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LM27761: How to have output voltage to be stable at -2.7V when the output current is 200mA and input voltage is 3V

Hao Wang14
Expert 4415 points
Part Number: LM27761
Other Parts Discussed in Thread: , TPS62825

Hi Team,

The output voltage of LM27761EVM is set to -2,7V by changing R1 to 200k and R2 to 160k, when the input voltage is 3V and output current is 200mA, the output voltage will increase to -2.4V which cannot be accepted by the load.

May I ask if there is any work-round solution to have LM27761 output voltage to be stable at -2.7V when the output current is 200mA and input voltage is 3V? The input voltage cannot be increased for this application.

If not, could you please help recommend alternate device? 

Thanks and Best Regards!

Hao

  • 0
    TI__Mastermind 22840 points

    Hi,

    Could you please share the schematic?

    Regards,

    Febin

  • 0 in reply to Febin
    TI__Expert 4415 points

    Hi Febin,

    It's tested on LM27761EVM. Thanks!

    Best Regards!

    Hao

  • 0 in reply to Hao Wang14
    TI__Mastermind 22840 points

    Hi Hao,

    On the EVM, it should be regulating at -2.7V. I am not sure why it doesn't.

    Could you please share the measurements plots when it is regulating and not regulating? Have you tried to change the IC and try again?

    Regards,

    Febin

  • 0 in reply to Febin
    TI__Expert 4415 points

    Hi Febin,

    I have tested on two LM27761EVM, both of them have the same results, maybe you can double confirm by checking with EVM, it seems the output voltage will increase when the output voltage is set to be close with input voltage, because if the input voltage is 3.3V, then the output will be kept at -2.7V with 200mA output.

    So is there any recommendation for the application which the output voltage to be stable at -2.7V or at least less than -2.7V(such as -3V) when the output current is 200mA and input voltage is 3V?

     Thanks and Best Regards!

    Hao

  • 0 in reply to Hao Wang14
    TI__Mastermind 22840 points

    Hi Hao,

    Okay! I misinterpreted your first message. From my experience with this device, with a Vin of 3V, I would expect approximately -2.5V. This is because we need to account for the losses in the converter and provide headroom for LDO and charge pump losses.

    You might want to increase the Vin or choose a different Vout setting. 

    Unfortunately, we do not have any devices to support a negative rail in our current portfolio. But you may use any DCDC buck converter by reversing the polarity if your application requirements are satisfying.

    Please let me know if I can help further.

    Regards,

    Febin

  • 0 in reply to Febin
    TI__Expert 4415 points

    Hi Febin,

    Is there any recommendation for the DCDC buck? The total solution size should be within 5mm X 5mm?

    Thanks and Best Regards!

    Hao

  • 0 in reply to Hao Wang14
    TI__Mastermind 22840 points

    Hi Hao,

    Are you looking for a negative output rail?

    Regards,

    Febin

  • 0 in reply to Febin
    TI__Expert 4415 points

    Hi Febin,

    Yeah, the same requirements for the application which the output voltage to be stable at -2.7V or at least less than -2.7V(such as -3V) when the output current is 200mA and input voltage is 3V?

    Thanks and Best Regards!
    Hao

  • 0 in reply to Hao Wang14
    TI__Mastermind 22840 points

    Hi Hao,

    So as I mentioned earlier, we do not have a buck for a negative voltage in our current portfolio.

    But you could look into TPS62825 with 1% accuracy in 1.5x1.5 QFN, 1A output current. But we have not tested or characterized this device for a negative voltage. Please test and verify the suitability of reversing the polarity on your application.

    Regards,

    Febin