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LM34936: No output voltage when enabled

Radomir Pans
Prodigy 90 points
Part Number: LM34936

Hi, 

I'm trying to make a 24V buck-boost power supply and can't find the problem why it isn't working.

I already found that:

  1. Q4 and Q7 are placed in the wrong direction.
  2. R40 has the wrong value. 

The 2 NMOS had been replaced and soldered correctly. The resistor R40 is replaced by a 24kR. 

VBAT is 29V during the test. The voltage at the EN/UVLO pin is around 5V and should be enough to activate the LM34936. There is no switching happening at the gates of the MOSFETs.

 

Does someone have an idea why it's not working?

Regards,

Radomir 

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  • +1
    TI__Guru 58215 points

    Hello Radomir,

    As the transistors were connected wrongly, do you possibly have a second board that you could use to check with corrected transistor connection to check? Just to be sure that the IC did not get damaged due to the wrong connections.

    Please check the VCC voltage when you enabled the device. It should be in the range 7.5V.

    On the schematic, I cannot see anything obviously suspicious other than the boost MOSFETs and the RT resistor.

    Please check the different signals on the pins when you enable the device. Especially FB, COMP, VCC, CS with respect to CSG.

    Best regards,
    Brigitte

  • 0 in reply to Brigitte
    Prodigy 90 points

    Hi Brigitte,

    Yes, I still have an unused board. I will apply the patch and let you know how it went. 

    Regards,

    Radomir 

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  • 0 in reply to Radomir Pans
    Prodigy 90 points

    Hi Brigitte,

    The LM34936 Is defective after booting the PCB with the wrongly connected Mosfets. Thanks for the help

    Regards,

    Radomir  

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  • 0 in reply to Radomir Pans
    TI__Guru 58215 points

    Hello Radomir,

    Great, that you have been able to solve the issue.

    Have a good weekend,
    Brigitte