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Problem with LM5085

Guenter Fritsch
Prodigy 90 points
Other Parts Discussed in Thread: LM5085

Dear all,

I need a Buck-Converter to get 5V with Load-current up to 250mA. The input voltage varies from 6V to 73V, which is a wide range.

I did the Design with Webench, and found the LM5085.

Now I placed all parts on my PCB, to do some tests and measurements. Unfortunately it doesn't work.

I tried several options. The first option, proposed by Webench, with external Sense-Resistor and reduced-Ripple Configuration, doesn't provide enough output power. Only 40mA! When my board needs 100mA, the output voltage of LM5085 breaks down to 0,1V.

I also tried to build the circuit without Rsense, just using the MOSFET's Rdson, and I tried different Ripple configurations. Nothing is working properly in the full range. Besides, the output is below 5V for low Input voltage (around 6V), altough it should be possible.

Am I stupid, or did I miss anything? Help is welcome!

Kind regards and thanks a lot,

Guenter