LCD 2 Digit Display Driver - Capacitor AC Coupling

[Klaus Kragelund]

Hi

I'm working on a project in which I need to add a 2 digit LCD (old
style reflective type)

Normally one would drive the display common with a square wave and
drive the segments seperately with another square wave that is either
in-phase or out-of-phase depending upon if the segment must be On/Off.

LCD manufactors state a maximum of approx. 50mW DC applied to the
segment to avoid degradation of the segment. But I was wondering if one
could ground the common pin and drive the segment with a square wave
through an ac coupling capacitor. Is there some reason why this will
not work? (so in this case I don't need to worry about precisely 50%
dutycycle)

Thanks

Klaus

 

[Wolfgang Mahringer]

Hi Klaus,

I'm working on a project in which I need to add a 2 digit LCD (old
style reflective type)

Normally one would drive the display common with a square wave and
drive the segments seperately with another square wave that is either
in-phase or out-of-phase depending upon if the segment must be On/Off.

LCD manufactors state a maximum of approx. 50mW DC applied to the

^^^^
should be 50mV...

segment to avoid degradation of the segment. But I was wondering if one
could ground the common pin and drive the segment with a square wave
through an ac coupling capacitor. Is there some reason why this will
not work? (so in this case I don't need to worry about precisely 50%
dutycycle)

You can do that, but keep in mind that the effective voltage swing
over a LCD pixel cell is only half of the value it will be when driving
with two square waves. Contrast will certainly be degraded.

HTH
Wolfgang

 

[Tim Shoppa]

But I was wondering if one could ground the common pin

and drive the segment with a square wave
through an ac coupling capacitor. Is there some
reason why this will not work?

The reason it doesn't work is that you have no control over the DC
level on the other side of the capacitor. If you aren't driving it and
the capacitor is charged with +5V, then you will have 5VDC on that LCD
segment for a very long time. If you then do drive it with 5VAC, then
you've got 5VDC with a superimposed 5VAC.

Now if you start adding pull-down resistors on the LCD side of the
capacitor, you lose all the advantages of low-power operation. And
besides you've now got a few dozen C's and a few dozen R's where before
you needed none.

Even with the pull-down resistor, you now have slowed down the turn-off
time for a segment to a couple times the RC time constant. And every
time you turn it off you're applying (decaying) DC now...

so in this case I don't need to worry about
precisely 50% dutycycle

Wow, dozens of discretes to avoid a single flip-flop. You need to find
better things to worry about, like that luggage retrieval system at
Heathrow.

Tim.

 

[John Fields]

Hi

I'm working on a project in which I need to add a 2 digit LCD (old
style reflective type)

Normally one would drive the display common with a square wave and
drive the segments seperately with another square wave that is either
in-phase or out-of-phase depending upon if the segment must be On/Off.

LCD manufactors state a maximum of approx. 50mW DC applied to the
segment to avoid degradation of the segment. But I was wondering if one
could ground the common pin and drive the segment with a square wave
through an ac coupling capacitor. Is there some reason why this will
not work? (so in this case I don't need to worry about precisely 50%
dutycycle)

---
What's wrong a divide-by-two flip-flop to generate the 50% duty
cycle square wave and a couple of 4543's:

http://focus.ti.com/lit/ds/symlink/cd74hc4543.pdf

to drive the displays?

Digi-Key and Mouser both have them in stock for $0.65 each, qty 1.