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Original question:

UCC21750-Q1: Clarification required for APWM pin (UCC21750-Q1)

UCC21750-Q1: UCC21750-Q1: AIN Input Voltage limit of 5V

Varunesh Kumar
Intellectual 930 points
Part Number: UCC21750-Q1

Tool/software:

Hello

The AIN Input has a 200 uA of current from Vdd which is +18V in my application.

While using an NTC of say 10K @ 25*C with Beta of 3984, at low  temperatures

the Resistance is over 30K resulting in a voltage of 6 V & above at AIN.

The Specs mention a Max voltage as 5V.

Above 4.5 V the PWM may go above 88%, which is of concern to me.

But otherwise do I need to bother about exceeding 5V at AIN

such as zener clamping?

Thanks and regards

Varun

  • 0
    TI__Expert 6220 points

    Hi Varun, 

    The device's absolute maximum value on AIN is 5V, meaning that voltage above 5V might permanently damage the device and/or affect other device characteristics. 

    I would recommend using a voltage division schematic such as below to control the AIN voltage and make sure it does not exceed 5V. 

    Thanks, 

    Vivian

  • 0 in reply to Vivian Qi
    Intellectual 930 points

    Hi Vivian

    Am pleasantly surprised at your prompt reply.

    Merry Christmas to you, your family and all your colleagues.

    My NTC is 10K @ 25*C with Beta of 3984; the operating temp

    is -40*C to +125*C resulting in a Resistance range of 415K to

    345 Ohms.

    If I keep Rp as 24K and Rs as zero my resulting voltage will be

    always less than 4.8V and at higher temperature drop to 0.69V

    giving me a goodish range of measurement.

    Alternatively using a zener or a TVS clamp to 5V ?

    Which do you think would be better?

    Thanks once again

    Varun

  • 0 in reply to Varunesh Kumar
    TI__Expert 6220 points

    Hi Varun, 

    0.69V to 4.8V of sounds good, although I would say it's recommended to keep the AIN value <=4.5V to adhere to datasheet recommendation and for better duty cycle accuracy.

    However if something were to happen with the thermistor and it becomes open-circuited, the voltage on AIN might exceed 5V, since max current source is 209uA according to datasheet and 209uA*24kOhm = 5.02V. Thus if you want to keep the Rp value I would suggest adding a zener diode to clamp the voltage at <5V as well. 

    Thanks, 

    Vivian