LM25117: Fet turn on

Peter Almond

Part Number: LM25117

I am using the LM25117 to produce a 10A 12V converter and while most of this seems OK, I get some switching that is not 'regular'. If I set the converter to run at 200kHz, it runs in at 100kHz with times when it tries to turn on after 5us but it then immediately turns off. I have tried changing the Rt to run at a higher frequency but again it seems to run at half frequency with a turn on for a very short time at the correct point.

The picture attached shows this. I think the pulse by pulse current limit is turning the FET off but I am not certain why the frequency seems to be wrong

over 6 years ago

0 Katelyn Wiggenhorn over 6 years ago

TI__Genius 11040 points

Hello,

One possible cause could be an instability with the loop response. Would you be able to share the schematic?

Thank you,
Katelyn

0 Peter Almond over 6 years ago in reply to Katelyn Wiggenhorn

Prodigy 70 points

lm(2)5117_quick_start_hv_calculator V1.5 - design.xlsIt's not easy to share the schematic but I have used your design spreadsheet to track the design, that will use component names etc that you would work with.

0 Katelyn Wiggenhorn over 6 years ago in reply to Peter Almond

TI__Genius 11040 points

Hello,

Does the spreadsheet match your design component choices? I can also share my e-mail if you are able to share the schematic via e-mail.

Are you able to measure the loop response of your board design? This would show us if this problem is related to loop stability.

Best Regards,
Katelyn

0 Peter Almond over 6 years ago in reply to Katelyn Wiggenhorn

Prodigy 70 points

Katelyn,

If you could send me you email address I am not keen on supplying the schematic on a relatively open forum. I don't have the ability to measure the loop response but I am fairly sure that what I have put on the spreadsheet matches the components I am using. I don't see any signs of instability it just seems that the circuit is running at half frequency the bulk of the time but has these occasions when the chip try's to turn on the FET's and full frequency but the turn on is terminated almost immediately, I'm guessing by the pulse by pulse current limit.

Do you have my email address from my profile or do you need me to supply it here?

0 Katelyn Wiggenhorn over 6 years ago in reply to Peter Almond

TI__Genius 11040 points

Hello,

You can e-mail me at k-wiggenhorn@ti.com.

Also, what is the load range at which it operates at half frequency?

Best Regards,
Katelyn

0 Katelyn Wiggenhorn over 6 years ago in reply to Katelyn Wiggenhorn

TI__Genius 11040 points

Hello,

Since we are communicating via e-mail now, I am going to close this thread.

Best Regards,
Katelyn

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LM25117: Fet turn on