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

[barkerben]

Looking through the data sheets, I might be able to get away with running eveything from 3.3v (which would make eveything very nice and simple. 3.3v regulators easy to get, everything running at same voltage)- it all depends on whether the GSM module (GR47 from erricson) can run at this level. It says 3.4v minimum, but it might work :-p

Ben

[Ron Marcus]

Looking through the data sheets, I might be able to get away with running eveything from 3.3v (which would make eveything very nice and simple. 3.3v regulators easy to get, everything running at same voltage)- it all depends on whether the GSM module (GR47 from erricson) can run at this level. It says 3.4v minimum, but it might work :-p

Ben

It's the ZX 4125 from Wonde Proud. It will operate up to....get this...3.6 volts! I am using one for a remote logging device. The input pins should not go above 3.3 volts. Slip a shottky diode in series with the PIC B+ line to drop it a bit for the I/Os. The GSM unit will be very finicky about voltage sags, so feed it well!

[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.

:-)

[Ron Marcus]

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.

:-)

OK. That will work, but if it is a linear regulator, it dissipates the difference voltage in the form of heat, and is very wasteful. As far as lithium rechargables, they have a very flat discharge voltage until they are out of gas. This might be your best bet as you can lose the regulator all together. Look at the Maxim website for switching regulators. I would personally use a boost regulator with a couple of alkaline "C" cells if you have the room. I have also used two pairs of AA cells in parallel. Maxim has a couple of small regulators that will put out up to 3 amps until your batteries have been drained to under 2 volts (deader than dead?). With all of the switchers, noise is an issue. Shielded inductors and low ESR caps are a must. They are fairly easy to work with and maximize your battery life.

[barkerben]

Ok -I may in fact run most of the circuit off 3.3v, with the GSM off a seperate regulator, as the first section with just GPS and LCD is going to be ready first, and the system is modular.

3.6v seems a really annoying voltage - non standard and a pain to generate :-)
I could use a variable output voltage switching regulator, as you suggested.

Will look into it,

Cheers,

Ben

[barkerben]

Came across: LP2966

Single chip, provides 3.3 and 3.6v outputs, with up to 150mA on each.
This isn't enough for the GSM module, but if I'm only reporting position via GSM every so often, I could possible use the regulator to charge a capacitor, and then the capacitor to drive the module. As long as the module is off a lot more than it is on, this should work...

Ben

[Ron Marcus]

Came across: LP2966

I could possible use the regulator to charge a capacitor, and then the capacitor to drive the module. As long as the module is off a lot more than it is on, this should work...

Ben

Unless that cap is relatively gargantuan, it won't work. If you don't have size issues, get a higher voltage battery pack (8 AA's?), and use a step down adjustable regulator. Set it to 3.5 volts. You will be amazed at the capacity. Many of these gems run at 95% efficiency, so drawing an amp at 3.5 volts will only draw 300 mils at 12 volts. This as opposed to running a linear regulator which will draw an amp, and become a circuit board warmer!

Hope this helps,
Ron

[barkerben]

Maxim has a couple of small regulators that will put out up to 3 amps until your batteries have been drained to under 2 volts

I've had a look around, and can't find any step ups that do this - the MAX711 will only provide around 300mA. The peak current drawn by the GSM module claims to be up to 2A.

Hmmm... will keep looking

Thanks for all your help,

Ben

[barkerben]

...ah. The MAX1771 looks like it might do it. It needs a fair few external components, but it looks quite straightforward.

Cheers,

Ben

[Ron Marcus]

MAX 1771
www.maxim-ic.com/quick_view2.cfm/qv_pk/1030/ln/

Simple to use, and easy to solder in an 8 pin package. What more could you want out of life?

Ron