LM25085: MOS turned off and back on unexpectedly

Kevin Lin

Part Number: LM25085

Hi there,

I am having trouble with LM28085; it turns off the MOS for 100ns and turns it back on. Is there any reason LM25085 behaves like that? Thank you

regards,

Kevin

over 8 years ago

0 Frank De Stasi over 8 years ago

TI__Guru 72625 points

We will need to see your schematic, input and output values and a snapshot of the pcb layout.

0 Kevin Lin over 8 years ago in reply to Frank De Stasi

Expert 1445 points

Hi Frank,

Here is the schematics and layout. Please let me know if you see inappropriate in the schematics and layout. Thank you for your help.

regards,

Kevin

0 Arief Hernadi over 8 years ago in reply to Kevin Lin

TI__Mastermind 23845 points

Hi,

What is your value for injection circuit (R489, C406)? Also what is your VIN and VOUT?
I think your circuit might need more ripple for the control loop. A couple things to try:
1) Reduce inductor value
2) Just use aluminum electrolytic capacitor at the output instead of ceramic and electrolytic capacitor
3) If you want to use only ceramic, you need to inject a ripple by populating R489,C406

Thanks
-Arief

0 Kevin Lin over 8 years ago in reply to Arief Hernadi

Expert 1445 points

Hi Arief,

Thank you for the tips.
My VIN is 24V and output is 5V/6A
I tried reducing the inductor value, but the results remain the same.
By the way, if I put a 220 ohms in series on the Pgate pin, LM25085 doesn't turn off the MOS unexpectedly anymore. What is the reason a 220 ohms in series would help?

Thanks

regards,
Kevin

0 Arief Hernadi over 8 years ago in reply to Kevin Lin

TI__Mastermind 23845 points

Hi Kevin,

It could be that it reduce the switching noise during switch transition. But normally we only put around 10 ohm or so on the gate of the FET.
Make sure the FET is not too hot because the slowing down of the switch node.

Thanks
-Arief

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LM25085: MOS turned off and back on unexpectedly