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Trying to understand VEE

fliptronics

New member
I thought I had figured out what I needed when I selected
the CFAH1602C-YYH-JP , but I discovered that there is also
the CFAH1602C-YYH-JPV

From looking at other forum discussions, it seems that VEE
is somehow related to temperature range.

I downloaded both data sheets and compared them line for line.

On page 1, they show the different part numbers.
On page 3, the JPV includes a ref to negative voltage.
Page 5 shows identical temperature and voltage ranges.:(
Page 7 shows a VEE pin shared with the LED Anode
Page 8 shows identical schematics. :confused:
All other pages are identical.

If VEE is related to temperature range, data sheet does not show it? What's the story?

Is an external VEE supply required if the JP (not JPV) version is selected?

If the JPV version is selected, how is the VEE output used, and how does that affect how the LED is driven

In my application, the temp will be a normal work environment,
and my available power will be 3 or 4 AA batteries. I am planning
to make the LED switched, to save power. Can I run the (JP)
version on 3 cells (4.5V dropping over time to maybe 3.0V) or
4 cells (6.0V dropping to 4.0V over time), or do I need to regulate
the supply. Do I need the JPV, or an external negative generator.

How do I find out the stock availability on this display.

What other questions should I be asking.



Thanks for any insight.
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CF Tech

Administrator
Vee is not directly related to wide temperature operation.

What varies over temperature is the voltage needed for Vlcd, which is controlled by the voltage applied to Vo.

For office enviroments, you will typically need Vlcd to be about 4v. For a 5v supply, that would mean Vo should be set to 1v. If your Vcc is 3v, then Vo would need to be at -1v.

How you vary the voltage at Vo is up to you. Typically the wiper of a potentiometer is connected to Vo and ant the ends of the pot are connected to supplies that are such that you can set the voltage on Vo to a level that makes the dispaly readable.

OK, so now back to Vee. Vee is an output that is basically an inverted voltage of Vcc. So if your Vcc is 3v, Vee would output about -3v, or if Vcc is 5v, Vee would output about -5v.

Typically if you get a "-V" display that has the Vee output active then you hook a pot between Vcc and ground or Vcc and Vee.

It will be very annoying to have to adjust the contrast as the battery voltage drops. A better design might be to use two AA batteries, and a small switcher supply to kick the voltage up to a stable value over the life of the battery (maybe like this).
 

fliptronics

New member
Thank you for a very clear reply.

No good deed goes un-punished, so here are some followup
questions.

The app note you pointed me to (fairchild AN-42008) shows
several regulator configurations for battery powered systems,
all based on the ML4861. A detailed search at www.fairchild.com
only found this device in old app notes. No data sheet is
available, and it doesn't even come up in the obsolete parts
search.

Can you recommend any other such regulator?
Typical current for the LCD (not the LED) is 1.2 mA, what
is the Max that it can be ? 3mA?

As an alternative, since I don't want to have a contrast knob on
the front panel of my device, is it reasonable to control Vo with
either a low power DAC, or a digital pot (such as MAX5471).
I would need to know how much current is needed on Vo, and
is this source or sink. (4V from a 10K pot across 5V suggests
that this is about 500 uA).

Lastly, I have been also looking at "competitive" products from
other vendors (I'll be nice and not name them), and they have
nearly identical pinout, and the same controller protocol. Is it
reasonable to assume that they have the same Vo requirements?


Thanks
Philip
 

CF Tech

Administrator
The ML4861 is a regulator I used long ago on a project. You would have to find something more modern. Try Linear or Maxim. Look for something that is designed for the battery chemistry and number of cells that you plan to use.

Maybe try poking around on Linear's site starting here:
http://www.linear.com/pc/viewCategory.do?navId=H0,C1,C1003,C1042,C1031,C1061

This one looks interesting, it could use a single cell:
http://www.linear.com/pc/productDetail.do?navId=H0,C1,C1003,C1042,C1031,C1061,P2130

Or this one:
http://www.linear.com/pc/productDetail.do?navId=H0,C1,C1003,C1042,C1031,C1061,P2461

There are lots of possibilities.

The idea would be to use the regulator to power your entire system--not just the LCD. In this way, nothing changes over the life of the battery, and you can run the battery completely flat before shutting down. With the right design, the user can drop in alkaline, NiCd or NiMH with no changes, the switcher just takes whatever you give it and produces a nice 3.3v (or 5v) supply.

It is perfectly reasonable to use a DAC output to drive the contrast. It can also be done with a PWM (including negative voltage if you are clever with some diodes & caps). A digital pot will work fine, but is probably overkill.

Most of the character displays are all based on "HD44780" compatible chips. Ours are typically a KS0066. In any case they are all very similar. The range of Vlcd can vary quite a bit for different LCD configurations (duty cycle, viewing angle, temperature range, specific formulations of the LC chemicals). If your Vo pin can be adjusted over a reasonable range, you will most likely be able to switch displays quite easily. The best approach is to find a couple of displays early in the project, make sure your circuit works for all of them, then pick the best looking one that has the best price and availability . . . which will be the Crystalfontz one, of course ;)
 

fliptronics

New member
Thanks for your help,

For the benefit of others, I thought I should list my goals, and
the choices I have made.

In my application, I will have a battery operated unit that
I would like to have operate for several hundred hours on
what will probably be 3 or 4 AA batteries. The unit will have
both a low power mode (back light LEDs off) and a higher
power mode with the LEDs on. The longer the user uses the low
power mode, the longer the battery life.

The CFAH1602C-YYH-JP runs (typ) 1.2 mA with LEDs off, and
(typ) 130 mA with LEDs on. AA batteries (rechargeable) have
1200 to 2000 mAHours capacity. The LCD needs 5.0V, so if
I start with 4 cells, I have 6.0V and I need a step down
regulator, and as the battery voltage drops below 5.0V I
need a step up regulator.

In shopping for a suitable regulator, I looked at availability,
price, Quiescent Current, efficiency, inductor prices, footprint
compatibility.

My design will allow me to try each of the following without
changing my PCB (all for Linear Tech):
LT1615, LT3460, and LT1930.

In the SEPIC configuration they handle both step up and step
down conversion, are available from digikey, have varying levels
of low power operation, as well as the ability to be efficient in
both my low power and high power modes of operation. The
data sheets do not give enough info for me to figure out
which has the best power consumption efficiency for my
application, so I will build a few boards with each of these and
make some measurements.

I hope the above is useful to others.
 
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