cin has an ESR

denoted as Resr. MOSFET and input capacitor current

waveforms, ESR voltage ripple and input voltage ripple

are shown in Figure 9.

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 R1a = R1b =

R and C = C = C , then R and C will be frequency-

1

1a

1b

1

eq

eq

independent and

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.

Req = 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.

The RMS value of the capacitor current is approximately

ICin = Io

D[(1+

δ2

12

)(1

−

D )2

η

+

D

η2

(1−

D) ].

The power losses at the input capacitors is then

P = I 2R .

Cin Cin 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.

Figure 8. Buck converter input model

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

is

 2005 Semtech Corp.

16

www.semtech.com

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