SLVSEQ3A September 2018 – December 2018 TVS1801
If the TVS1801 is in place to protect the device, the voltage will rise to the breakdown of the diode at 24.4 V during a surge event. The TVS1801 will then turn on to shunt the surge current to ground. With the low dynamic resistance of the TVS1801, even large amounts of surge current will have minimal impact on the clamping voltage. The dynamic resistance of the TVS1801 is around 50 mΩ, which means a 25-A surge current will cause a voltage raise of 25 A × 50 mΩ = 1.25 V. Because the device turns on at 24.4 V, this means the module input will be exposed to a maximum of 24.4 V + 1.25 V = 26.9 V during surge pulses, close to the LM2734 absolute maximum. Because this is a transient pulse, this will likely be safe for the system.
In addition, the TVS1801 provides protection against reverse voltage application that could accidentally be caused by shorts between pins. If –12 V is applied to the VBUS pin, the LM2734 will not be harmed because the series diode will prevent the voltage from being applied to the input, and the TVS1801 will not shunt current because the reverse working voltage is –18 V. If the TVS1800 or a unidirectional device is used in this case, a –12-V short would cause the device to shunt current until it fails.
Finally, the small size of the device also improves fault protection by lowering the effect of fault current coupling onto neighboring traces. The small form factor of the TVS1801 allows the device to be placed extremely close to the input connector, which lowers the length of the path fault current going through the system compared to larger protection solutions.