Current source design (tricky?)

Since this appears to be a vehicle application there may be limited facility for a large heat sink.

In normal operation the MOSFET must be able to dissipate (15-11)*2.5 Watts. Outside of that it may be useful to implement a current shutdown/turndown, either as a foldback (based on the 11 ohm load line), or temperature sensed (based on a switching PTC).

That addition definitely makes for an improved transient response- by a factor of 50x from what I can see. The big 1000u across the regulator limits the current perturbations to under 5% when subjected to things like that +30/-300V 20u/30u transient, and the 22 ohm resistor makes for a nice low pass attenuating Volts-transient to mV-transient applied to the circuit and on a time scale it can follow. Since we can't put a component in series with the main current path, the MOSFET is on its own, with the body diode saving it during negative transients, but no protection from positive transients. The part numbers suggested thus far are shy on Vbr,dss.

Hey, I said "slab" of metal (firewall) not "block" of metal!

....at 150 C when the engine is running, on a hot day.

Why do you think most automotive Electronic Control Units are located under the driver's seat or in the kick panel to the left of the "clutch" foot?

Under-hood is VERY inhospitable... when I was in the automotive electronics biz I designed for a normal high of +140°C.

...Jim Thompson

Indeed. The OP didn't say anything about operating inside an actual automobile, let alone in the engine compartment, but if he has this in mind, he should speak up and let us deal with it!

In article , Fred Bloggs wrote: [.. saving band width ..] Go back one level to see where I added the capacitor.

I think the 50x factor assumes a tighter tolerance than we can expect in real life. The slightly mismatched capacitors we are likely to have in real life will still match near exactly on the initial step but the ext(-t/T) curves will not match.

It would be nice if we could make the circuit not rely on the match. If we had a rail-to-rail op-amp, or something like the LM301, we could shift both inputs of the op-amp up near the positivve rail. As it is, the LM741 will work with its inputs up within about 4V of the positive supply. If we changed the resistor dividers to make them a 1/3,2/3 ratio, we would get an improved transient performance.

Since the LM741, IIRC, only draws about 3mA, the 22 ohm resistor in the minus supply to that section could be increased to lets say 100.

We can put a small inductance in series. It won't help much except to round off very short timed pulses. I don't think such things are common in automotive power systems though.

The problem with looking for a higher Vbr(dss) is that Rds(on) runs about as the square of Vbr(ds) for the same sized part.

Practically anything in the engine compartment gets very hot. But it was just a hint to consider Murphy's Law.