SC2447 View Datasheet(PDF) - Semtech Corporation

Part Name

Description

Manufacturer

SC2447

Dual-Phase, Single or Dual Output Synchronous Step-Down Controller

Semtech Corporation 

SC2447

POWER MANAGEMENT

Application Information (Cont.)

charge balance between the load and the converter

output. The third term is voltage ripple due to ESL and

the fourth term is the voltage ripple due to ESR. The

total output voltage ripple is then the vector sum of the

last three terms.

The voltage rating of aluminum capacitors should be at

least 1.5Vo. The RMS current ripple rating should also

be greater than

δIo .

23

Since the inductor current waveform is a triangular with

peak-to-peak value δ*I , the ripple-voltage caused by

o

inductor current ripple is

∆v C

≈

δIo

8Co fs

,

the ripple-voltage due to ESL is

∆v ESL

= Leslfs

δIo

D

,

and the ESR ripple-voltage is

∆vESR = Resr δIo.

Aluminum capacitors (e.g. electrolytic, solid OS-CON,

POSCAP, tantalum) have high capacitances and low ESLs.

The ESR has the dominant effect on the output ripple

voltage. It is therefore very important to minimize the

ESR.

When determining the ESR value, both the steady state

ripple-voltage and the dynamic load transient need to be

considered. To keep the steady state output ripple-

voltage < ∆V , the ESR should satisfy

o

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

Co

>

10

2πfsR esr

.

In many applications, several low ESR ceramic capacitors

are added in parallel with the aluminum capacitors in

order to further reduce ESR and improve high frequency

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.

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.

R esr1

<

∆Vo

δIo

.

To limit the dynamic output voltage overshoot/

undershoot to within α (say 3%) of the steady state

output voltage) from no load to full load, the ESR value

should satisfy

R esr 2

<

αVo

Io

.

Then, the required ESR value of the output capacitors

should be

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.

Resr = min{Resr1,Resr2 }.

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 ESRs either. Instead

© 2006 Semtech Corp.

17

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