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LM27761: -5V generation from a Li-Po battery

Fatih DAGSEVER
Prodigy 20 points
Part Number: LM27761

I've used LM27761 to generate -5V from a typical Li-Po battery (3.7v). However, I can just obtain -3.47V at the output of LM27761. I share my circuit below;

As shown in the application information below from the datasheet, using a positive voltage in the range of 2.7 V to 5.5 V to a negative output voltage configurable with external gain setting resistors, I tried to get -5V.

However, it does not work even though I use the calculation in the datasheet to determine the resistor values and follow the typical application design;

I wonder whether to generate -5V from a typical Li-Po battery by using the component? Did I miss something?

Thank you in advance for your help

  • 0
    TI__Genius 16010 points

    Hello Fatih,

    thanks for reaching out in E2E.

    The LM27761 is a combination of inverting charge pump and an LDO. Means, the charge pump is inverting the input voltage in the first stage which results in -Vin at the capacitor C3. Then the LDO is regulating VOUT according to the resistor voltage divider. If your input voltage is 3.7V then the charge pump will generate -3.7V from it (best case without load). The LDO also has some voltage drop from CPOUT to VOUT, so |VOUT| is always less then |Vin|.

    Probably the LM2682 in combination with an LDO would be an option for you. The LM2682 datasheet is showing another alternative application with an OPV in Figure 7.

    Let me know if you have any questions.

    Best regards,

    Andreas.

  • 0 in reply to Andreas Mueller
    Prodigy 20 points

    Dear Andreas,

    Thanks very much for your reply and useful infomation I really appreciate.  Your suggestion "LM2682 in combination with an LDO" sounds proper solution. I will definetly consider.

    On the other hand, I figured out and observed how LM27761 works. However, there are something confusing about this product. In the Mouser website, it seems LM27761 has "boost topology" as shown in the picture below;

    I also see boost swticthing frequency in the datasheet;

    When you read Application information, it seems like you can invert a positive voltage in the range of 2.7 V to 5.5 V to a negative output voltage configurable with external gain setting resistors. 

    This is because, in the equation of output voltage setting, VOUT depends on feedback voltage and resistors only.

     

    It seems like VOUT is independent from VIN. It is like if you have a positive voltage in the range of 2.7 V to 5.5 V, you can configure your VOUT by using resistors.

    I just would like to hihglight this confusion

    For your information

    Thank you again for your help

    Kind regards,

    Fatih

  • 0
    TI__Genius 16010 points

    Dear Fatih,

    thanks a lot for your valuable feedback. I will forward this information and ask if the datasheet can be updated to avoid such confusion.

    Thanks and best regards,

    Andreas.

  • +1
    TI__Genius 16010 points

    Hello Fatih,

    may I also suggest to evaluate the TPS63700 device, which might fit to your application needs.

    Please let me know if you have any questions.

    Best regards,

    Andreas.

  • 0 in reply to Andreas Mueller
    Prodigy 20 points

    Dear Andreas,

    The TPS63700 device looks like perfectly meets my application needs. I can directly implement typical application circuit in the datasheet to obtain -5V from a Li-Po battery.

    Thanks very much for your suggestion and your interest.

    I really appreciate your help

    Have a great week!

    Kind regerds,

    Fatih

  • 0
    TI__Genius 16010 points

    You are welcome.

    Also have a nice week.

    Best regards,

    Andreas.