The typical 24-V battery protection application circuit shown in Figure 10-3 uses two uni-directional TVS diodes to protect from positive and negative transient voltages.

Figure 10-3 Typical 24-V Battery Protection with Two Uni-Directional TVS

The breakdown voltage of the TVS+ must be higher than 48-V jump start voltage, less than the absolute maximum ratings of anode and enable pin of LM74700D-Q1 (65 V), and must withstand 65-V suppressed load dump. The breakdown voltage of TVS– must be lower than 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 action translates to 12 A flowing through the TVS–. The clamping voltage of the TVS– cannot be same as that of 12-V battery protection circuit. Because during the ISO 7637-2 pulse, the Anode to Cathode voltage seen is equal to (–TVS Clamping voltage + Output capacitor voltage). For a 24-V battery application, the maximum battery voltage is 32 V, then the clamping voltage of the TVS– must not exceed, 75 V – 32 V = 43 V.

Single bi-directional TVS cannot be used for 24-V battery protection because breakdown voltage for TVS+ ≥ 65 V, maximum clamping voltage is ≤ 43 V and the clamping voltage cannot be less than the breakdown voltage. Two uni-directional TVS connected back-back must be used at the input. For positive side TVS+, TI recommends SMBJ58A with the breakdown voltage of 64.4 V (minimum), 67.8 (typical). For the negative side TVS–, TI recommends SMBJ26A with breakdown voltage close to 32 V (to withstand maximum reverse battery voltage –32 V) and maximum clamping voltage of 42.1 V.

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