LM27402: LM27402 negative voltage on LG

Yang Wu

Part Number: LM27402

Hi Team,

I have 2 questions when using LM27402.

Q1: Datasheet says the minimum voltage on LG can't exceed -0.3V, while both customer board and EVM show around -1V (period 4ns) at the beginning of turning on Low Side FET as below.

Obviously, it exceeds the datasheet spec. Could you help explain it why it has undershoot to -1V at the beginning of turning on time? Since EVM also has this phenomenon, is it normal? if it can have negative overshoot, what the maximum allowable time?

Q2: What's the Absolute Maximum Ratings for HG? it doesn't shown in datasheet.

Thanks a lot.

Best regards,

Yang

over 4 years ago

0 David_Daniels over 4 years ago

TI__Genius 16660 points

I reviewing your post. I will respond shortly.

0 David_Daniels over 4 years ago in reply to David_Daniels

TI__Genius 16660 points

Where are you measuring the voltages? Below are two plots, left one is on the LG and GND pins of the ICs. The right plot is one the gate of the low fet and ground.

I believe the undershoot is caused by the sw node falling slew rate and the gate to drain fet capacitance.

Slowing the fall time will minimize the undershoot.

I have not found any information on the HG or of the duration of an acceptable undershoot.