Voltage Limiter Generates Supplies That Improve Standard Diode Technique

Electronic Design Europe 10.02.11

Yossi Barkan

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


Figure 1.	*A very simple overvoltage protection technique uses diodes connected to V_CC and Gnd.*

If the circuit is sensitive and can handle only voltages between Gnd and VCC, 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.


Figure 2.	*A better limiter generates V_P and V_N, which can be common to all the protection diodes on the same printed-circuit board.*

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