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LM2776: Start up issue in the high temperature

Part Number: LM2776
Other Parts Discussed in Thread: TPS60400

Hi Experts:

Please see the schematic below, my customer is using the LM2776 to convert from 5V to -5V ,which is for the amplifier's bias.

 the LM2776 could work normally if we increasing the ambient temperature from 25degree to 60degree.  

However, The LM2776 can't start working if we turn off and turn on input power in the case of 60degree.

We check the fly cap and input/output cap , there are all X7R.

We also capture the waveform of Vout, C+/C- , could you please help on it ? it seems some protection take effect to block the device working.

  • Hi Jason,

    1. Firstly, can you please disconnect the load and try to take the measurements again.

    2. The load in the application is an opamp- The positive rail can bleed through the load onto the negative output and pull it positive. A schottky diode would clamp this.  This might help the startup.

    You may also want to consider TPS60400 family. It is safe with startup but not pin to pin. This has an inbuilt diode.

    3. How are you powering LM2776?

    Please let me know if this works.

    Regards,

    Febin

  • Hi  Fenin:

    Appreciate for your comments, the issue has been resolved if we add the schottky on the output .

    Can i know the reason why the issue only appear in the case of high temperature , in the normal room temperature , the device could also start up even the output is clamp to the positive voltage?

  • Hi Jason,

    The technical reason behind the startup issues of LM2776 are not available. In general, the device has common issues during startup (even in normal room temperature). 

    We have the TPS60400 family, which integrates this diode for a proper startup.

    Since this issue is resolved, I would like to close this thread. Please contact us for any further support.

    Regards,

    Febin