Lpc2000

Guillermo Prandi wrote:

>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).
>
>  
>
Keep in mind that most off-the-shelf supplies are intended for general 
purpose use / short run / prototypes.  Anyone doing significant 
production quantities is not willing to accept the built-in premium cost 
for the convenience of not designing your own.  I stopped looking at 
off-the-shelf supplies for my boards / systems a long time ago.

My reaction from what you were saying was that you need multiple 
supplies rather than a single supply.  Different regulator types for 
each current load profile.  Most switching supplies require a minimum 
load on them otherwise they can go into conduction (switching gate left 
on), you hear a term called "discontinuous mode", that helps prevent 
stalling.

What it sound like to me is that you need a modest supply to run the 
processors circuitry, then a larger supply to drive the heavier loads.  
The larger supply could be one that can be shutdown, thereby reducing 
standby current (minimum load to avoid stalling).

FWIW, 400 units, and assuming a $10 premium (wholesaler profit), that 
comes out to $4000.  I think that if you approach a good power suppy 
consultant, he would be more than happy to take $4000 to design you a 
supply which could only cost you one half of what an off-the-shelf model 
would be.

I know of one in my area (Pennsylvania) and deal with him from time-time...

Regards,

TomW

-- 
Tom Walsh - WN3L - Embedded Systems Consultant
http://openhardware.net, http://cyberiansoftware.com
"Windows? No thanks, I have work to do..."
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