In the below circuit, the DC component (6V) of a bridged car head unit speaker wire turns on a LM53601 buck converter. The LM53601 accepts voltages up to 42V. The +12V input into Q2 is protected to withstand the pulses described in ISO 7637. I have used this circuit in my car for some months now, and it works fine.
But now I want to extend the functionality so that the LM53601 is turned on either with the +6V speaker signal or when the ignition is turned on. The EN signal of the LM53601 accepts up to 42V, but I believe that I will have to protect this input for the pulses of ISO 7637 as well. My proposed solution is shown in blue where I suggest to use TVS-diodes and a reverse polarity diode. This seems like an overkill solution to me, but I have not found any other way to protect the input.
My question is regarding Q2. Should I put a diode (STPS2200U) or a resistor (or both) in the point where I have drawn a red arrow? The 3906 PNP transistor specifications are listed below.
Update: The transient protection circuitry was taken from a TI reference design using a similar buck converter that has the same voltage input range (see below). This passed ISO 7637 testing so that is why I thought it was smart to use the same design. The design uses a smart diode controller (LM74610) for reverse polarity protection, but I have used a 200V Schottky instead which I believe should be ok.