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LM27761: Overheating IC and wrong output

Arturo Rios
Prodigy 20 points
Part Number: LM27761

Hello, I having troubles with LM27761 it's overheating and making a little noise. I use it for a -5V power supply (low current, less than 100mA) and I get -1,8 and the IC is overheating. Vin is 5V and here is the schematic:

Layout:

I measured a few points:

Vout:

Vout aprox -2V instead of -5V

Vin:

That little noise in Vin it's beacause of LM27761 overheating.

Cpout:

C1+ and C1-:

I don't know what's wrong here, my schematic design or my layout desing, I made 4 PCB prototype with this circuit and the same problem it's happening in all of them. Hope I get some help. Thanks.

  • 0
    TI__Guru*** 148870 points
    Hi Arturo,

    Those FB resistors are very large. Try reducing them by 10x each and see if you get better results. Noise may be coupling in.

    As well, your layout is rather poor. None of the cap's GNDs connect directly back to the IC.

    Finally, this IC is an inverting charge pump plus an LDO. You will not get -5.0Vout from +5.0Vin. Vout must be closer to GND than Vin due to the output impedance and LDO drop. This is explained at the top of page 13 of the D/S.
  • 0 in reply to Chris Glaser
    TI__Guru*** 148870 points
    Hi Arturo,

    Were you able to resolve this?
  • 0 in reply to Chris Glaser
    Prodigy 20 points
    Hi, unfortunately the LM27761 still have the same issue. I tried changing the FB resistor like you suggested and wirewrap Cap's GND. I don't know what's wrong with this. Thanks for your time.
  • 0 in reply to Arturo Rios
    TI__Guru*** 148870 points
    Hi Arturo,

    Can you capture a waveform of both Vin and Vout, right after you apply Vin for the first time (with the IC cool)?

    Have you already checked if the thermal pad was soldered? Sometimes there is not an opening in the paste mask and so there is no solder paste placed at all.