LCD 2 Digit Display Driver - Capacitor AC Coupling

Nov 23, 2005 6 Replies
K

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



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T

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...

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.

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J

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.
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W

^^^^ should be 50mV...

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

From-address is Spam trap Use: wolfgang (dot) mahringer (at) sbg (dot) at
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K

John Fields skrev:

Well. I was trying to see if anything could be saved. 0.65$ for a chip is to much for me - so I think I'll go for using some extra pins of the micrcontroller and drive it directly

Thanks

Klaus

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K

Tim Shoppa skrev:

Thanks - I will abandon that idea

Regards

Klaus

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J

Good luck! If you\'re going to use one pin to drive the backplane and it\'s a static display, you\'re going to need fourteen pins to drive the segments and you\'re going to have to write the code so that the segments are driven properly, in the background, while your µC is taking care of real-time business. For two drivers, at 65 cents each, is it worth it?
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