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LM5145: Oscillation. Fault OCP trigger

David ojea
Prodigy 10 points
Part Number: LM5145
Other Parts Discussed in Thread: LM70860, LM70660

Tool/software:

Hello expert,

I designed a Buck by using LM5145. 48Vin 24Vout, 4.6Aout.

In general it is working well but at a certain loads it starts to oscillate. It looks like to me the OCP protection is triggering.

Here a scope capture at 1.9Aout. 

Blue and red are high and low side mosfets gates.

Yellow is Rs current. I use a 5mOhm resistance in series with mosfets plus Rlim

It is weird because at 2.5Aout it is stable again.

What do you think it is happening?

Regards,

David

  • +1
    TI__Guru 73402 points

    Hi David,

    I advise checking stability, as the resultant oscillation may be engaging current limit. Please complete the quickstart calculator https://www.ti.com/tool/LM5145DESIGN-CALC, with particular caution on entering the output cap value and its ESR. Note that the capacitance of a ceramic will derate with voltage, e.g. a 10uF/50V/X7R/1210 ceramic cap would likely be ~3uF at 24VDC. Take a look at the application example in the LM5145 data sheet as well.

     

    PS: we have converters (with integrated FETs) that can do 4.6A, e.g. LM70860 80V, 6A or LM70660 65V, 6A converters. These would likely give a smaller solution size with no need for external FETs.

    Regards,

    Tim