AX3902

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Rev.1.2 Nov.04, 2019

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. 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:

△VESR = change in output voltage due to ESR

△IOUT = load transient.

ESRCAP = maximum ESR per capacitor (specified in manufacturer’s data sheet).

ESRMAX = maximum allowable ESR.

High frequency decoupling capacitors should be placed as close to the power pins of the

load as physically possible. For the decoupling requirements, please consult the capacitor

manufacturers for confirmation.

Output Voltage

The output voltage is set using the FB pin and a resistor divider connected to the output as

shown in AP Circuit below. The output voltage (VOUT) can be calculated according to the

voltage of the FB pin (VFB) and ratio of the feedback resistors by the following equation, where

(VFB) is 0.8V:

Thus the output voltage is:

Choose R1=40kΩ~200kΩ to ensure feedback loop noise immunity. It is optional to add a

feed-forward capacitor CFF=4.7~33pF in parallel with R1 to achieve better transient response

performance. The T-type network shown in below figure can also be used.