Roughly dropping voltage to charger IC

[CheekyRobot]

I need to charge a 1cell 2000mAh Lipo on my device. Normally, it's strapped to a vehicle and can keep itself topped up using the vehicle's battery and I'd like to be able to charge it via a USB charger when it's taken off.

I might use something like a MAX1551 http://www.farnell.com/datasheets/1675715.pdf It gives me a dual input from USB as well as an ac adapter input which I could connect to the vehicle's 12v battery to keep the LiPo topped up.

Now... what this question is about is whether it's a better idea to just use a resistor to drop the 12v input from the vehicle's battery to the 4-7v that the charger IC would like (23r ish at 1.8w if I'm drawing the 300mA that the datasheet suggests) or to pop in a regulator. I'm a little loathed to use a 7805 or similar given how inefficient linear regulators are or should I go all fancy and put in a buck regulator? (expensive?).

The device itself only draws about 200mA absolute max spike current, so we're not really pushing a lot of power about here.

Any thoughts or suggestions?

[Zero999]

If you want cheap and cheerful, you could use a Black regulator:
http://www.romanblack.com/smps/smps.htm

[CheekyRobot]

If you want cheap and cheerful, you could use a Black regulator:
http://www.romanblack.com/smps/smps.htm

Cute! Though that big-old inductor is gonna kill me. This all fits into a VERY small box