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

3.3 Stability

External capacitors that are connected to the shunt regulator and IKA affect the stability of the 431 comparators. The 431 devices are stable for a full range of IKA if there is no capacitor connected on any of the pins. The designer may want to connect a capacitor to suppress noise and support high inrush. A load capacitor is connected between cathode to GND to support the load inrush current. Designers may add a capacitor at the reference pin to suppress noise on the input pin. 431 comparators may need Type 1 or Type 2 compensation in power supply designs to make whole loop stable with the pole inserted by the optocoupler. These capacitors introduce extra poles in the system and may lead to unstable VOL if the pole is located at an undesired location. This section explains how to select the capacitors so that VOL is stable.