A second criteria before finalizing the C

the RMS current capability. The necessary RMS current rat-

ing of the input capacitor to a buck regulator can be estimated

by

With this high AC current present in the input capacitor, the

RMS current rating becomes an important parameter. The

maximum input capacitor ripple voltage and RMS current oc-

cur at 50% duty cycle. Select an input capacitor rated for at

least the maximum calculated I

Additional bulk capacitance with higher ESR may be required

to damp any resonance effects of the input capacitance and

parasitic inductance.

Output Capacitor Selection

In general, 22 µF to 100 µF high quality dielectric (X5R, X7R)

ceramic capacitor rated at twice the maximum output voltage

is sufficient given the optimal high frequency characteristics

and low ESR of ceramic dielectrics. Although, the output ca-

pacitor can also be of electrolytic chemistry for increased

capacitance density.

Two output capacitance equations are required to determine

the minimum output capacitance. One equation determines

the output capacitance (C

The second equation determines C

sient characteristics. Select the largest capacitance value of

the two.

The minimum capacitance, given the maximum output volt-

age ripple (

ΔV

ing equation:

Where the peak to peak inductor current ripple (

Δi

to:

R

value of the internal power inductor, where L = 2.2 µH, and

f

The minimum output capacitance requirement due to the

PWM ripple voltage is:

Three miliohms is a typical R

The following equation provides a good first pass capacitance

requirement for a load transient:

Where I

I

ΔV

±20 mV.

Therefore the capacitance requirement for the given design

parameters is:

In this particular design the output capacitance is determined

by the load transient requirements.

Table 1 lists some examples of commercially available ca-

pacitors that can be used with the LMZ10503.

www.national.com

10