SC2544 datasheet(11/24 Pages) SEMTECH

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ã 2005 Semtech Corp.

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SC2544

POWER MANAGEMENT

The first term is the DC voltage across C

o at time

t=0. The second term is the voltage variation caused

by the 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 a

vector sum of the last three terms.

Since the inductor current is a triangular waveform

with peak-to-peak value G *Io, the ripple-voltage

caused by inductor current ripples is

the ripple-voltage due to ESL is

The voltage rating of aluminum capacitors should be

at least 1.5V

o. The RMS current ripple rating should

also be greater than

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 voltage ripple caused by

the ESR. To guarantee this, the capacitance should

satisfy

HVU

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.

HVO

(6/

HVU

(65

and the ESR ripple-voltage is

Aluminum capacitors (e.g. electrolytic) 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. 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-voltage spec, the ESR should satisfy

To limit the dynamic output voltage overshoot/

undershoot within a (say 3%) of the steady state

output voltage from no load to full load, the ESR

value should satisfy

Then, the required ESR value of the output capacitors

should be

esr = 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 10 P F, 4m : ceramic capacitor is

connected in parallel with 2x1500 P F, 90m :

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 100 P F, 2m : 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 100 P F, 2m : 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.

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 they should be calculated using the following

formula.

Applications Information (Cont.)

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