Voltage Limiter Generates Supplies That Improve Standard Diode Technique

Diodes

The circuit

Some systems require a voltage limiter to protect the input circuit. Designers can connect diodes (Fig. 1) to V_CC and Gnd to create a simple solution. This simplicity, however, comes at a disadvantage: the input circuit can see voltages within a range of Gnd – V_Dup to V_CC + V_D.

If the circuit is sensitive and can handle only voltages between Gnd and V_CC, the circuit in Figure 2 is a better alternative. The improved limiter includes circuitry to generate two voltage power supplies:

V_P = V_CC – V_D

which can sink current, and:

V_N = Gnd + V_D

which can source current.

The circuit assumes that V_D = V_BE. In this case, the protection diodes at the input will clamp the input voltage at no more than V_CC and no less than Gnd.