Teach you a reliable new method to implement overvoltage protection for electronic signal inputs

Overvoltage protection is a key design consideration and challenge because protecting a system from overvoltage often requires additional components, but these additional components often have an impact on the system and, in the worst case, can cause error signal. In addition, these components add additional cost and further exacerbate space constraints. Therefore, when designing protection circuits, traditional solutions often require a compromise between system accuracy and protection level.

Typically, a common and simple design approach is to use an external protection diode, usually a transient voltage suppressor (TVS) diode, installed between the signal line and the power line or ground line. TVS diodes are useful because they react quickly to transient voltage spikes. This type of external overvoltage protection is shown on the left side of Figure 1.

Figure 1. Traditional overvoltage protection design using additional discrete components

If a positive transient voltage pulse overvoltage occurs, current will flow to VDD through D1 to clamp this positive transient voltage pulse, and the voltage is therefore limited. The clamping voltage is equal to VDD plus the forward voltage on the diode. If the pulse is negative and less than VSS, then the same function described above applies, but instead it is clamped to VSS through D2. However, if leakage current caused by overvoltage is not limited, the diode may be damaged. For this reason, a current-limiting resistor is added to the path. Bidirectional TVS diodes at the input are often used for increased protection under very harsh environmental conditions.

Disadvantages of this type of protection circuit include increased signal rise and fall times and capacitive effects. Additionally, no protection is provided when the circuit is de-energized.

Real devices, such as analog-to-digital converters (ADCs), operational amplifiers, etc., often have built-in protection features. As shown on the right side of Figure 1, this protection function consists of a switching architecture. Figure 1 also shows that there are input and output protection diodes on both sides of the power rail. The disadvantage of this setup is that when the floating signal is present in the powered-down state (no power to the IC), the switch may act as if it were active (even if set to off), with current flowing through the diode and the supply rail. This phenomenon will allow current to pass through the signal line, causing the signal line to lose protection.

Fail-Safe Switching Architecture

One way to solve the above problem is to use a fault-protected switch architecture with a bidirectional ESD cell, as shown in Figure 2. Instead of using the input TVS diode, the ESD cell now clamps the transient voltage by constantly comparing the input voltage to the voltage on VDD or VSS. In the event of prolonged overvoltage, the downstream switch will automatically open. This way, the input voltage is no longer limited by the protection diodes clamped to the power rail, but by the maximum voltage rating of the switch. In addition, higher system robustness and reliability can be achieved without affecting the actual signal and its accuracy. In addition, the leakage current when the switch is turned off is very low, so no additional current-limiting resistor is required.

Figure 2. Overvoltage protection with integrated bidirectional ESD unit

The ADG5412F four-channel single-pole single-throw (SPST) switch provided by Analog Devices (ADI) uses this input structure. This switch can withstand permanent overvoltage up to ±55 V, regardless of the size of the existing power supply. Integrated ESD cells on each of the four channels can clamp transient voltages up to 5.5 kV. During an overvoltage condition, only the affected channel will open, other channels continue to operate normally.