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Rev1.5 Dec.11, 2019

AX3075/B

Inductor Selection

The inductor maintains a continuous current to the output load. This inductor current

has a ripple that is dependent on the inductance value:

Higher inductance reduces the peak-to-peak ripple current. The tradeoff for high

inductance value is the increase in inductor core size and series resistance, and the

reduction in current handling capability. In general, select an inductance value L based on

ripple current requirement:

RIPPLE

OUTMAX

LX

IN

OUT

IN

OUT

K

I

f

V

)

V

V

(

×

V

=

L

－

where VIN is the input voltage, VOUT is the output voltage, fLX is the switching frequency,

IOUTMAX is the maximum output current, and KRIPPLE is the ripple factor. Typically, choose

KRIPPLE = 30% to correspond to the peak-to-peak ripple current being 30% of the maximum

output current.

With this inductor value, the peak inductor current is IOUT × (1 + KRIPPLE/2). Make sure

that this peak inductor current is less than the controller’s current limit. Finally, select the

inductor core size so that it does not saturate at the peak inductor current.

Input Capacitor

The input capacitor needs to be carefully selected to maintain sufficiently low ripple at

the supply input of the converter. A low ESR capacitor is highly recommended. Since large

current flows in and out of this capacitor during switching, its ESR also affects efficiency.

The input capacitance needs to be higher than 100μF. The best choice is the ceramic

type, however, low ESR tantalum or electrolytic types may also be used provided that the

RMS ripple current rating is higher than 50% of the output current. The input capacitor should

be placed close to the VIN and GND pins of the IC, with the shortest traces possible. In the

case of tantalum or electrolytic types, they can be further away if a small parallel 1μF ceramic

capacitor is placed right next to the IC. Especially C8 capacitor should be placed as close as

possible to the VIN pin.

Output Capacitor

The output capacitor also needs to have low ESR to keep low output voltage ripple.

The output ripple voltage is:

OUT

2

LX

IN

ESR

RIPPLE

OUTMAX

RIPPLE

LC

f

×

28

V

+

R

K

I

=

V

where IOUTMAX is the maximum output current, KRIPPLE is the ripple factor, RESR is the

ESR of the output capacitor, fLX is the switching frequency, L is the inductor value, and COUT

is the output capacitance. In the case of ceramic output capacitors, RESR is very small and

does not contribute to the ripple. Therefore, a lower capacitance value can be used for

ceramic type. In the case of tantalum or electrolytic capacitors, the ripple is dominated by

RESR multiplied by the ripple current. In that case, the output capacitor is chosen to have

sufficiently low ESR.

For ceramic output capacitor, typically choose a capacitance of about 470μF. For

tantalum or electrolytic capacitors, choose a capacitor with less than 50mΩ ESR.