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LM27762: Only Positive rail output is present. Not the negative rail.

Yash Gupte
Prodigy 120 points
Part Number: LM27762
Other Parts Discussed in Thread: TPS63050

Hello,

I am designing a Li-ion battery to +/- 3.3V supply using the LM27762. I made the schematic and selected components as per the WEBENCH guidelines, screenshot of which I have attached.

I have also attached the Altium Schematic which I have made for reference. The issue I am facing is that when measuring, Positive rail(OUT+) to ground I get a 3.3V signal from a 3.7V Li-ion battery.

However, the negative rail(OUT-) provides a 0V reading when measured between OUT- and ground. There is no electrical short between the Ground and OUT-.

If you could tell me what could be the mistake I would be grateful.

Thank You

  • 0
    TI__Guru**** 151400 points
    Hi Yash,

    You should first check if this occurs on all boards or just one, to rule out a manufacturing error.

    You can also check the C1+ pin to see if there is any switching going on.

    The Cfly cap part number looks odd. This does not seem to be a normal ceramic cap. We just need to use normal, standard ceramic caps with this IC. You might try another cap and see if it works.
  • 0 in reply to Chris Glaser
    Prodigy 120 points

    Thank You Chris.

    I tried both the solutions you offered:

    1. The C+ pin was giving a proper 2MHz oscillation as expected. 
    2. Replaced the Cfly capacitor with a normal ceramic SMD capacitor but to no avail.

    I remade the PCB and I am able to get the desired voltage. The circuit is designed to work between Vin values of 3V - 4.2V.

    VIN(V) OUT+(V) OUT-(V)
    4.2 3.301 -3.285
    4.0 3.301 -3.285
    3.6 3.301 -3.285
    3.5 3.254 -3.285

    But, at Vin=3.5V  the OUT+=+3.3V drops to about +3.254V and the negative(OUT-) stays put at -3.285V which is the expected behavior.

    Could you please suggest ways as to improve these characterisics/ resolve issues?

    I am attaching the following for reference:

    1. Schematic
    2. WEBENCH design
    3. PCB image

  • 0 in reply to Yash Gupte
    TI__Guru**** 151400 points
    Instead of making your own PCB, next time just order the EVM. It is much simpler and is a proven circuit.

    As the LM27762 is an LDO + an inverting charge pump + LDO, it will not give +/-3.3Vout from a 3Vin source. It cannot boost Vin. Each output voltage will drop based on the LDO's dropout voltage and the charge pump's output impedance (which both depend on the load current).
  • 0 in reply to Chris Glaser
    Expert 1280 points

    Hi Chris,

    Thanks for this useful information.
    While inputting the design parameter to select the passive component in the webench , there should be some warning which says +3.3V cannot be given from 3 V input source.

    Mr. Yash and I are working on application that requires +/- 3.3V output. The battery source is Li-ion battery, voltage level of Li-ion battery varies from 3V to 4.2V. If LM27762 voltage drops at 3.4V, then the IC is not good for us, as their is sufficient charge left in the battery.

    Requirement:

    Input voltage: 3 to 4.2V
    Output voltage: +/- 3.3V
    Current: 200 mA

    Is there any other IC which better suit our application, or we have to tweak input design parameter for LM27762 in the webench.

    If we limit the current to 100 mA while using LM27762, voltage drop will not occur until Vin reaches 3.35V ? I need your suggestion on this.

    Regards,

  • 0 in reply to TapasXplore
    TI__Guru**** 151400 points
    Hi Tapas,

    I can check with the Webench team on this, but the LM27762 is a positive LDO combined with an inverting charge pump followed by another LDO. Thus, it cannot boost Vin. I hope this is at least clear from the D/S.

    I suggest using the TPS63050 buck-boost from the battery to some voltage higher than 3.3V. Then, you can use LM27762 from this voltage to create your +/- 3.3V, taking into account the output impedance of the charge pump and LDO dropout voltages.