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UCC28065: Any limitation in pull up resistor of the VINAC, HVSEN and VSENSE

Part Number: UCC28065

Tool/software:

May I know the pull-up resistor of the VINAC, HVSEN and VSENSE have any limitation? If we set the pull-up resistor around 1M ohm is workable or not?

We don't have light load Eff need, but we don't prefer use the resistor high than 1M ohm because of the reliability concern. 

BR,

Gary

  • Hello GaryC, 

    There is no limitation on the value of the upper resistor of the VINAC, HVSEN and VSENSE resistor-dividers.  The lower resistor values are chosen to follow the same ratio targets based on the thresholds at each input.  

    Higher resistances (> 1Meg) are used successfully in countless designs, but do make the signals a little more prone to noise pickup.  I don't see a reliability concern until >10Meg, and it is not the resistor reliability, but the possibility of leakage currents around the resistor due to pcb surface contamination and/or moisture that can be unreliable. 
    Lower resistances (< 1Meg) can also be used successfully and help reduce the noise sensitivity of the signals, but do increase stand-by power loss. 
    In your case, apparently higher stand-by power is not a concern. 

    Regardless of upper resistor value chosen, the noise filtering R-C time constant (involving the lower resistance) for VSENSE and VINAC should not exceed 150us and is ideally around 100us or less.  For HVSEN, this time constant can be ~1ms or so, to avoid nuisance triggering of the higher-level OVP shutdown.   

    Note: the upper resistance in the VINAC divider is used to adjust the Brown-in/Brown-out threshold for the AC input voltage, by way of a 2uA current sink into the VINAC pin.  
    With lower value upper resistance, the fixed 2uA current may not adjust the BI/BO difference as much as you may require.  A simple solution, in this case, is to insert a resistor in series with the VINAC pin connected to the divider node.  The series resistor value is calculated based on the total resistance needed for the BI/BO hysteresis minus the upper resistor value. 

    Regards,
    Ulrich