TPS54622: boot-SW ringing when Rboot is set to large value

Given Ding

Part Number: TPS54622

Hi Team,

During test, we set Rboot=20ohm, Cboot= 0.1uf, then we found there is large oscillation on Boot-SW ( up to ~9V) during the period when LS FET is closed. When decreasing the Rboot, the oscillation is small. So want to check with you why there is oscillation. Thanks.

BRs

Given

over 3 years ago

0 Gerold Dhanabalan - Online Design Tools over 3 years ago

TI__Genius 14455 points

Hi Given

20 ohm is definitely a large value to be using for Rboot. We would recommend between 1 ohm to 2 ohm.

Regards,

Gerold

0 Given Ding over 3 years ago in reply to Gerold Dhanabalan - Online Design Tools

TI__Expert 7405 points

Hi Gerold,

Thanks for your reply.

20ohm is large but since datasheet doesn't mention the Rboot value, customer hope to know the reason why there is such oscillation. Please kindly check. Thanks.

CH1：PVN-PH， CH2：PH-BOOT， CH3：PH-GND

0 Gerold Dhanabalan - Online Design Tools over 3 years ago in reply to Given Ding

TI__Genius 14455 points

Hi Given

Those spikes look like the boot charge circuit. With the 20 Ohm resistance, when the boot pin outputs current to charge the boot cap the voltage shoots up so the boot charge turns off right away. When the voltage drops enough the boot charge tries again to charge the boot cap Looks like there is about a 2 V jump which is 100 mA. I would recommend against using such a large boot resistance and use a 1 ohm to 2 ohm resistance.

As we are discussing this by email, I will go ahead and close it.

Regards,

Gerold

Power management

Power management forum

TPS54622: boot-SW ringing when Rboot is set to large value