Driving LCD displays

[Mosaic]

Hi with ref to this:
**broken link removed**

Is it fair to say that the 'backplane' is equivalent to a common cathode or anode in a 7 segment LED.
Thus a 4 digit LED that has 4 common cathodes permitting sequenced multiplexing of the 7 data segments is equivalent to a 4 digit LCD with 4 independent backplanes?

Also, is it fair to say that LCDs require a much higher voltage to switch compared to LEDs and much less current.

Thus if I wanted to drive a multiplexed 6 digit, 7 segment LCD panel, I'd need a driver chip that accommodated 6 backplanes and 8 segment (incl. DP) drive lines?

I guess using a uC to do the job isn't on as the voltages need stepping up.

 

[jbeng]

The backplane signal is basically the common for the LCD display, so it's quite like the common cathode or anode of an LED display. The part about the LCD displays requiring higher voltage isn't necessarily the case, however. In most all driver systems I've worked with, 5 volts has been the supply voltage, sometimes it's as low as 3 volts. The current requirement for the actual display segments are often orders of magnitude lower than LEDs, eg. 10-20 mA for LEDs, where LCDs generally only require microamps (or less) per segment.

The main thing about driving LCD displays is that they must use an alternating (square wave) drive signal. DC is not used, as it will damage the display by electroplating away the microscopically thin, metallic electrical areas of the segment. Eventually, the display segment would stop functioning altogether.

You could certainly use a micro to control LCDs, but it may be easier to use a chip specifically designed to drive them.

 

[dr pepper]

As I understand it great care is taken in drivers to prevent dc from getting to the display, even short delays between transitions reduces the life of the display.

 

[blueroomelectronics]

Correct, LCDs don't need much voltage, but they don't like DC.

Read the PIC16F917 data sheet for a good description of directly driving LCDs.
**broken link removed**

 

[granddad]

The AY0438 IC will drive 4 digit LCD using a serial 3 signal input . data . clock, load . Takes all the phase control out of any micro system. ( its a 40 pin chip lots of connections , but with 7 seg LCD you have that anyway )