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

3.3.1 Load Capacitor (Cathode to Anode)

The 431 family does not need any load capacitor for stability in the comparator mode. However, load capacitors can be connected at the cathode pin in case there is high inrush current demand in the system. Load capacitors increase the propagation delay and affect the stability of the system. Stability analysis was done on the circuit shown in Figure 3-1.

Figure 3-1 Test Circuit for Stability of TL431 in Comparator Mode

Vin is ramped up from 0 V to 3 V to turn on the comparator. Stability of the comparator mode is checked for by sweeping IKA from IKA minimum to IKA maximum for TL431, TL431LI. Figure 3-2 shows the unstable VOL behavior when 0.1 uF capacitor was connected between the cathode to anode terminal or TL431 for IKA of 1mA. Figure 3-3 shows stable VOL for same IKA condition when there is no capacitor .

Figure 3-2 Unstable Output
Figure 3-3 Stable Output

The Comparator mode operation follows the stability plot of VREF =VKA as specified in the data sheet. Figure 6 shows the stability plot of TL431 specified in the data sheet. It follows the stable region for plot A in Figure 3-4. The capacitors and current combination out side of the curve A (VKA = VREF) is stable in comparator mode. Further recommendations are given in the app note slva482 for choosing a stable capacitor across the temperature.

Figure 3-4 TL431 Stability Plot

Note: A design that has a capacitor at the reference pin of the 431 device to suppress the input noise. This adds one extra pole in the system and alters the stability curve recommended in the data sheet. TI does not recommend this and the designer must be very careful and qualify this on silicon.