Lpc2000

Hi! I am in trouble trying to find a voltage regulator suitable for my 
project; my project features an LPC2138, a GPS and a radiomodem. When 
idle, it draws around 20-25 mA from the 3.3V source, 500 mA when 
working hard and with short (50 mS) peaks of up to 1.8A. The supply is 
a lead-acid 12V battery (can't change that), and occasionally, when the 
battery is charging, the input voltage will rise up to ~20V. The 
problem is the regulator efficiency. We were going for the LM2676-3.3V 
switching regulator, because it seemed simple and in range, but two 
things made it a poor choice: it draws about 4.3 mA from the battery 
just for its own supply, and its efficiency when the load is in the 20-
25 mA range is awful (less than 50%, not counting Iquiescent). I looked 
everywhere but I couldn't find any other switcher with better 
performance in these conditions (12V in, 3.3V out, 20-25 mA average 
load, but >1.5A capability). And there's a catch! The radiomodem is 
extremely sensitive to VCC getting lower than 3.3V especially in the 
current peaks! Also, it should be relatively easy to find in ~400 
quantities (i.e., Digikey, Mouser or your suggestion).

Any ideas?

Guille