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LM27762: -VE OUTPUT PROBLEM

A SHORT
Prodigy 20 points
Part Number: LM27762

WE HAVE A DESIGN WITH +5V VIN TO GENERATE +/- 3.5V OUTPUT. THE DESIGN HAS BEEN IMPLEMENTED ON WEBTOOLS TO THE LETTER. THE PROBLEM WE ARE SEEING IS THE -VE O/P IS FLUCTUATING BADLY BETWEEN -2V TO APPROX -4.5V THAT LOOKS LIKE A SAWTOOTH WAVEFORM. ALL CONNECTIONS AND COMPONENTS HAVE BEEN CHECKED AND REPLACED. THERE IS APPROX 50mA DRAIN ON +VE O/P AND ONLY 2mA ON THE -VE O/P. i HAVE ADDED EXTRA LOAD TO BALANCE THE -VE BUT PROBLEM STILL EXISTS, THE FREQUENCY OF THE SAWTOOTH JUST INCREASES SLIGHTLY. THIS IS A 3RD PARTY DESIGN SO I CAN'T SUBMIT SCHEMATICS. ANY HELP WOULD BE APPRECIATED.

  • 0
    TI__Mastermind 25430 points

    Hi,

    It is quite challenging to find out the root cause without the schematic, layout, measurement results, etc.

    But anyways, I can suggest a few things and you may try again.

    1. The choice of the external capacitors (type/ effective-value/ etc) is very critical for a charge pump. Make sure it is as per the datasheet recommendations for the intended load.

    2. Please use a stable input supply (like lab DC supply) and ensure that the input is constant and stable.

    3. Please do the test with and without load and you should notice a difference.

    4. If you have the chance, I would suggest to swap the IC with our EVM.

    Please let me know if you need further support.

    Regards,

    Febin

  • 0 in reply to Febin
    Prodigy 20 points

    Hi Febin

    I have checked and replaced the external capacitors with ceramic X5R deices as per datasheet, no change. I have removed the load from the supplies. The +3.5v is still 3.5v but the -3.5v has changed to approx -4.6v but the sawtooth waveform is no longer evident. The current drain with load was  approx 50mA for the +ve supply and approx 2mA for the -ve supply. Does this change with no load give you any ideas.

    Regards

    A Short

  • 0 in reply to A SHORT
    TI__Mastermind 25430 points

    Hi A Short,

    Were you able to swap IC with EVM? I think this is a good try.

    Regards,

    Febin

  • 0 in reply to Febin
    TI__Mastermind 25430 points

    Hi A Short,

    Could you solve the problem? I did not hear from you.

    Regards,

    Febin

  • 0 in reply to Febin
    TI__Mastermind 25430 points

    Hi A Short,

    I hope you could solve the issue. I would like to close this thread due to inactivity. 

    Please contact us for any further support.

    Regards,

    Febin