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jecottrell
Joined: 16 Jan 2005 Posts: 559 Location: Tucson, AZ
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3v LCD Source? |
Posted: Sun Aug 28, 2005 7:23 pm |
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Hello All,
(Pardon the non CCS topic, but you guys have been such a great help with everything I've run into.)
Tomorrow (Monday) I'll be back on the phone trying to locate the following animal:
3v LCD Character Display, 16x2, 16x4, 20x2, or 20x4.
Design issues are:
3V Radio is required and implies that to keep things simple everything else should be 3V. (I can deal with level shifting and dual supplies if required, but I would like to avoid that.)
Secondary issue is power. My options are 2AA or 4AA. That leaves me with 2.4V (2AA NiCad) up to 7.2V (2AA Li-Ion). I would prefer a 2AA or 4AA Alk-Mang option to eliminate the recharging circuitry (start to get the simplicity drive here?)
So....If I can find a source for the 3V LCD I can use the 4AA Alk-Mang option with a simple 3V LDO and a couple of caps and just live with the inefficiencies of the LDO. (Or more likely a 3V switcher.)
If I can't locate a 3V LCD then things get trickier (I think). My options would be:
1. 2AA Li-Ion, recharging circuitry, 3V LDO and 5V LDO.
2. 2AA (Ni-Cad or Alk-Mang) with 3v Switcher and 5V Switcher, and recharging circuitry if required.
3. Some other topology that hasn't occured to me yet. (I'm not an EE, I don't play one on TV, and I didn't stay at Holiday Day Inn Express last night.)
What I have attempted so far:
Searched the CCS forum for simialr problems. (With no luck.)
Googled 3V LCD. Vishay lists many "5V (3V available)" but trying to find a knowledgeable technical sales rep late on Friday afternoon was impossible.
Brainstormed through all sorts of power configurations, to the point of experiencing a cranial overheat.
Finally, broke down and posted an off topic message in a forum that has provided great ideas in the past.
Thanks,
John
BTW, ran into an EE question while trying to come up with a power solution. Could someone tell me if it is possible to configure a switcher to provide step down regulation when the supply is above the regulated value and step up when it goes below? i.e. 4AA supplying 6V nominal to a 5V switcher. At some point, early in the life of the 4AA Alk-Mang cells the V will drop below the 5V of the switcher and require stepping up.
Or does a switcher inherently provide this functionality? |
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newguy
Joined: 24 Jun 2004 Posts: 1903
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Posted: Sun Aug 28, 2005 7:47 pm |
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Many moons ago someone on this board mentioned a very low power consumption LCD that ran on a 3V supply - and it stuck, luckily for you.
http://ca.digikey.com/scripts/DkSearch/dksus.dll?Detail?Ref=450832&Row=178928&Site=CA
It's a COG (chip on glass) 16x2 lcd module. The digikey entry lists its power supply as +3/-7, but I haven't read the datasheet to see what the -7V would be for.
In the original discussion/mention of the thing, someone said that its current consumption was low enough that it could be powered from one of the pic's output pins. I remember that. |
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jecottrell
Joined: 16 Jan 2005 Posts: 559 Location: Tucson, AZ
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Posted: Sun Aug 28, 2005 10:12 pm |
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Thanks for the info. Sorry to add conditions after the fact.....
I would like to use a module (needs to go in an enclosure that is configured for "standard" LCD dimensions. So COG would complicate mounting.
The LCD typically requires Vdd-Vo = 4.5V so that can be statisfied in a 3V system with a simple negative voltage generator. That is typically where the -V number comes into play. (I think.)
John |
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jecottrell
Joined: 16 Jan 2005 Posts: 559 Location: Tucson, AZ
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PCM programmer
Joined: 06 Sep 2003 Posts: 21708
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jecottrell
Joined: 16 Jan 2005 Posts: 559 Location: Tucson, AZ
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Posted: Mon Aug 29, 2005 12:45 am |
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Excellent.
I'm getting closer.
Previously I was concerned about the 3V PIC being able to drive the 5V logic in the LCD controller. But I had gooned up the numbers. PIC should put out Vdd - 0.7V for Voh and LCD controll needs 2.2V for the input. So I should be safe. (I had pulled incorrect numbers from the data sheet earlier.)
I think I may even be able to live without the negative voltage generator LCD module (if one isn't available in the size/config I need) if I use dedicated neg volt generator ( Microchip TC1142) to get the Vdd/Vo bias great enough to have a readable contrast.
Also found a good part for my power dilemma:
http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1742
Just what the doctor ordered.
Thanks! |
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