3.6v regulation

[barkerben]

Hi,

I'm building a circuit a PIC running off 3.3v. Another part of the circuit needs between 3.6-4 v.

Using NiMh or NiCd this is fine - 1.2*3=3.6. So run the 3.6v section straight off the batteries, and the 3.3v section through a regulator.

But if alkaline batteries were used, the voltage would be 4.5v. This is too high for my 3.6-4v circuit (might be ok, rather not try though)

If I put a regulator in for the 3.6v supply, it will always be in dropout, as 4.5-3.6 is only 0.9v, and I can't get a dropout that low at 350mA. Even if I could, when rechargeables were used, the regulator would cut out again.

There must be a very simple solution....?

[jmgelba]

3 batteries at 1.2v will only give you 3.6v for a short while depending upon how much current your circuit requires. You will hit 3.3v fairly soon.

Can you use 4 batteries?
4 x 1.5 = 6V Plenty of headroom for an LDO
4 x 1.2 = 4.8V Still plenty of room for an LDO

Is this high volume, whats your cost for the circuit? Is there enough budget to use a switcher?

Here is a simple circuit using Linear Technologies SwitcherCAD program.
3.2 to 6v input, 4.0v output at up to 350mA.

[Ron Marcus]

<<Here is a simple circuit using Linear Technologies SwitcherCAD program.
3.2 to 6v input, 4.0v output at up to 350mA.>>

There's something missing with the above circuit. You have a straight line from input to output through the diode. If you go over 4.3 volts input, the voltage will be over what you need.

Why must you run the PIC circuit at 3.3 volts? It won't work at 3.6?

[barkerben]

Hi,

The PIC will run at 3.6v. However, my circuit contains 3 main modules:

PIC 16F628
GSM module (GR47)
GPS module (ZX4120)

The PIC can run at a very wide range of voltages (2.2-5.5 I think)
The GSM module can run from 3.4-6v (including ripple)
According to the datasheet, the GPS can run from 3.3v +-5%. However, in the same data sheet, it is specified that it can run up to 3.6v.

In this case I can run eveything from a 3.6v supply, which would be good :-)

Maybe I should email the manufacturers and ask them about the descrepancy...


Ben

[Ron Marcus]

Who makes the GPS module?

[barkerben]

It is sold by Crownhill (same people who run the forum it seems). I think it is made by Nemerix - they certainly make the internal chipset, although I'm not entirely sure :-)

Ben

[jmgelba]

<<Here is a simple circuit using Linear Technologies SwitcherCAD program.
3.2 to 6v input, 4.0v output at up to 350mA.>>

There's something missing with the above circuit. You have a straight line from input to output through the diode. If you go over 4.3 volts input, the voltage will be over what you need.

Why must you run the PIC circuit at 3.3 volts? It won't work at 3.6?

I believe the cap provides isolation?
All I did was create the i/p and o/p specifics in Switcher CAD and it produced the above circuit. The image was a screen shot of the schematic.

EDIT: Nevermind, I posted the wrong schematic.

[barkerben]

That's great - do you reckon the 120 and the 125 are the same? It seems to me that one just has an aerial module bolted to the top, so the insides are likel the same...Assuming they are, I can run the whole circuit off 3.6v.

Any idea what is the voltage stability of a LiIon battery like? Since phones use them, it must be ok I guess. If the GSM module is fussy about voltages, and I get a voltage drop from the nominal 3.6 as soon as I load the circuit, that is not much good :-)

I could of course use a 3.6v linear regulator, and a higher voltage battery, except that they don't seem to exist as far as I can see!

[barkerben]

hmm...I have found a 3.6v regulator. It can supply up to 1.5A, which I hope will be more than enough. The GSM module specs is a bit vague - it says consumption is <350mA when transmitting data, with peaks of <2A. Should do the job... and I can run the regulator off 4 AA cells.

:-)