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Author Topic: Is there a rule of thumb for LCD bias selection?  (Read 729 times)
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Signal
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« on: March 04, 2015, 08:52:19 20:52 »

Speaking more specifically, I have the LCD module (RX2002) with duty factor 1/16 and selectable bias: 1/4 or 1/5. Both variants work - after adjustment of V0 (different values) I can not see the difference in contrast. But this is not an objective test and the only one sample was tested. In the example of init procedure from module datasheet the 1/5 bias is used, but in LCD driver ic datasheet both noted as possible.

I understand that "bias" setting is about a number of voltage levels for COMx and SEGx signals. I noticed that for a greater duty factor (i.e. multiplexing) more levels are required. I also know that multiplexing reduces contrast. Does it mean that I should select least possible (still working) number of levels (1/4 here)?

I can draw waveforms for both variants for ON and OFF segments but do not know the physics of LCD enough, so do not know what to compare. How to calculate effective LCD voltage - average, average square, just peak of amplitude or peak multiplied by duty?
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Signal
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« Reply #1 on: March 05, 2015, 09:35:06 09:35 »

The answer - there is the optimum bias for each duty factor.

Effective voltage for LCD is RMS. Actual contrast depends on LCD panel and "discrimination ratio" - ratio between VrmsON and VrmsOFF.
According to http://www.nxp.com/documents/data_sheet/PCF8576C.pdf this ratio is
  D =  sqrt((b^2)+m-1)/((b-2)^2 + m -1)),
where bias is 1/b and duty is 1/m.

For 1/16 duty the optimum is 1/5 bias with D = 1.291.
This is magically the same bias as stated (though not explicitly described as optimum) in Raystar datasheet :).

Discrimination for 1/4 bias is 1.277 - almost the same - that is why I did not see the difference. Transmission curve of LCD is sharp enough for 1.277 too. On the other hand a temperature drift of this curve will be visible (slightly) earlier with 1/4 bias.

(for duty factor 1/8 the optimum bias is 1/4 with D=1.446, and for 1/5 bias D is 1.414 - more considerable difference)

There is another factor - absolute voltage values. Bias 1/4 requires less V0 than 1/5 bias does:
 Von(1/4)= V0*0.348, Von(1/5)= V0*0.316; (1/16 duty)
In case of RX2002 this is not an issue.

UPDATE: actually - the issue. There is no voltage margin. Vdd=4.75 V is not enough even for optimal contrast (for some view angles) using 1/5 bias. In my case an optimal VrmsON is about 1.5 V. So for 5V application of RX2002 an 1/4 bias is a practical choice.
« Last Edit: March 05, 2015, 02:11:49 14:11 by Signal » Logged

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