I have had some experience in the past designing control sticks that would be used in machine, truck, and automotive applications. We had the best luck using switch mode VRs (voltage regulators) that were protected in several ways. One part of the scheme was to use a VR that was capable of functioning with input voltages up to 60V. Another part sometimes considered was to add a switching circuit using a high voltage FET at the input to disconnect the VR momentarly whenever the input voltage would go over say 55V as in the event of a load dump which can result in inputs as high as 120 volts. When the switch is used a combination of L and C input filters would be used to keep the VR operational through the time that the FET switch was off during the load dump pulse from the alternator. Another part that was used was to use a liberal design of input filtering including L/C pi filter networks to cut spikes and then follow this with high energy capacity transzorb devices designed to start their clamp action at around 45 volts or so.

Engine cranking is indeed a troublesome issue when you try to deal with converting VBAT in a vehicle to local operational voltages that may indeed at times may be higher than the input voltage!!

There is a really nice switch mode VR made by Linear Technology that provides a workable solution taking the above ideas into account. Their LT3433 is a buck-boost multi-mode VR that can work from 4 to 60 volts input and supply an output of 3.3V to 20V at about half an ampere. Take a look here:

http://www.linear.com/pc/produc...1790,P2332

I've used this part and can attest to it working quite nicely.

Michael Karas