Optimum Liquid Crystal Drive Voltage
Optimum LCD Drive Voltage
In general, when driving an LCD, the most important design consideration is the drive voltage necessary for proper operation. There are two very different cases.
- Direct Drive Displays
- Multiplexed Displays
Direct Drive Displays
For a direct drive display, the situation is very easy, use 5 volts and everyone will be happy. Almost all fluids used by manufacturers today will work very well at 5 volts in a direct drive mode. However..
What if you only have 3.3 volts or 3.0 volts available? The answer here is also very easy. Tell us that you only have 3.3 or 3.0 volts available, and we will select the correct fluid for your design. This is very important, as not all of our fluids will work with only 3 volts available, so we need to select from a much shorter list of options. It's really that easy, as long as we know in the beginning.
Multiplexed displays can be much more complicated than direct drive displays, a little math is required.
|
The optimum RMS on to off voltage ratio for driving an LCD is calculated as follows:
|
|||||||||||||||||||||||||||||||||||||||||
|
This calculation shows the theoretical maximum on to off voltage ratio for any given multiplexed ratio, where n is the multiplex rate of the display. The ratio given by the above equation, when multiplied by the threshold voltage of a given LCD fluid, will give the equivalent steady state RMS voltage necessary to properly drive a multiplexed display. So the first step in designing your drive circuit it to make sure that you have enough DC voltage to properly drive the LCD. Some designs have problems down the road because this voltage, which can be quit high, is not available. We suggest that you do this calculation before you do anything else, as it may require additional components to generate the required voltage. Adding a DC to DC converter to a PCB that is already laid out can be downright challenging. We now move on to optimizing the driver chip support network for maximum contrast. Maximizing Contrast in a Multiplexed Display In real life, maximizing the contrast and viewing angle of a multiplexed display is a bit tricky the first time you try it, however, once you understand the overall goal it's not really very mysterious. The diagram below shows the resistor ladder network that generates the various LCD multiplex drive voltages. The Rx 's are the individual resistors, and the Vr 's are the voltage drops across each one individually. You can find an equivalent circuit in the data sheet for every driver chip.
Regardless of the information or formulas given in the data sheet, in practice the strategy is simply to make the voltage drop across each resistor Vr equal. By making each Vr equal, the individual voltage levels in our multiplex scheme will be equal, giving us the maximum voltage swing between the individual voltage steps. It is also not obvious from our simple DC circuit shown above, that the complex feedback inside the driver chip dictates that the Values of R1, R2, R3, and R4 NOT be equal in value. Some experimentation with your individual circuit will show this to be true. Collecting empirical data, making a single Rx value change, then recording more data, will usually show the resistor pattern change necessary for uniform Vr. Our engineers will be glad to help you with your multiplexed circuit designs. Most problems can be handled over the phone. Just give us a call between 9:00 am and 5:00 pm EST. |
|||||||||||||||||||||||||||||||||||||||||
