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LM5069: -2 variant of part trips at much lower current than -1 variant

Brad Luyster
Prodigy 20 points
Part Number: LM5069

Hello, we're using an LM5069 in a design as an electronic fuse. We've validated our design with the -1 variant of the part, using the design spreadsheet supplied by TI. After validating the design, we realized we wanted the automatic retry behavior of the -2 variant. We swapped this part in, without changing any of the related passive components. This single change has dramatically lowered the trip point of the circuit.

The circuit before was validated to a trip minimum of 6.1 amps (with the -1 variant on this board tripping at 6.3 amps). Swapping in the -2 variant, the trip point is 1.87 amps.

Find attached our circuit design and spreadsheet values.

Why is this circuit tripping at a much lower current when using the -2 variant?

  • 0
    TI__Genius 10370 points

    Hi Brad,

    Welcome to E2E! Let's see if we can figure this out. I didn't see any output cap on the schematic, but 220uF was listed on excel sheet. Is the output cap on another part of the schematic?

    Also, can you please provide waveforms, probing just five signals:  VGATE, VTIMER, VOUT, VIN, and IIN (all referenced to GND).  If you only have 4  probes, then could they take TWO separate waveforms showing the same event replacing VTIMER with IIN. It's probably best to trigger the event that's causing the 1.8A current limit issue. If it's during startup, then trigger falling edge of VGATE. I hope this helps :-)

    Best Regards,

    Aramis P. Alvarez 

  • 0 in reply to Aramis P. Alvarez
    Prodigy 20 points

    I solved my own problem! Or rather, a colleague solved the problem. Gathering the data you asked for, I noticed some odd behavior on the retry cycle. Upon calling in my colleague, he noticed that the bench supply we were using to drive this contraption was, for a tiny moment, entering current limit mode, dropping the voltage, and interacting oddly with the timeout of the LM5069 by happenstance of the two protection mechanisms working at the same time.

    Some had mucked with my lab supply without my knowing! Problem solved by increasing the current limit on the bench supply.

    Some days you eat the bear, some days the bear eats you.