X

0

Q

© 2012 Exar Corporation

25/28

Rev. 1.2.2

I1 is the step load low current

Vos is output voltage including the overshoot

Vout is the steady state output voltage

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Here,

out

os

V

V

V

−

=

∆

is the overshoot voltage deviation.

Select ESR such that output voltage ripple (Vrip) specification is met. There are two components in

Vrip. First component arises from the charge transferred to and from Cout during each cycle. The

second component of Vrip is due to the inductor ripple current flowing through the output

capacitor’s ESR. It can be calculated for Vrip:

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Where:

Irip is the inductor ripple current

fs is the switching frequency

Cout is the output capacitance

Note that a smaller inductor results in a higher Irip, therefore requiring a larger Cout and/or lower

ESR in order to meet Vrip.

•

Input Capacitor Selection

Select the input capacitor for Voltage, Capacitance, ripple current, ESR and ESL. Voltage rating is

nominally selected to be at least twice the input voltage. The RMS value of input capacitor current,

assuming a low inductor ripple current, can be approximated as:

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Where:

Iin is the RMS input current

Iout is the DC output current

D is the duty cycle

In general, the total input voltage ripple should be kept below 1.5% of VIN. The input voltage

ripple also has two major components: the voltage drop on the main capacitor

Cin

V

∆

and the

voltage drop due to ESR -

ESR

V

∆

. The contribution to Input voltage ripple by each term can be

calculated from:

2

)

(

in

in

s

out

in

out

out

Cin

V

C

f

V

V

V

I

V

−

=

∆

)

5

.

0

(

rip

out

ESR

I

I

ESR

V

+

⋅

=

∆