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Original question:

UCC28711: NTC pin

UCC28711: NTC Pull-down

Cameron Perl
Expert 5375 points
Part Number: UCC28711

Hey team,

Should there be any concerns about directly tying the NTC pin of the UCC28711 to ground? What is the "pull-up current" of 90-125uA in the datasheet?

Is this an accurate representation of a shutdown circuit or is the 100kOhms resistor superfluous?

 

Thanks,

Cam

  • 0
    TI__Mastermind 24375 points

    Hi Cameron,

    Thank you for the query on UCC28711.

    Tying the NTC pin to ground will disable PWM action of the IC. From the datasheet there is an internal threshold of 0.95V below which the shutdown will initiate. The "pull up current" mentioned above is the internal bias current (current source) of 105uA (nominal) which requires a minimum resistor of 9.5kohm connected below which protective action will be taken.

    There is no issue in using 100k NTC, If you use a NTCS0805E3 which is used in our EVM, the NTC threshold will be reached at close to 105C where the resistance is below 9.5k and PWM trips. Are you trying to accomplish an external forceful shutdown from MCU using optocoupler in addition to NTC?

    Regards,

    Harish

  • 0 in reply to Harish Ramakrishnan5
    TI__Expert 5375 points
    Harish Ramakrishnan5 said:
    Are you trying to accomplish an external forceful shutdown from MCU using optocoupler in addition to NTC?

    Hey Harish,

    Correct - they basically want to use it as an EN pin - it should be noted that is simply a thermistor rather than an NTC. "there is some desire to be able to shut down the AC-DC converter" Any issues doing this?

    Thanks for your comments on the resistance value.

    Thanks,

    Cam

  • 0 in reply to Cameron Perl
    TI__Mastermind 24375 points

    Hi Cameron,

    Yes, the NTC input can be pulled to GND directly with the photo-coupler transistor as shown, without any series resistor, as the NTC current is limited to 105uA. So it can be used to shutdown the device from MCU.

    Thank you

    Regards,

    Harish