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Rev.1.1 Nov.05, 2019

AX3984

the input capacitance. Due to large di/dt through the input capacitor, electrolytic or ceramics should

be used. If a tantalum must be used, it must be surge-protected. Otherwise, capacitor failure could

occur.

Output Inductor Selection

The output inductor selection is to meet the requirements of the output voltage ripple and affects

the load transient response. The higher inductance can reduce the inductor's ripple current and

induce the lower output ripple voltage. The ripple voltage and current can be approximately

calculated approximated by the following equations:

Vin

V

L

F

V

V

I

out

S

out

in











ESR

I

Vout









Although the increase of the inductance reduces the ripple current and voltage, it contributes to the

decrease of the response time for the regulator to load transient as well. Increasing the switching

frequency (Fs) for a given inductor also can reduce the ripple current and voltage but it will

increase the switching loss of the power MOS.

The way to set a proper inductor value is to have the ripple current (

△I) be approximately

10%~50% of the maximum output current. Once the value has been determined, select an

inductor capable of carrying the required peak current without going into saturation. It is also

important to have the inductance tolerance specified to keep the accuracy of the system controlled.

Using 20% for the inductance (at room temperature) is reasonable tolerance able to be met by

most manufacturers. For some types of inductors, especially those with core made of ferrite, the

ripple current will increase abruptly when it saturates, resulting in a larger output ripple voltage.

Output Capacitors Selection

An output capacitor is required to filter the output and supply the load transient current. The high

capacitor value and low ESR will reduce the output ripple and the load transient drop. These

requirements can be met by a mix of capacitors and careful layout.

In typical switching regulator design, the ESR of the output capacitor bank dominates the transient

response. The number of output capacitors can be determined by the following equations:

OUT

ESR

MAX

I

V

ESR





