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Rev.1.1 Nov.01, 2019

AX3703A

current and avoid output overshoot.

Power Good Indication

AX3703A has open drain Power GOOD indicator PGOOD pin. When PGOOD is connected with

external pull up resistor, it will be pulled up if output voltage is higher than 90% of regulation,

otherwise PGOOD is pulled down by the internal open drain NMOS.

Application Information

Setting the Output Voltage

External feedback resistors are used to set the output voltage. Refer to typical application circuit

on page1, the top feedback resistor R1 has some impact on the loop stability, so its recommended

range is between 100kΩ-300kΩ. For any chosen R1, the bottom feedback resistor R2 can be

calculated as:

Inductor Selection

The recommended inductor value for AX3703A is between 0.33uH to 1uH. Usually the inductor

value is chosen to satisfy a desired ripple current:

where

is the inductor ripple current.

With the chosen

, the peak inductor current will be:

Input Bypass Capacitor Selection

The input current to the step-down converter is discontinuous with very sharp edges, therefore an

input bypass capacitor is required. For best performance, it’s recommended to use low ESR

ceramic capacitors and place them as close to the input pin as possible. For lowest temperature

variations, use X5R or X7R dielectric ceramic capacitors.

The RMS current of the input capacitor is approximately:

From the equation, it can be seen that the highest RMS current occurs when D is 0.5:

Choose the capacitor with RMS current rating higher than 1/2 IOUT

The power dissipation on the input capacitor can be estimated with the RMS current and the ESR

resistor. Electrolytic or tantalum capacitors can also be used, but due to their significantly higher

ESR, a small size ceramic capacitor should be placed as close to the IC as possible.

The voltage ripple on the input capacitor, neglecting the ESR impact, can be calculated as: