SC2441AEVB(2006) View Datasheet(PDF) - Semtech Corporation

Part Name

Description

MFG CO.

SC2441AEVB
(Rev.:2006)

1.8V to 20V Input 2-Phase Synchronous Step-down Controllers with Step-up Converter

Semtech Corporation 

SC2441A

POWER MANAGEMENT

Applications Information

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23

Usually it is necessary to have several capacitors of the

same type in parallel to satisfy the ESR requirement.

The voltage ripple cause by the capacitor charge/

discharge should be an order of magnitude smaller than

the voltage ripple caused by the ESR. To guarantee this,

the capacitance should satisfy

Req (w)

:=

R1aR1b (R1a

(R1a

+ R1b )w 2C1a2C1b2 + (R1bC1b2 + R1aC1a2 )

+ R1b )2 w2C1a2C1b2 + (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 17).

Co

>

10

2pfsR esr

.

C1a

C1b

Ceq

In many application circuits, several low ESR ceramic

capacitors are added in parallel with the aluminum

capacitors to further reduce ESR and improve high

frequency decoupling. Since the capacitances and the

ESR’s of ceramic and aluminum capacitors are different,

the following remarks are made to clarify some practical

issues.

Remark 1: High frequency ceramic capacitors may not

carry most of the ripple current. It also depends on the

capacitor value. Only when the capacitor value is set

properly, the effect of ceramic capacitor low ESR starts

to be significant.

For example, if a 10µF, 4mΩ ceramic capacitor is

connected in parallel with 2x1500µF, 90mΩ electrolytic

capacitors, the ripple current in the ceramic capacitor is

only about 42% of the current in the electrolytic

capacitors at the ripple frequency. If a 100µF, 2mΩ

ceramic capacitor is used, the ripple current in the

ceramic capacitor will be about 4.2 times of that in the

electrolytic capacitors. When two 100µF, 2mΩ ceramic

capacitors are used, the current ratio increases to 8.3.

In this case most of the ripple current flows in the

ceramic decoupling capacitor. The ESR of the ceramic

capacitors will then determine the output ripple-voltage.

R1a

R1b

Req

Figure 17. Equivalent RC branch.

R and C are both functions of frequency. For rigorous

eq

eq

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) in Step-down Sections

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 18.

Remark 2: The total equivalent capacitance of the filter

bank is not simply the sum of all the paralleled capacitors.

The total equivalent ESR is not simply the parallel

combination of all the individual ESR’s either. Instead

they should be calculated using the following formulae.

Ceq(w) :=

(R1a + R1b )2 w 2C1a2C1b2 + (C1a + C1b )2

(R1a2C1a + R1b2C1b )w2C1aC1b + (C1a + C1b )

Figure 18. Buck converter input model

As shown in Fig. 18, the internal DC input voltage source

 2006 Semtech Corp.

28

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