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

Rev.1.4 Sep.27, 2019

AX2020

Capacitor selection

A low ESR capacitor should be used for input decoupling, as the ESR of this capacitor

appears in series with the supply source impedance and lowers overall efficiency. This

capacitor has to supply the relatively high peak current to the coil and smooth the current

ripple on the input supply. A minimum value of 4.7uF is acceptable if the input source is

close to the device, but higher values will improve performance at lower input voltages,

especially when the source impedance is high. The input capacitor should be placed as

close as possible to the IC.

For maximum stability over temperature and voltage, capacitors with X7R, X5R, or

better dielectric are recommended. Capacitors with Y5V dielectric are not suitable for

decoupling in this application and should not be used.

Inductor selection

Recommended inductor values for the AX2020 are in the range 47uH to 120uH.

Higher values of inductance are recommended at lower output current in order to minimize

errors due to switching delays, which result in increased ripple and lower efficiency. Higher

values of inductance also result in a smaller change in output current over the supply voltage

range. The inductor should be mounted as close to the device as possible with low

resistance connections to the SW and VIN pins.

Diode selection

For maximum efficiency and performance, the rectifier (D1) should be a fast low

capacitance Schottky diode with low reverse leakage at the maximum operating voltage and

temperature.

They also provide better efficiency than silicon diodes, due to a combination of lower

forward voltage and reduced recovery time.

It is important to select parts with a peak current rating above the peak coil current and

a continuous current rating higher than the maximum output load current. It is very important

to consider the reverse leakage of the diode when operating above 85°C. Excess leakage

will increase the power dissipation in the device and if close to the load may create a thermal

runaway condition.

The higher forward voltage and overshoot due to reverse recovery time in silicon

diodes will increase the peak voltage on the SW output. If a silicon diode is used, care

should be taken to ensure that the total voltage appearing on the SW pin including supply

ripple, does not exceed the specified maximum value.