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Topic: Best way to regulate 4.2v to 3.3v (question) (Read 16476 times) previous topic - next topic

vwlowen

Jan 16, 2013, 03:51 pm Last Edit: Jan 16, 2013, 03:58 pm by vwlowen Reason: 1
Hi,

I'm building a small portable battery-operated device using the Nokia 5110 display which is a 3.3v device.  I've decided to use a small 110 mAh Lithium-Polymer battery to get the most capacity for its size.

Although the battery is nominally 3.7v (which I think would be ok for the display) when fully charged, it's 4.2v which I think is getting a bit too high.

I'm using a LDO 3v3 regulator which works fine. But, to conserve battery power I put the ATmega328 into Power Down mode. In this state, it takes less than 1.5uA but the regulator itself uses 0.5mA.  The project "awake" draws about 8.5mA so it's still a good saving but not as good as it could be without the regulator.

So, my question is does anyone know of a more efficient way to clamp the voltage to the display (and the ATmega328) to 3v3 without using the LDO regulator. To complicate things a little, I am very short of space in the project's enclosure.

Thanks
John.

CrossRoads

I'd say you need a switching regulator like this
http://www.pololu.com/catalog/product/2120
Not sure if the No Load Quiescent Current helps you much tho.
It does offer a ShutDown pin, can use it to turn off the regulator and prevent over-discharge of the battery.

110mA only offers 12 hours of total on-time. Are you recharging every night?

Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

vwlowen

It's actually a slave flash controller and "intervalometer" (http://en.wikipedia.org/wiki/Intervalometer) so it will only be used intermittently but, yes, I am anticipating recharging regularly. 

Thanks for the link. The efficiency at the sort of currents I'm using doesn't seem very good but I don't know how that compares with a standard LDO regulator.  It's worth getting one to play with anyway :)

The battery claims to have built-in protection http://www.technobotsonline.com/lipo-polymer-lithium-ion-battery-110mah.html ... The project is still being developed  on a 3v3 mains power supply (although the case size has been decided!) so I'll be able to put the battery through its paces once I get to that stage.

John.


John_S

A silicon diode in series will drop the voltage by about 0.7 volts.

CrossRoads

Yes, but as the battery drops down towards 3.7, the voltage will drop down towards 3, and if one is trying to maintain 3.3 ...
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

BillO

I just read the spec sheet for that display (PCD8544).  Looking at page 17 it seems that the absolute maximum Vdd rating is 7V.  Sure 3.3V is ideal, but apparently it will work fine at 4.2V.  Unless you have something other than the display that is limiting the voltage, I do not think the 4.2V to 2.8V range of the lithium battery will be a problem.

dc42

You can use a micropower regulator such as http://uk.farnell.com/microchip/mcp1702-3302e-to/ic-v-reg-ldo-250ma-to-92-3/dp/1331485, which has only 2uA quiescent current. I use the 5V version to power the mcu in designs that run from a 9V battery and have no on/off switch.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

vwlowen

As I understand it, the PCD8544 is the controller, not the display. As far as I know, the display itself is rated at 3.3v.   I have read a few articles on the 'net about people running them at up to 5v and getting away with it but, as I don't have a spare display, I don't want to chance it.  Also, when finished, I'll be writing it up as an article for my own website and, being a bit of a purist, I like to do it properly rather than what I manage to get away with :-)

Yes, I thought about putting a diode in series but, as has been pointed out, I'd be missing out on a lot of the battery voltage as it runs down. The LDO regulator I'm using only needs 0.2v differential so is half a volt better than a diode in that respect.

I can live with the 0.5mA consumption - I just wondered if someone had come across the issue of powering 3v3 devices with a 4v2 battery and found a quick and easy solution.  Given that most 4v2 batteries have built-in over and under-volt protection, I'm surprised they don't fit a regulator inside them as well !

John.


vwlowen


You can use a micropower regulator such as http://uk.farnell.com/microchip/mcp1702-3302e-to/ic-v-reg-ldo-250ma-to-92-3/dp/1331485, which has only 2uA quiescent current. I use the 5V version to power the mcu in designs that run from a 9V battery and have no on/off switch.


Ah, I like that :)

I'm using similar at present http://www.hobbytronics.co.uk/datasheets/LE33CZ.pdf but its Quiescent Current is quoted at 0.5mA (typical) - which is exactly what I found.  Yours is only 2uA and would be a simple direct replacement.... thanks!

John.

retrolefty

Quote
Given that most 4v2 batteries have built-in over and under-volt protection,


I would never make that assumption or statement. Only by verification of the specific batteries datasheet can one tell what if any internal protection it offers. Buyer beware. Many Li-Po cells have no such internal circuitry added and rely on it being provided external to the cell.

Lefty


BillO

It is, of course, your decision.  However, the LCD itself is supplied between 6V and 8.5V by the PCD8544 controller (see Vlcd2, page 18).  In this particular application, I believe PCD8544 generates these voltages itself to supply to the LCD from Vdd.  The PCD8544 is also capable of taking an external LCD power input, but that needs to be between 6 and 9V also.


As I understand it, the PCD8544 is the controller, not the display. As far as I know, the display itself is rated at 3.3v.

vwlowen

Yes, I must admit that I'd seen the datasheet but Adafruit ( http://learn.adafruit.com/nokia-5110-3310-monochrome-lcd/power-requirements ) seem so specific that it's a 3v3 device that I prefer to err on the side of caution!

Unfortunately, I based my PCB design on a display I ordered from the Far East (with the usual uncertainty about delivery) and it was only afterwards that I realized there are several different pinouts for what is ostensibly the same display. The bottom line being that I'm reluctant to push my luck until I can get a pin compatible spare :)



John.


BillO

#12
Jan 16, 2013, 08:57 pm Last Edit: Jan 16, 2013, 09:32 pm by BillO Reason: 1
The two outside ones look as if they have the resistors for the LEDs.  The middle one, well I can't see them, my guess is that you'd have to add an LED limiting resistor for that one.  Maybe they are top-side though.  Which one did you get?  

vwlowen


Quote
Given that most 4v2 batteries have built-in over and under-volt protection,


I would never make that assumption or statement. Only by verification of the specific batteries datasheet can one tell what if any internal protection it offers. Buyer beware. Many Li-Po cells have no such internal circuitry added and rely on it being provided external to the cell.

Lefty


Yes, that's true.  I asked specifically about my battery before I ordered it so I know it is protected.  Perhaps I'd have been better saying that those batteries which are protected with internal circuitry could have an internal voltage regulator at very little extra cost :)

John.

vwlowen


The two outside ones look as if they have the resistors for the LEDs.  The middle ones, well I can't see them, my guess is that you'd have to add an LED limiting resistor for that one.  Maybe they are top-side though.  Which one did you get? 


The left hand one. I'm driving it via a 120R resistor from a PWM pin on the Arduino so I can control its brightness. On the left hand one, the LED input is active low - ie analogWrite(PWM_pin, 0) turns it on full.  The middle one is active high. I don't know about the right hand one - that's just a photo from the Adafruit site - I haven't got one of those.

The middle one has a (missing) diode and a capacitor on the top side.


John.

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