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LTC7803 Current Sense calculation

Hi,

I am currently using LTC7803EUD to convert 24V to 12V/25A. I use a 1.5mOhm current shunt to set the 25A trip threshold but the current limit was actually 17A. I see that in the datasheet (page 27) the design example which sets the current limit at 20A calculated the Rsense to be around 2mOhm, and looking at the EVB, which is an identical circuit using 1mOhm instead. I tested the EVB and indeed the current limit is 20A. So now I am a little confused, where did I get wrong?

My MOSFETs settings are identical to the EVB, the inductor I use is Wurth 744364047B and I set the switching frequency to 120Khz.

I have tried tinkering with the SENSE RC filter but still don't get much out of it. Am I missing out on anything?

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  • Hi FENCO,

    Can you provide us with some waveforms such as inductor current waveform and Rsense voltage in one scope shot?

    Using the inductor current ripple formula found on page 16 of the datasheet, your current ripple is about 10A. With 25A load, the peak current would be 30A. A 1.5mOhm Rsense would very little margin. That being said, 17A should work properly since the peak current would be 22A.

  • Hi Ron,

    Sorry about the late reply, totally forgot about it.

    The problem was actually an layout issue, so our new revision did fix the problem and 1.5mOhm did actually clamp the output current at around 25A which is quite interesting, I didn't expect the to have this much accuracy. 

    Still, thank you for the reply. Much appreciated

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  • Hi Ron,

    Sorry about the late reply, totally forgot about it.

    The problem was actually an layout issue, so our new revision did fix the problem and 1.5mOhm did actually clamp the output current at around 25A which is quite interesting, I didn't expect the to have this much accuracy. 

    Still, thank you for the reply. Much appreciated

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