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

2.2 Output Levels (VOH , VOL)

The comparator operation for shunt devices operates in 2 modes.

  • VIN = 0: The device stays in the off condition in this state. The cathode current IKA = 0 and output voltage VKA (VOH) = Vsup when the input voltage is Vin = 0 for the device.
  • 0 < VIN < VIT: The reference pin starts taking a few nA current as soon as the input voltage (Vin) goes above 0 V and this will establish IKA in the range of few uA which will be much lower compared to IKAmin of the device . Figure 2-1 circuit is tested with Rin = 100 kΩ, Rsup = 10 kΩ and IKA vs Vin. Figure 2-3 shows the typical variation of IKA when VIN < VIT. This plays an important role in deciding the upper limit of Rsup for VOH.
  • VIN ≥ VIT: The reference pin is saturated around VIT and Iref increases with Vin. The cathode pin turns on and VKA gets saturated around a fixed voltage (VOL).

Figure 2-3 Vin Vs IKA for Different 431 Devices

Table 2-2 lists typical VOL across different shunt regulators –

Equation 1. VKA = V S u p   ,           V I N < V R E F   V O L   ,           V I N > V R E F
Table 2-2 Typical Low Level (VOL) Voltage When Shunt Regulator is ON
Devices Typical VOL (V)
TL431, TL431LI, ATL431, ATL431LI, 2
ATL431 1

Please refer to app note slva987 for more details about the VOL level.