On occasion, miswiring faults occur when field transmitters are installed in industrial environments. If the OUT pin of the XTR200 is mistakenly connected to the power supply, and the VSP pin is grounded, the on-chip ESD diode between these pins is forward biased. The forward bias causes a large current to flow and damage the IC. Figure 7-2 shows the addition of a miswiring protection diode in series with the XTR200 power supply. This diode prevents reverse current from flowing out of the VSP pin through the ESD diode. The supply bypass capacitor, C_BYP, placed at the VSP pin provides a low-impedance pathway to ground for high-frequency ESD events that forward bias the on-chip ESD diode. Include the forward voltage drop of the miswiring protection diode when calculating the minimum supply voltage to provide the necessary headroom for the XTR200.

Figure 7-2 Basic Miswiring Protection Diode in Series with Supply

Figure 7-3 shows a basic reverse polarity protection circuit to protect the XTR200 in the event the ground voltage exceeds the supply or load voltage. In normal operation, the NMOS transistor is turned on via resistors R1 and R2. Select resistor values which produce an appropriate gate voltage for the NMOS from the range of expected power supply voltages. Zener diode ZD1 protects the NMOS gate from over-voltage. If V_Supply is low but the voltage at the ground is high (reverse polarity condition) the NMOS is turned off, preventing current flow through the on-chip ESD diodes. Select an NMOS transistor with low R_DS(ON) to minimize ground potential differences.

Figure 7-3 Basic Reverse Polarity Protection

Figure 7-3 does not protect the XTR200 in the case that the ground connection is shorted to the power supply but the load is still grounded. Figure 7-4 adds additional protective circuitry to limit current through the ESD diode between the GND and OUT pins. In normal operation, when this ESD diode is reverse-biased, a current equal to the IDSS of JFETs Q1 and Q2 flows through resistor R3, turning-on PNP transistor Q3. This turns on the NMOS transistor in series with the ground connection. If the ESD diode becomes forward biased, Q1's gate is pulled below the source, pinching-off the channel and drastically decreasing the drain current, turning off Q3 as well as the NMOS in series with ground.

Figure 7-4 Miswiring Protection in Case of Ground Short to Power Supply

IO-Link transceivers produce large ground currents which can cause transient ground voltages when flowing through miswiring protection circuitry. However, many IO-Link transceivers such as TI's TIOL112, integrate internal miswiring protection and can therefore bypass the external protection circuitry described in this section as shown in Figure 7-5.

Figure 7-5 IO-Link Devices with Integrated Miswiring Protection Can Bypass Protection Circuitry