SC2545MLTRT View Datasheet(PDF) - Semtech Corporation

Part Name

Description

MFG CO.

SC2545MLTRT

High Performance Wide Input Range Dual Synchronous Buck Controller

Semtech Corporation 

SC2545

POWER MANAGEMENT

Applications Information (Cont.)

ω CReqe(q (ω))=:=

R1aR1b (R1a

(R1a

+ R1b )ω2C1a2C1b2 + (R1bC1b2 + R1aC1a2 )

+ R1b )2 ω2C1a2C1b2 + (C1a + C1b )2

ewlhecetrreolRyt1ica

caanpdacCi1toa rasr,

e the E

and R1b

SR and

and C1b

capacitance of

are the ESR and

capacitance of the ceramic capacitors, respectively

(Figure 5).

In Figure 6 the DC input voltage source has an internal

impedance Rin and the input capacitor Cin has an ESR of

Resr. MOSFET and input capacitor current waveforms, ESR

voltage ripple and input voltage ripple are shown in Figure

7.

i Q1

C1a

C1b

R1a

R1b

Ceq

i Cin

Req

Vesr

Figure 5. 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

= R1 and C1a = C1b = C1, then Req and Ceq will be frequency-

independent and

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

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.

VCin

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

ICin = Io

D[(1+

δ2 )(1−

12

D )2

η

+

D

η2

(1−

D) ].

The power dissipated in the input capacitors is then

21

L1

Rin

Resr

Q1

D1

VDC

Cin

PCin = ICin2Resr.

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

Co Ro 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.

Figure 6. A simple model for the converter input.

© 2005 Semtech Corp.

12

www.semtech.com

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