LM25085A: LM25085A

John DeCesare

Part Number: LM25085A
Other Parts Discussed in Thread: LM25085, LM5085

Hi,

I designed in this chip to convert 18V to 12V at 8A. I used power designer which provided me with a feedback network of 10K and 806Ohms as shown.

If I used the equation in the datasheet:

The output voltage should be 16.8V not 12. If I solve for Rfb1, in order to get 12V, I should have a 1.162K resistor. That is what I designed in thinking power designer has a bug. But the voltage is about 10V.

Above is my schematic and I don't used the C1, C2, R3 network for improved ripple and my output caps are ceramics. I do have considerable ripple (620mV) and when I tacked down a 100uF tantalum, the ripple decreased to about 320Ohm so it seems you need a little ESR to reduce the ripple but can you explain why the output voltage is way off?

Thanks!

John

over 3 years ago

0 Timothy Hegarty over 3 years ago

TI__Guru 54612 points

Hi John,

It's probably the reference voltage in power stage designer - I will check. In the meantime, please use the LM25085 quickstart calculator (available by download from the product folder) for component selection. Recommendation when using ceramic output caps is to use the type-3 ripple injection circuit as the controller needs triangular ripple in phase with the SW node voltage. Ceramic caps give sinusoidal ripple phase shifted by 90 degrees.

Regards,

Tim

0 John DeCesare over 3 years ago in reply to Timothy Hegarty

Prodigy 250 points

Thanks Tim. I missed the ESR requirements in the spec sheet but I did tack on a tant with some ESR and my ripple cleaned up well enough for this application. When the refrerence voltage is sorted out, I can try that as well. Appreciate the fast response, especially on a nice Saturday afternoon!

John

0 Timothy Hegarty over 3 years ago in reply to John DeCesare

TI__Guru 54612 points

No problem, John. Just ensure the voltage ripple at FB is triangular.

Regards,

Tim

0 John DeCesare over 3 years ago in reply to Timothy Hegarty

Prodigy 250 points

Hi Tim,

Will you be checking the Vref and associated equation to set the output voltage as you mentioned. That is more of a concern that the ripple which with the cap is triangular by the way.

John

0 Orlando Murray over 3 years ago in reply to John DeCesare

TI__Mastermind 22299 points

John,

Note the LM5085 and the LM5085A use different reference voltages (1.25V vs 0.9V).

Be sure you are using the correct datasheet and equation.

-Orlando

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LM25085A: LM25085A