This protection solution is based on the LTC4380 low supply current surge stopper, limiting the output voltage to 22.7V from input voltages as high as 100V at the input—sufficient protection against an ISO-16750-2 load dump as well as ISO 7637-2 pulses 1, 2a, 2b, 3a, and 3b. It also prevents current flow during reverse battery conditions, and provides continuous power during the ISO 16750-2 superimposed alternating voltage test at severity level 1 where the peak-to-peak AC voltage is 1V. (It may temporarily shut off power in the presence of larger AC voltages.) Continuous power is provided to the load when the input voltage drops as low as 4V to satisfy the minimum supply voltage requirements of ISO 16750-2.
The MOSFETs in this circuit are protected by limiting the time spent in high power dissipation conditions, such as when the input voltage surges high during load dump or when the output is shorted to ground. If a fault exceeds the conditions specified in ISO 16750-2 and ISO 7637-2, MOSFET M2 shuts off to protect the circuit, reapplying power after an appropriate delay.
For example, a sustained 100V input voltage, or a downstream short-circuit fault causes the surge stopper to self-protect by limiting the current in M2 and then completely shutting off if the fault persists. This method has a distinct advantage over shunt-type protection, which must dissipate continuous power—blowing fuses in the best case; lighting fires in the worst.
Load Dump and Overvoltage Protection
To understand the operation of the circuit in Figure 8, consider a simplified description of the LTC4380. During normal operation, the LTC4380’s internal charge pump drives the GATE pin to enhance M2. The voltage at GATE is clamped to a maximum of 35V above ground (when the SEL = 0V), thereby limiting the output voltage at M2’s source to less than 35V.
The circuit in Figure 8 further improves on that voltage limit by adding a 22V avalanche diode D3, in combination with R6, R7, R8, and Q2 to regulate the output voltage to a maximum of the avalanche diode voltage, 22V, plus the base-emitter voltage of Q2, roughly 0.7V. When the output voltage exceeds 22V + 0.7V = 22.7V, Q2 weakly pulls down on M2’s GATE to regulate M2’s source and the output voltage at 22.7V.