To program the cathode voltage to a regulated voltage a resistive bridge must be shunted between the cathode and anode pins with the mid point tied to the reference pin. This can be seen in Figure 8-5, with R1 and R2 being the resistive bridge. The cathode/output voltage in the shunt regulator configuration can be approximated by the equation shown in Figure 8-5. The cathode voltage can be more accurately determined by taking the cathode current in to account

V_O = (1 + R1 / R2) × V_ref – I_ref × R1 (1)

For Equation 1 to be valid, TLV431 must be fully biased so that it has enough open-loop gain to mitigate any gain error. This can be done by meeting the I_min spec denoted in Recommended Operating Conditions table.