Your maths is bad. You have a supply voltage of 14V with the engine running and claim to need 1.25A @12V into your regulator. That's 2V drop so your resistor cant be greater than 2/1.25 = 1.6 Ohms. I'm not even going to consider that you want 15W ( 1.25A @12V) as I know there isn't a snowflake's chance in hell of doing that with a reasonable size and cost series resistor so lets scale it back a bit
500mA @5V out is 2.5W. Assuming 90% converter efficiency
(probably a bit optimistic), that's about 2.8W which @14V in is 200mA. That's getting into the region where a pulse rated resistor isn't going to be cost-effective. Even if the MCU board only needs another 500mA (@5V), you'll probably run into trouble with the resistor, but lets run with 5.6W in for the sake of argument.
You *MUST* limit the TPSMA6L15A input voltage at no more than 20V (to stay under its 24V Abs. Max. rating and keep within its working range). Lets say you need 5.6W into the switching regulator. You don't want more than about 1V drop across the resistor in normal operation, to keep the dissipation reasonably low and the efficiency reasonably high. 5.6W @ (14V -1V) is 0.43A, so you can't make the resistor over 2.3 ohms.
If you run a sim with a worst case ISO 7637-2 Pulse 5B
# , with the vehicle's central clamping system voltage parameter Vs* set to 35V, into a 2.2 ohm resistor feeding a 5.6W constant power load, with 1000uF of bulk input capacitance clamped at 20V, you get a peak dissipation of 68W and about 22.5 joules dissipated in the resistor during the pulse. To be certain your generic wirewound resistor survives that, its going to need to be a 14W one (x5 overload factor). If you get a brand name pulse rated one (x10 overload for 1s), you could use a 7W one. If you dig deep into the manufacturers datasheets for I
2t ratings, as the total overload duration is only about 400ms, you can probably find a 5W one that will survive and *MAY* even find a 3W one.
However if you test against a worst case ISO 7637 pulse 5A,
(that's the one with no clamping), the peak dissipation is 755W and about 140 joules dissipated in the resistor, so the odds are your resistor's element is now a rapidly expanding cloud of metal plasma and its body is hot ceramic shrapnel bouncing around inside your device.
YMMV with the results above as I cant run LTspice XVII to use its ISO 7637-2 pulse models I'm stuck with LTspice IV so have to use my carefully hand crufted ISO 7637-2 Pulse 5A and 5B sim. Its as close as I can get it to the spec. but hasn't been independently verified. Sim attached.
Also its *really* difficult to do high energy clamping at such a low voltage and still pass the x1.5 sustained input overvoltage test. You need a TVS that's totally off under 18V, with a worst case clamping voltage while carrying the maximum possible peak current the supply can produce (which is equal to the maximum output current the alternator can produce unless you can limit it with a series resistor) that's under 24V. If the clamping voltage is too high, your switching regulator fries. If its rated holdoff voltage is too low, the TVS diode can fail during a jump start or if the alternator regulator fails and overvoltages the whole electrical system. If you are lucky it fails shorted and the supply fuse to your device will blow. If you are unlucky, the TVS diode ruptures and no more protection.
TLDR: Above a couple of watts of load, series resistor + shunt clamping is impractical for such a low clamping voltage. You'll need to use series MOSFET + over-voltage disconnect controller protection. Stick a small 100V TVS diode shunting the power in in front of the MOSFET to tame fast HV spikes to what the MOSFET can handle.
# Yes, I know ISO 7637-2 Pulse 5A and 5B have been superseded, but its what I have sims for.*WHOOPS!*
I just spotted an error I made in the sim while tidying up the currently commented out 24V pulse 5B. It should be:
Code: [Select]
.param Uss 70
and the also commented out 12V pulse 5A should be
Code: [Select]
.param Uss 1K
They got swapped.