If it shuts down "gracefully" (i.e. not letting the smoke out) I'm okay with that... 99% of the time I expect the LEDs to be used when the car is off anyways; I just want to make sure that IF they're used when the car is on, I don't smoke the LEDs or regulators. If an overvoltage just results in a little LED flicker I can live with it, with the idea that the filtering should make it infrequent enough to not really be noticeable.
I tried to do the thermal calculations to the best of my meager ability, but I'm also trying to keep this all in a small(ish) ventilated enclosure. Since I'm just using the regulators to "take a little off the top" (~14V -> 12V) I'm figuring the per-regulator thermal dissipation should be no more than:
[LATEX]P_D = IV = 0.6{\text{A}} \times 2{\text{V}} = 1.2{\text{W}}[/LATEX]
If I'm reading the datasheet right, and assume that the temperature in the car will very seldom exceed 140°F (especially while we're actually using the lights), I should be able to go by the following:
[LATEX]T_{R(max)} = T_{J(max)} - T_{A(max)} = 125^{\circ}{\text{C}} - 60^{\circ}{\text{C}} = 65^{\circ}{\text{C}}[/LATEX]
thus, the maximum allowable junction-ambient thermal resistance is
[LATEX]\theta_{(J-A)} = T_{R(max)} / P_D = 65^{\circ}{\text{C}} / 1.2{\text{W}} = 54.17^{\circ}{\text{C}}/{\text{W}}[/LATEX]
Since the datasheet says "If the maximum allowable value for θ
(J−A) is found to be ≥ 53˚C/W for the TO-220 package [...] no heatsink is needed since the package alone will dissipate enough heat to satisfy these requirements" any heatsinking is really just superfluous. Of course I like a little margin of error, so I threw in some heatsinks
I got two sets of LEDs because they're cheap enough, and if one looks like crap or fries on the first test run, I have a fallback. One set of LEDs are 12V/300mA units, and the other are "3 watt", so I assume 12V/~250mA. Both sets are allegedly designed as automotive lamps, but given that they're Chinese eBay specials, I don't want to assume too much about any built-in noise/voltage/current regulation. The second set actually appears to have some filtering/regulation hardware built in (I can spot an inductor, what looks to be a quad set of diodes as a bridge rectifier, and what I think is a small SMT regulator with attendant resistor and capacitor); the first set though appears to just be a bunch of LEDs stuck on a board with the connector attached. Even if the second set has all the necessary regulation built in, I'd rather have something a little more robust than whatever they managed to fit on a 0.5" x 1.5" PCB... call me crazy