SLVAF37 January   2021 ATL431LI

 

  1.   Trademarks
  2. 1Introduction
    1. 1.1 TL431 as a Type-2 Amplifier
    2. 1.2 TL431 Solution with Isolation
  3. 2Design Procedures
    1. 2.1 Design Considerations for Optocoupler
    2. 2.2 Design Considerations for Biasing Circuit
    3. 2.3 Schematic Analysis and Waveforms
  4. 3Summary
  5. 4References

1.1 TL431 as a Type-2 Amplifier

Figure 1-1 is a general current-mode control converter. This type of non-isolated converters integrate the EA (error amplifier) and Vref as parts of feedback control loop. And Type-2 amplifier network provides optimal compensation in the loop.

GUID-20210119-CA0I-CTHV-J69Q-9GZHDRF0HLGL-low.png Figure 1-1 Feedback Loop in Non-Isolated Current-Mode Control Converter

On the basis of the composition of TL431, Figure 1-2 shows how it can be used as a standard EA.

GUID-20210119-CA0I-DSVK-0WR7-FW9VGT7J3JB3-low.png Figure 1-2 TL431 Used As A Type-2 Amplifier

The pullup resistor (Rbias) value should be chosen to meet the minimum cathode current requirement, this is to provide sufficient bias current to TL431 under all circuit conditions.