Figure 25 shows the TLV431, TLV431A, or TLV431B used in a 3.3-V isolated flyback supply. Output voltage VO can be as low as reference voltage VREF (1.24 V ± 1%). The output of the regulator, plus the forward voltage drop of the optocoupler LED (1.24 + 1.4 = 2.64 V), determine the minimum voltage that can be regulated in an isolated supply configuration. Regulated voltage as low as 2.7 Vdc is possible in the topology shown in Figure 25.
The 431 family of devices are prevalent in these applications, being designers go to choice for secondary side regulation. Due to this prevalence, this section will further go on to explain operation and design in both states of TLV431 that this application will see, open loop (Comparator + Vref) and closed loop (Shunt Regulator).
Further information about system stability and using a TLV431 device for compensation can be found in the application note Compensation Design With TL431 for UCC28600 (SLUA671).