Simple "Duty Cycle" Explanation

[Chris Parr]

Hello,

I'm hoping that somebody out there will be able to help me. I am writing an "idiots guide" to LCD products for my local sales force. I am finding great difficulty explaining "DUty Cycles" in a clear and concise simple form. The explanations I have written so far are difficult for complete novices to understand. I'm also having the same problem with the Bias explanation. Can anybody explain what they both mean in fairly short simple statements. I certainly know I cannot.

Thanks very much

CP

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[CF Tech]

Chris:

Here is a short Duty Cycle explanation:

A pixel or segment in an LCD is turned on by an AC voltage being applied to it. On a very simple display, each segment that needs to be on can be driven by its own AC voltage. In this case there needs to be one connection from the LCD controller to the LCD for every segment. For simple displays with 100 or so segments, this method gives great performance. This is called 1/1 duty cycle, each segment is driven 100% of the time when it is on.

A modest 20x4 character display would require 3200 connections between the LCD controller and LCD if it were to be driven at 1/1 duty cycle. It is not practical to route that many connections between the controller and the glass.

What is done is called "multiplexing". The same 20x4 display driven by at 1/32 duty cycle allows it to have a much more reasonable 132 connections. Essentially, the display is divided into 32 separate displays--one per horizontal line. Each line gets to be driven by its AC signal in its turn. That means that each "on" pixel gets driven 1/32 of the time or 3.125%.

The challenge is to create an LCD that will work well when it is only being driven a small fraction of the time.

Here is a really short one:

Low duty cycle (1/1, 1/2) = better performance but constrained to simpler displays

High duty cycle (1/32, 1/64) = lower performance but allows more complex displays

Here is a short bias explanation:

When a display is created using multiplexing, the pixels that are not supposed to be on still see some voltage across them. The bias is a way of dividing up the available bias voltage so the pixels that are supposed to be on are dark, while the pixels that are supposed to be off are still light. The bias levels are set either internally to the controller (on modern controllers) or with an external resistor chain (on older controllers). The bias is set by the LCD designer to work optimally with the particular LC fluid in the LCD.

Here is a really short one:

The optimal bias is selected by the LCD designer.

Here are a couple of LCD tutorials that I like, they may help some:

http://crystalfontz.com/links/external/bruck_scott/index.html
http://crystalfontz.com/links/external/case_western/index.html

I hope that helps!