Hi,
For my application the current needs are 20mA when active and 01.156mA when sleep.I am in look out for 11V to 3V dc-to dc converter.
i am not able to decide whether to go for switching regulator or linear regulator...
our application has analog section too....
I am drawing power from battery so in order to avoid battery drainage i thought to proceed with switching regulator but its efficiencies are poor at low current outputs.....
whats the thumb rule to choose b/w the two.....

 

You might have a look at this regulator:
http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1042,P88521
Better than 90% efficiency at 1mA output, efficiency increases up to the limit of the regulator.

 

With an 8 Volt drop, there will be a huge difference in the efficiencies of a Linear Regulator vs. a Buck switching supply.

I'd suggest going with the Switching Supply (Buck), linked above by Wookie. Using a linear regulator, you'll be wasting more power (as heat) in the regulator that you'll be using by the circuit.

 

As thatoneguy says, with 11v in and 3v out using a linear regulator, better than 72% of your power dissipation will be in the regulator itself. There is also some "overhead" power that's consumed in the regulator itself. So, a linear regulator cannot be more than 27% efficient in your circuit.

A worst-case efficiency of better than 90% in a switching "buck" regulator is quite attractive by comparison.

You may require some additional filtering on the output of the switcher; your analog requirements are vague at best.

 

But, the current draw is only 20mA. How long must this run on one battery and how big is the battery? Maybe fooling around with a switching regulator isn't worth doing because--it isn't worth doing.

 

...how big is the battery?

You're asking us?

 

But, the current draw is only 20mA. How long must this run on one battery and how big is the battery? Maybe fooling around with a switching regulator isn't worth doing because--it isn't worth doing.

It's probably a fairly hefty Li-Po battery. Such a battery could last quite a while with a 20mA run/1mA sleep cycle if it was well planned out.

 

battery we choose is Li-ion battery with 6.4Ah , 11.1Vnominal voltage.

Power consumption is an issue. As there are other systems running in with the same battery in parallel.
Charge pumps will have moderate efficiencies at low power operations wont they??
Especially when current is low ....
But here too I should stick to filtering I presume.

 

Forgot to mention that Other system which is in parallel is so power hungry that the whole battery drains out in 6days. It needs continuous current of 60mA at 5V no sleep cycle. here we use a buck regulator to derive 5V.

Thought of not to add the burden to battery..

 

Forgot to mention that Other system which is in parallel is so power hungry that the whole battery drains out in 6days. It needs continuous current of 60mA at 5V no sleep cycle. here we use a buck regulator to derive 5V.

Thought of not to add the burden to battery..

Well if you want a linear, why not derive it from that 5V with a low dropout regulator? There are some nice 3.3 and 3V micropower ones with low voltage drops (less than 385mV) and the efficiency will be better. Still not as good as a switcher but much better than deriving it from 11V would be.

If you can't get the LT device referenced earlier, I like these http://www.maxim-ic.com/datasheet/index.mvp/id/3269 similar thing, just what your favourite stockist has in really.

 

I still havnt decided on which to choose.....may be a small analysis would help me decide....
Actually uC runs on 3V . 5V is used for a heater coil.....
So if the battery starts dying down uC will shutdown the coil .......
Even when the battery drains out uC will derive power from a secondary battery source....
I have not used Switching regulators before and usage of filters for clean power supply and what the imapct it would have in ADC read ??.... will there be any unstability factors coming up???.....
uC also has interfaces with wireless physical layer chipsets ....