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LM2731: Voltage OverShoot under Load

Part Number: LM2731
Other Parts Discussed in Thread: BQ24600, LM2733

Tool/software:

Hello,

I am currently using the LM2731 in a design. Input range 10.5-13.5Dvc and output at 16Vdc.

Unloaded, the boost converter has a solid 16V output, but if I put it under load (even 10mA) the output will shoot to 18-19Vdc and the chipset will begin to have some high pitch switching noise.

I have attached my schematic.

Thanks,

-Alex N

  • Hi Alex,

    There should be instabilities in your circuit when the load is added.

    From schematic, I see no concerns. Could you send me your layout and the part number of the feedforward capacitor C12 and output capacitor C14? 

    If available, could you also share what is your application? Thank you.

    Regards

    Lei

  • I have attached a image of my layout. I did have an input diode (D6), but I bypassed that for testing,

    C12 and C14 are both C0805C106K3PACTU

    I am wanting to use this boost converter to power a BQ24600 and slowly charge a 4S LiFePO4 battery pack at 250mA.

  • Hi Alex,

    C12 and C14 are both C0805C106K3PACTU

    Do you mean that the C12 you used is 10μF, not 180pF? If so that should be the problem.

    And your layout is not so optimized in the input and output power path, especially on the long path from the in/out cap GND to the chip GND.

    You may refer to the following layout concept to optimize the layout.

    And the Vin trace can be wider.

    Regards

    Lei

  • Hello, thank you for your reply. 

    I am sorry, but I didn't pay attention to the designations on the caps. C12 is 180pF.

    For the layout, it is not as optimized as it could be, but I tried to make it as close to the EVM as possible inside of my layout envelope. 

  • Hi Alex,

    Thank you for your information.

    What's the part number of C12/180pF? I'd like to check the effective capacitance of C2. Or you may try a big capacitance of C12 to make the effective capacitance close to 180pF.

    One more question, what's the device you used? LM2731 or LM2733? In the schematic it's LM2733.

    Regards

    Lei