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Backfeeding LM22672

Christian Engstrand
Prodigy 90 points
Other Parts Discussed in Thread: LM22672

Hi,


We have an application using LM22672MRE-5.0 to convert VIN (12 - 24 VDC) to output 5 VDC.

But this product also has the option to be powered by 5 VDC from USB or from a separate connector.


So the question is: With VIN floating, can SW be driven to 5 V without damage?


From other threads (e2e.ti.com/.../357881) I understand it is usual that the VIN will rise close to SW when doing this. That should not be a problem in our case. As soon as the decoupling capacitors on VIN are charged no more current would flow as VIN is created using a bridge rectifier.

Best regards
Christian

  • 0
    TI__Genius 15810 points
    Hello,
    When you have VIN floating and SW at 5V, then there is going to be some current flowing through the FET's body diode to charge the input capacitor. We do not recommend a condition that turns on the body diode because the current flow may damage the FET. There are high chances of the device malfunctioning after that.

    Regards,
    Akshay
  • 0 in reply to Akshay Mehta
    Prodigy 90 points

    Perfect, thanks for your reply Akshay!


    Follow-up questions:
    By "turning on the body diode" you mean this current flowing from SW to VIN, right?

    What would be the recommended solution to avoid this?

    If a series diode is needed I guess the best placement is right before VIN (after the decoupling capacitors) due to the lower current on primary side.

    Christian

  • 0 in reply to Christian Engstrand
    TI__Genius 15810 points
    Hello,
    A series diode is going to add to the power dissipation. I would instead suggest a parallel Schottky diode from SW or VOUT to VIN. This way the body diode gets bypassed and you don't hurt the device.
    Regards,
    Akshay
  • 0 in reply to Christian Engstrand
    TI__Genius 11085 points

    Yes, the risk is currnet flows from SW to VIN via body diode of the HS FET. If VIN is floating, it is ok to charge it to 5V. If you want to add a diode, it should be added before the input bypass caps. The bypass caps should be directly connected to the VIN and GND pins as close as possible.

    Akshay's suggestion of a schottky diode from SW to VIN is a good way to bypass the body doide of the HS FET. The risk is during the time  when the other 5V is coming up and the current spike going through the HS body diode can be pretty high, even if you have a diode on the VIN side. Just think when you add a 5V DC voltage directly to a diode to charge a cap, the initial current can be quite large.

    I would suggest to add the 5V not through SW, but VOUT of the LM22672. The SW and VIN will still be charged up to 5V. But the inductor of the LM22672 should be able to limit the initial current spike when 5V comes up. Also, after VIN is charged up to 5V, no more current would go through the inductor to generate unnecessary loss and heat.

    Hope this helps.

    -Yang

  • 0 in reply to YangZhang
    TI__Genius 15810 points
    That's a good point. I had conveniently assumed that the 5V was being applied to the output.
  • 0 in reply to YangZhang
    Prodigy 90 points
    Perfect, thanks Akshay and Yang for good solution and explanation!
    I have added the parallell diode from VOUT (after the inductor) to VIN to relief the LM22672!