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UCC21750: If APWM is not been used, Can it add external 1nf to GND?

Part Number: UCC21750

Hi  team,

I have below questions about UCC21750 APWM pin.

If UCC21750 APWM is NOT been used,  Can this pin (pin16) add external 1nf to GND? what effects it'll have?  Like Iq turn bigger?

If UCC21750 APWM is  been used,  Can this pin (pin16) add external 1nf to GND? What effects it'll have?  


  • Hi Liang,

    Thanks for your interest towards UCC21750 part. The UCC21750 features an isolated analog to PWM signal function from AIN to APWM pin, which allows the isolated temperature sensing, high voltage dc bus voltage sensing, etc. The UCC21750 encodes the voltage signal VAIN to a PWM signal, passing through the reinforced isolation barrier, and output to APWM pin on the input side. The PWM signal can either be transferred directly to DSP/MCU to calculate the duty cycle, or filtered by a simple RC filter as an analog signal. The AIN voltage input range is from 0.6V to 4.5V, and the corresponding duty cycle of the APWM output ranges from 88% to 10%. 

    So the APWM pin will always give out the PWM signal corresponding to AIN pin voltage. [if device is powered and enabled]. It can be directly sensed by the controller or through a RC filter to measure corresponding DC voltage as explained in the following link.  

    Also refer the following e2e link about leaving it unconnected.

    Hope it answers your question. 

  • Hi Thangam,

    Thanks for your help!

    I  Know APWM can be floating when it not used.

    I want to know if APWM  add external 1nf in to GND,  what effects it'll have?  


    Hopefully get your help!

  • I believe that you want to understand the differences between APWM floating vs APWM with 1nF cap- will share the details by tomorrow CST.

  • Hi Liang,

    Keeping APWM floating vs having 1nF cap doesn't impact much on the VCC/VDD current, when AIN pin is not used.  

    However I am curious to understand why 1nF cap planned when APWM is not in use. It will be good to understand the system requirement. 

    Let us know if it answers your question.  



  • Thanks for your help! Besides VCC/VDD current, what else effects it'll have?

  • Hi Liang,

    There will not be any difference in keeping the pin floating Vs using a 1nF capacitor.  As its an output pin, you don't really need a capacitor especially if you are not sensing it. But adding it will not have any impact. [when AIN is not used]

    You can also plan 1nF capacitor on the Ain pin as well when not in use, to decouple the switching noise.