FAC2786FE

2015.01.26

12/13

Revision No : 0

Output Capacitor Selection

The load transient requirements are the function of the slew

rate (di/dt) and the magnitude of the transient load current.

These requirements are generally met with a mix of

capacitors and careful layout. High frequency capacitors

initially supply the transient and slow the current load rate

seen by the bulk capacitors. The bulk filter capacitor

values are generally determined by the ESR and

voltage rating requirements rather than actual capacitance

requirements.

High frequency decoupling capacitors should be placed as

close to the power pins of the load as physically possible. Be

careful not to add inductance in the circuit board wiring that

could cancel the usefulness of these low inductance

components. An aluminum electrolytic capacitor’s ESR value

is related to the case size with lower ESR available in larger

case sizes. However, the Equivalent Series Inductance (ESL)

of these capacitors increases with case size and can reduce

the usefulness of the ca- pacitor to high slew-rate transient

loading.

Inductor Value Calculation

The operating frequency and inductor selection are

interrelated in that higher operating frequencies permit the

use of a smaller inductor for the same amount of inductor

ripple current. However, this is at the expense of efficiency

due to an increase in MOSFET gate charge losses. The

equation shows that the inductance value has a direct effect

on ripple current.

Accepting larger values of ripple current allows the use of

low inductances, but results in higher output voltage ripple

and greater core losses. A reasonable starting point for

noticed that the maximum ripple current occurs at the

maximum input voltage. The minimum inductance of the

inuctor is calculated by using the following equation:

Where