Zener diodes can also be used to protect data lines from overvoltage events. A controller area network (CAN) bus has two data lines – CANH and CANL. In the recessive state both lines are biased to approximately 2.5V. In the dominant state CANH is increased by 1V to approximately 3.5V while CANL is reduced by 1V to approximately 1.5V, therefore creating a approximately 2V differential signal between the two data lines. ISO 11898 standard specifies CAN data lines bus voltages between -2V to 7V, but CAN transceivers are typically rated for much wider common mode voltage. For instance, TI CAN signal improvement capability (SIC) transceiver TCAN1462-Q1 is rated for 8Mbps maximum data rate, with CANH and CANL pins rated for +/- 58V absolute maximum voltage. In this example we will look for a zener diode to verify the voltage stays below 58V. For CAN-SIC the suggested maximum bus capacitance is <6pF.

The selected zener diode is MMBZ30VCL-Q1 that has a 30V working voltage, 34.8V maximum breakdown voltage, 30kV contact ESD in a SOT23 package. MMBZ30VCL-Q1 has 4.5pF typical capacitance across both internal diodes. Each line has 4.5pF / 2 = 2.25pF rated capacitance when connected in a bi-directional configuration. Figure 6-6 shows the typical circuit implementation.