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

3.3.2 Compensation in Comparator Mode in Feedback of PWM Controller

The 431 family devices are commonly used for sensing the output of the SMPS and providing feedback to the PWM controller IC through the optocoupler to improve the efficiency of the power supply design as shown in the Figure 3-5.

Figure 3-5 Power Supply Design Example

The external components and devices such as the optocoupler and feedback compensation network of the PWM controller affects the stability of the system. TL431 is regarded as a trans-conductance amplifier, so high trans-conductance gain is required in compensation application. A feature of TL431 is that when IKA is low, it’s trans-conductance gain is also low, however, when IKA rises, trans-conductance gain grows quickly, hence this sets minimum limitation of IKA. A compensation circuit of second order is widely used to compensate the original poles and of the feedback system and improve the phase margin thus stability of the system. Application note SLUA671 explains the calculation of the compensation components {R8, C1, C8, R10}.