In selecting the TVS, important specifications are breakdown voltage and clamping voltage. The breakdown voltage of the TVS+ must be higher than 24-V jump start voltage and 35-V suppressed load dump voltage and less than the maximum ratings of LM74722-Q1 (65 V). The breakdown voltage of TVS– must be beyond the maximum reverse battery voltage –16 V, so that the TVS– is not damaged due to long time exposure to reverse connected battery.

Clamping voltage is the voltage the TVS diode clamps in high current pulse situations and this voltage is much higher than the breakdown voltage. In the case of an ISO 7637-2 pulse 1, the input voltage goes up to –150 V with a generator impedance of 10 Ω. This translates to 15 A flowing through the TVS– and the voltage across the TVS is close to its clamping voltage.

The next criterion is that the absolute maximum rating of cathode to anode voltage of the LM74722-Q1 (85 V) and the maximum V_DS rating MOSFET are not exceeded. In the design example, 60-V rated MOSFET is chosen and maximum limit on the cathode to anode voltage is 60 V.

During ISO 7637-2 pulse 1, the anode of LM74722-Q1 is pulled down by the ISO pulse, clamped by TVS–, and the MOSFET Q1 is turned off quickly to prevent reverse current from discharging the bulk output capacitors. When the MOSFET turns off, the cathode to anode voltage seen is equal to (TVS clamping voltage + output capacitor voltage). If the maximum voltage on output capacitor is 16 V (maximum battery voltage), then the clamping voltage of the TVS– must not exceed (60 V – 16) V = –44 V.

The SMBJ33CA TVS diode can be used for 12-V battery protection application. The breakdown voltage of 36.7 V meets the jump start, load dump requirements on the positive side and 16-V reverse battery connection on the negative side. During ISO 7637-2 pulse 1 test, the SMBJ33CA clamps at –44 V with 12 A of peak surge current and it meets the clamping voltage ≤ 44 V.

SMBJ series of TVS are rated up to 600-W peak pulse power levels and are sufficient for ISO 7637-2 pulses.

If 40-V rated MOSFET is choosen then maximum voltage across C (Drain of the MOSFET) and A (Source of the MOSFET) must not exceed 40-V. For 40-V MOSFET selection, two back-to-back connected uni-directional TVS diodes are required to protect against input transient events. On the positive side, the SMBJ33A TVS diode can be used for 12-V battery protection application. However on the negative side, TVS has to withstand 16-V reverse battery connection and clamping voltage has to be –(40 V - 16 V) = –24 V. SMBJ16A can be used.