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LM25085: Strangely high ADJ current sink

Ryan.Bishop
Expert 3900 points
Part Number: LM25085
Other Parts Discussed in Thread: LM5085

Hello,

We are seeing the switcher go into what appears to be current-limiting at a lower current than expected. The design current limit threshold is 8.2A. We are seeing current limiting start at 6A on the bench. That leaves me with a bit less margin at high temps than what I would like.

R-adj is 4.7K nominal (4.724K measured). I’m not sure why the part is current-limiting when the voltage drop across the FET is about 128mV and across R-adj is about 470mV. The MOSFET is SI4401BDY.

The first thing I noticed is the 470mV is way too high. The internal current sink on ADJ is supposed to be 48uA max, but we are seeing 100uA.

Also, if the R-adj voltage is too high, I would expect the current limit to be too high, but instead we are seeing the current limit trigger too low.

One known problem we have is ringing on the switch node / ISEN pin that lasts longer than the 100ns blanking time. This is perhaps why we still trigger the current limit even though R-adj voltage is abnormally high. We should fix the ringing obviously, but I'm still perplexed by the large voltage across R-adj. The regulator isn’t heavily loaded in this measurement; maybe 2A. Vin is a regulated 7V, and doesn’t droop or dropout. The R-adj voltage measurement was taken with a Tektronix P6247 differential probe that gets calibrated annually.

Thanks!

Regards,
Ryan B.

  • 0
    TI__Mastermind 39980 points

    Hello Ryan,

    The issue is likely noise on the CS input. can you please post schematic?

    Thanks.

  • 0 in reply to David Baba
    TI__Expert 3900 points

    Hi David,

    Thanks for the quick reply. I have shared the schematic with you privately.

    I agree the ringing on the switch node (which is connected to ISEN pin 5) is a huge factor here. 

    However, the large voltage across the R-adj resistor made me think other things might be wrong.

    Regards,
    Ryan B.

  • 0 in reply to Ryan.Bishop
    TI__Mastermind 39980 points

    Hello Ryan,

    Whether is on the CS or the ADJ pin which sets the reference, its noise causing the issue. 

    Layout is also going to play a big part here.  Make sure the customers layout is ideal, if not, may need to modify.  Ideal means small input loop and deferentially routed traces for the current sense input referenced to Vin. 

    Note: If I were doing an 8A design using the LM5085, I would have used an Rsense, this makes it much easier to routing CS deferentially to Vin and to insert an RC filter to the input to the CS pin much easier to implement. 

    Hope this helps?