NCV8842

http://onsemi.com

11

Input capacitor type selection is determined by design

constraints and emphasis (such as cost, performance, or

size). Aluminum electrolytic capacitors are widely available

in a broad selection of values, and are generally economical.

Their ESR and ESL (effective series inductance), however,

are relatively high. Multiple capacitors are often connected

in parallel to reduce ESR, and ceramic capacitors added in

parallel to reduce high frequency noise. OS−CON types

exhibit lower ESR. Solid tantalum capacitors combine low

ESR with small physical size.

Output Capacitor

In a buck converter, the requirements on the output

capacitor are not as critical as those on the input capacitor.

The current to the output capacitor comes from the inductor

and thus is triangular. In most applications, this makes the

RMS ripple current not an issue in selecting output

capacitors.

The output ripple voltage is the sum of a triangular wave

caused by ripple current flowing through ESR, and a square

wave due to ESL. Capacitive reactance is assumed to be

small compared to ESR and ESL. The peak to peak ripple

current of the inductor is:

IP * P +

VO(VIN * VO)

(VIN)(L)(fS)

across the ESL is proportional to the di/dt of the output

capacitor. It is realized that the di/dt of the output capacitor

is the same as the di/dt of the inductor current. Therefore,

ripple voltage induced by ESL can then be derived from

VRIPPLE(ESL) + ESL(

VIN

L

) ) ESL(

VIN * VO

L

) + ESL(

VIN

L

)

Figure 15. Output Voltage Ripple Using Two 10 mF

Ceramic Capacitors in Parallel

Figure 16. Output Voltage Ripple Using One 100 mF

POSCAP Capacitor

Figure 17. Output Voltage Ripple Using One 100

mF OS−CON

Figure 18. Output Voltage Ripple Using One 100 mF

Tantalum Capacitor