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

Rev.1.1 Oct 02, 2019

AX3901

External Bootstrap Diode

When the condition Duty Cycle > 65% occurs, it is strongly recommended to add an external

bootstrap diode (such as IN4148 or BAT54) between an external 5V and BST pin for efficiency

improvement. The external 5V should be lower than 5.5V.

Layout Consideration

For proper operation of the converter, some layout rules should be followed. It is necessary to

understand which pin of AX3901 is sensitive and which is insensitive. Please refer the following

for the location where noise comes from on the circuit and where the clear ground is for the small

signal ground.

1. First, put the input capacitor (CIN) as close as possible to the VIN pin.

2. Secondly, place the Cs, Rs, Cp, Css and R2 as close as AX3901 and connect these analog

grounds (Clear AGND) to AX3901 GND pin. It is recommended to use a dot short for these

AGND pins or connect the GND pin via contact.

3. The large current loop shown in bold lines in the above figure circuit should be routed as short

and wide as possible and the switch node is a high dv/dt. It easily couples noise to other

traces by the capacitive path. Therefore the sensitive signals like FB, COMP and AGND

should be routed away with this noise source.

4. The feedback network resistors (R1 & R2) should be routed away from the inductor and

switch node to minimize noise and EMI issue. And the R1 resistor should be sensed the

output capacitor or device loading, not the inductor’s output node.