Figure 23 shows the TLVH431, TLVH431A, or TLVH431B 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 23.
The TLVH431 family of devices are prevalent in these applications, being designers go to choice for secondary side regulation. Due to this prevalence, this section explains operation and design in both states of TLVH431 that this application will see, open loop (Comparator + VREF) and closed loop (Shunt Regulator).
Further information about system stability and using a TLVH431 device for compensation see Compensation Design With TL431 for UCC28600, SLUA671.