SNVAA58 December   2022 ATL431 , ATL431LI , TL431 , TL431LI

 

  1.    Abstract
  2. 1Trademarks
  3. 2Comparator Mode
    1. 2.1 Input Threshold Voltage (VIT)
    2. 2.2 Output Levels (VOH , VOL)
    3. 2.3 Propagation Delay
  4. 3Design Parameters for Comparator Mode
    1. 3.1 IREF (Reference Current)
    2. 3.2 IKA (Cathode Current)
    3. 3.3 Stability
      1. 3.3.1 Load Capacitor (Cathode to Anode)
      2. 3.3.2 Compensation in Comparator Mode in Feedback of PWM Controller
  5. 4Design Example
    1. 4.1 Input Parameters
    2. 4.2 Specs of Comparator Mode
  6. 5Conclusion
  7. 6Revision History

3.1 IREF (Reference Current)

IREF does not vary as long as VIN stays below the threshold voltage. Once VIN crosses the comparator threshold, voltage at reference pin (VREF) remains constant around the threshold voltage and IREF starts increasing with VIN.

Equation 2. I R E F = V I n - V R E F R I n

Choose the Resistor RIn so that IREF does not go more than a few μA for maximum possible VIN. This affects VOL, VIT and the switching time at the cathode terminal.