Buck regulator maximum supply voltage VS. voltage spikes at input where we have a TVS diode
Normally, VCL (clamping voltage) of the TVS should not exceed the abs max input voltage of the device to be protected. Note that VCL is given for a maximum IPP (peak pulse current) value. For higher peak currents that VCL may not be valid. Also, VRM (standoff voltage) should be higher than the nominal input voltage. For two-battery vehicles (coach, truck, etc) the nominal battery voltage is 28.7VDC. According to these considerations, SMAJ30CA may not be suitable for you.

As a former designer for automotive electronics, I've never used TVS diodes anywhere but sensitive analog and digital inputs. Instead, I had put a varistor (a.k.a. VDR or MOV) following a fuse (if not present on the power line externally) in my designs.







simulate this circuit &ndash; Schematic created using CircuitLab

The advantage of using a VDR is that the VDR can absorb the excess energy created by the overvoltages or spikes and it can lead the fuse to blow, so the end-user can replace the fuse and the device can work again. This may not be possible when using a TVS since the TVS can get damaged even before the fuse to blow.

VDR serves good also for Load Dump. For better protection, Texas Instruments has a greatly detailed application note.