A typical 24-V battery protection application circuit shown in Figure 9-3. This circuit uses two unidirectional TVS diodes to protect from positive and negative transient voltages.

The breakdown voltage of the TVS+ must be higher than the 48-V jump start voltage, less than the absolute maximum ratings of the anode and enable pin of the LM74703-Q1 and LM74704-Q1 (65 V) and must withstand 65-V suppressed load dump. The breakdown voltage of TVS- must be lower than the maximum reverse battery voltage –32 V, so that the TVS- is not damaged due to long time exposure to reverse connected battery.

During ISO 7637-2 pulse 1, the input voltage goes up to –600 V with a generator impedance of 50 Ω. This behavior translates to 12 A flowing through the TVS-. The clamping voltage of the TVS- can not be the same as that of the 12-V battery protection circuit because during the ISO 7637-2 pulse, the ANODE to CATHODE pin voltage seen is equal to (-TVS Clamping voltage + Output capacitor voltage). For a 24-V battery application, the maximum battery voltage is 32 V, which indicates that the clamping voltage of the TVS- must not exceed, 75 V – 32 V = 43 V.

Single bidirectional TVS can not be used for 24-V battery protection because breakdown voltage for TVS+ ≥ 65 V, maximum clamping voltage is ≤ 43 V and the clamping voltage can not be less than the breakdown voltage. Two unidirectional TVS that are connected back-to-back need to be used at the input. For the positive side TVS+, the SMBJ58A with the breakdown voltage of 64.4 V (minimum), 67.8 (typical) is recommended. For the negative side TVS-, the SMBJ26A with breakdown voltage close to 32 V (to withstand maximum reverse battery voltage of –32 V) and maximum clamping voltage of 42.1 V is recommended.

For 24-V battery protection, a 75-V rated MOSFET is recommended to be used along with the SMBJ26A and SMBJ58A connected back-to-back at the input.