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Axelite Confidential Materials, do not copy or distribute without written consent.

Rev.1.5 Oct.15, 2019

AX3117

Where is inductor Ripple Current. Large value inductors lower ripple current and

small value inductors result in high ripple currents. Choose inductor ripple current

approximately 20% of the maximum load current 2A, ΔIL=0.6A. The DC current rating of

the inductor should be at least equal to the maximum load current plus half the ripple

current to prevent core saturation (2A+0.3A).

Input Capacitor Selection

This capacitor should be located close to the IC using short leads and the voltage

rating should be approximately 1.5 times of the maximum input voltage. The RMS current

rating requirement for the input capacitor of a buck regulator is approximately 1/2 the DC

load current. A low ESR input capacitor sized for maximum RMS current must be used. A

220~470µF low ESR capacitor for most applications is sufficient.

Output Capacitor Selection

The output capacitor is required to filter the output and provide regulator loop stability.

The important capacitor parameters are; the 100 KHz Equivalent Series Resistance (ESR),

the RMS ripples current rating, voltage rating, and capacitance value. For the output

capacitor, the ESR value is the most important parameter. The ESR can be calculated

from the following formula.

ESR

I

V

L

RIPPLE







= 0.6A x 80mΩ= 48mV

An aluminum electrolytic capacitor’s ESR value is related to the capacitance and its

voltage rating. In most case, higher voltage electrolytic capacitors have lower ESR values.

Most of the time, capacitors with much higher voltage ratings may be needed to provide

the low ESR values required for low output ripple voltage. System stability is depending on

output capacitor’s ESR, Correct to choose output capacitor’s ESR is very important. It is

recommended to using a 220~470µF, the ESR values range is 40~130mΩ.

Thermal Considerations

The SOP-8L-EP package needs a heat sink under most conditions. The heat sink

connect exposed pad of AX3117/A/B to obtain best effect. The size of the heat sink

depends on the input voltage, the output voltage, the load current and the ambient

temperature.