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WEBENCH® Tools/LMZM23601: Connection of MODE/SYNC in Inverting Buck-Boost configuration

Vishal Kakade
Intellectual 930 points
Part Number: LMZM23601

Tool/software: WEBENCH® Design Tools

Hello.

I am using LMZM23601 in inverting buck-boost configuration.

Vin: 4.5V - 5.5V

Vout: -5V @ 0.5A.

After I generated with a design from webench,  MODE/SYNC is left floating. But in the datasheet, it is mentioned to not to float this pin. Kindly let me know where to connect this pin?

Regards

Vishal Kakade

  • 0
    TI__Mastermind 19891 points

    Hello Vishal,

    Connect the MODE/SYNC pin to the return (negative VOUT for inverting) if you want PFM operation at light load.

    Connect the MODE/SYNC pin to VIN if you want forced PWM operation at light load. See below:

    Cheers,

    Denislav

  • 0 in reply to Denislav Petkov
    Prodigy 30 points
    Hello
    What happens if this pin is left open, as proposed in the Webench design.
    I'm asking because I made the design according to Webench and left it open, unfortunately...

    Now I have the issue, that the circuit correctly produces -12V from a +12 supply up to a certain load current. Above this current (constantly at 150mA) the output voltage changes to -4.8V, which are also regulated and stable for different loads... Is it possible, that the left open MODE/SYNC pin causes this behaviour?
    Best regards,
    Stefan
  • 0 in reply to Stefan Tschannen
    TI__Mastermind 19891 points

    Hello Stefan,

    Apologies for the delayed response. The MODE pin sets the mode of operation at light load. With MODE pulled high the part will go into forced PWM mode at all loads. With MODE pulled low it will go into auto PFM mode at light load to save power.

    If MODE is left floating by accident, I believe there is an internal pull down (although not very strong) that will keep the part into auto PFM mode operation at light load.

    What is the input and output current when the output voltage changes and is your input supply capable of providing the input current?

    Cheers,
    Denislav

  • 0 in reply to Denislav Petkov
    Prodigy 30 points

    Hello Denislav,

    thank you very much for the response. This information is very valuable for me.


    By accident I found out, that the input cap of my LMZM23601 was too far away from the IC! After moving it closer to the IC, I had a stable -12V supply up to >300mA, which is more than sufficient for me. I did not see any jumps to -4.8V anymore! So, my issue is solved.

    Best regards,
    Stefan

  • 0 in reply to Stefan Tschannen
    TI__Mastermind 19891 points
    Thank you Stefan for confirming.
    Yes, component placement and layout is quite important for any switching regulator. I will close this thread. If you have any additional questions, please don't hesitate to write here and we can re-start the conversation.

    Cheers,
    Denislav