SC2542TETRT View Datasheet(PDF) - Semtech Corporation

Part Name

Description

MFG CO.

SC2542TETRT

High Performance Wide Input Range Dual Synchronous Buck Controller

Semtech Corporation 

SC2542

POWER MANAGEMENT

Applications Information (Cont.)

Aluminum capacitors (e.g. electrolytic) have high capaci-

tances and low ESL. The ESR has the dominant effect

on the output ripple voltage. It is therefore very impor-

tant to minimize the ESR. Other types to choose are

solid OS-CON, POSCAP, and tantalum.

When determining the ESR value, both the steady state

ripple-voltage and the dynamic load transient need to be

considered. To meet the steady state output ripple-volt-

age spec, the ESR should satisfy:

RESR1

<

∆VO

δIO

To limit the dynamic output voltage overshoot/under-

shoot within a (say 3%) of the steady state output volt-

age from no load to full load, the ESR value should sat-

isfy:

RESR2

<

3%VO

IO

Then, the required ESR value of the output capacitors

should be:

Resr = min{Resr1,Resr2 }.

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 10uF, 4mÙ ceramic

capacitor is connected in parallel with 2x1500uF, 90mÙ

electrolytic capacitors, the ripple current in the ceramic

capacitor is only about 42% of the current in the electro-

lytic capacitors at the ripple frequency. If a 100uF, 2mÙ

ceramic capacitor is used, the ripple current in the ce-

ramic capacitor will be about 4.2 times of that in the

electrolytic capacitors. When two 100uF, 2mÙ ceramic

capacitors are used, the current ratio increases to 8.3.

In this case most of the ripple current flows in the ce-

ramic decoupling capacitor. The ESR of the ceramic ca-

pacitors will then determine the output ripple-voltage.

Remark 2: The total equivalent capacitance of the filter

bank is not simply the sum of all the paralleled capaci-

tors. The total equivalent ESR is not simply the parallel

combination of all the individual ESR either. Instead they

should be calculated using the following formula.

CEQ(ω )

=

( R1A + R1B )2 ω 2C1A2C1B 2 + (C1A + C 1B )2

( R1A2C1A + R1B 2C1B )ω 2C1AC1B + (C1A + C1B )

The voltage rating of aluminum capacitors should be at

least 1.5Vo. The RMS current ripple rating should also

be greater than: δ IO

23

Usually it is necessary to have several capacitors of the

same type in parallel to satisfy the ESR requirement. The

voltage ripple caused by the capacitor charge/discharge

should be an order of magnitude smaller than the volt-

age ripple caused by the ESR. To guarantee this, the ca-

pacitance should satisfy:

CO

>

2π

10

fs RESR

In many applications, several low ESR ceramic capaci-

tors are added in parallel with the aluminum capacitors

in order to further reduce ESR and improve high fre-

quency decoupling. Because the values of capacitance

and ESR are usually different in ceramic and aluminum

capacitors, the following remarks are made to clarify

some practical issues.

CEQ (ω )

=

R1A R1B ( R1A

( R1 A

+ R1B )ω 2C1A2C1B 2 + ( R 1B C1B2 + R1AC1A2 )

+ R1B )2ω 2C1A2C1B 2 + (C1A + C1B )2

where R1a and C1a are the ESR and capacitance of elec-

trolytic capacitors, and R1b and C1b are the ESR and

capacitance of the ceramic capacitors, respectively (Fig-

ure 13)

C1a

C1b

Ceq

R1a

R1b

Req

Figure 14. Equivalent RC branch.

 2005 Semtech Corp.

16

www.semtech.com

Share Link: