LM25085 not bucking

steve parker

Other Parts Discussed in Thread: LM25085A

We are testing a circuit designed with webench using the LM25085A. Vin 13.5-16V, Vout 12V, 5A. The circuit will pass through V when it is turned on but does not appear to be regulating. For example if we supply 14, 15 , or 16 V the output is the same as the input . Appreciate any suggestions. Thanks

over 11 years ago

0 Michael Mellitt over 11 years ago

TI__Intellectual 2060 points

Hi Steve,

Are you willing to post your schematic? Do you have a scopeshot at the SW node of the LM25085A?

Make sure that you are using a PFET and that it is oriented correctly.

Regards,

Mike

0 Vijay86929 over 11 years ago in reply to Michael Mellitt

TI__Genius 11490 points

Good points by Mike. Please also check FB pin connection is correct and the feedback resistor dividers are correct.

0 steve parker over 11 years ago in reply to Michael Mellitt

Prodigy 70 points

Hi Michael. Yes, I can send you the schematic. Where should I email? PFET is oriented correctly. Eval doc suggest that special probes are needed for scope. I do not have them. Are they required?

Thanks for your help.

0 Vijay86929 over 11 years ago in reply to steve parker

TI__Genius 11490 points

You can post it on the forum using the paper clip icon.

0 steve parker over 11 years ago in reply to Vijay86929

Prodigy 70 points

Fixed. IC is very output capacitor type and brand specific. Changed from a low esr smt electrolytic to a Sprague tantalum and works great now. esr on both the tantalum amd electro were the same but would not output w/o the tanty. Have used with several designs with different in and output. Great product once working.

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LM25085 not bucking