Controlling Vo Of A Fixed Output Voltage Regulator

Sometimes it is necessary to have a regulated output voltage different from that for which the regulator is designed. This may be accomplished with any three-terminal regulator, although it should be noted that the lowest obtainable voltage will be 2.5 V for the TL431 plus the voltage of the three-terminal regulator. In the circuit in Figure 6-45, the lowest possible regulated voltage would be 7.5 V (2.5 V for the TL431 + 5 V for the 7805). This particular circuit provides 9 V output using a uA7805 three-terminal regulator.

Figure 6-45. Fixed Output Shunt Regulator