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low voltage regulator bypass - is there a better way?

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m12lrpv:
I'm working on a battery powered circuit that I've tested down to 2.5v without a problem.
The whole circuit is made of components that can handle up to 5v but I will be running it at 3.3v via a 1117-3.3 linear regulator.
While the general current draw of the main circuit is very low. It can rise to as much as 30mA when the LCD which forms part of the project is switched on.
There are no issues with the main circuit.

What I am trying to do that has me asking for help is to bypass the regulator when the battery voltage drops to a certain level so as to extract every last bit of energy out of the batteries.

I've tested the circuit attached and it works and in fact works better than I initially expected when controlled by a micro (which it will be).


However I really want to reduce the 2mA to 3mA of wasted current through the base. If I could drop it down to 1mA or less I'd be happy


I looked at mosfets but they don't seem to be suitable for what i'm doing (I don't understand them well though).

Is there a better way or is the simple pnp bypass that i've been playing with the only way to go?

free_electron:
so you are concerned about a little bit of drop in the transistor, but at the same time you burn all this energy in the regulator...
you're solving the wrong problem... chuck the linear regulator in the bin and move to a switching regulator. if you go for a sepic construction you can really suck that battery dry ...

m12lrpv:

--- Quote from: free_electron on May 02, 2012, 05:21:04 am ---so you are concerned about a little bit of drop in the transistor, but at the same time you burn all this energy in the regulator...
you're solving the wrong problem... chuck the linear regulator in the bin and move to a switching regulator. if you go for a sepic construction you can really suck that battery dry ...

--- End quote ---

Regulator losses were going to be the next test just to see if I can live with it.

I took one look at all the extra components required to put in a switching regulator and really baulked at it.

Rufus:

--- Quote from: m12lrpv on May 02, 2012, 04:51:51 am ---Is there a better way or is the simple pnp bypass that i've been playing with the only way to go?

--- End quote ---

Not using such a junk LDO would be a start. The Microchip TC1014 has about 85mV drop out at 50mA rather than about 1v from an LT1117 for example.

Psi:

--- Quote from: m12lrpv on May 02, 2012, 05:35:03 am ---I took one look at all the extra components required to put in a switching regulator and really baulked at it.

--- End quote ---

It's not that bad, all you need is
- Switchmode IC (probably with built in driver) 
- Schottky diode
- Inductor
plus maybe a few caps and resistors to set the voltage and frequency

For example, quick google image search found this simple one.
The IC is even available in a 3.3v version (5v version shown)

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