PCA8576D Datasheet, PDF (7/42 Pages) NXP Semiconductors – Automotive 40 x 4 LCD segment driver for low multiplex rates up to 1:4

NXP Semiconductors
PCA8576D
Automotive 40 x 4 LCD segment driver for low multiplex rates
where the values for n are
n = 1 for static mode
n = 2 for 1:2 multiplex
n = 3 for 1:3 multiplex
n = 4 for 1:4 multiplex
The RMS off-state voltage (V_off(RMS)) for the LCD is calculated with Equation 2:
V_{offRMS}
=
V_LCD
--^a---²----^–----²----^a----⁺-----ⁿ---
n  1 + a²
(2)
Discrimination is the ratio of V_on(RMS) to V_off(RMS) and is determined from Equation 3:
D = -^V----^o---ⁿ--^--^R---^M----^S---^ = ^---^a-----+-----¹----^--²----⁺-----^--ⁿ-----^–----¹----^
(3)
V_{offRMS}
a – 1² + n – 1
Using Equation 3, the discrimination for an LCD drive mode of 1:3 multiplex with
1_{2 bias is 3 = 1.732 and the discrimination for an LCD drive mode of 1:4 multiplex with}
1_{2 bias is}
-----²---¹--
3
=
1.528 .
The advantage of these LCD drive modes is a reduction of the LCD full scale voltage V_LCD
as follows:
• 1:3 multiplex (¹ï‚¤₂ bias): V_LCD = 6  V_{offRMS} = 2.449V_{offRMS}
• 1:4 multiplex (¹ï‚¤₂ bias): V_LCD =
_---4_-----_---------3_----
3
= 2.309V_{offRMS}
These compare with V_LCD = 3V_{offRMS} when ¹ï‚¤₃ bias is used.
It should be noted that V_LCD is sometimes referred as the LCD operating voltage.
7.3.1 Electro-optical performance
Suitable values for V_on(RMS) and V_off(RMS) are dependent on the LCD liquid used. The
RMS voltage, at which a pixel will be switched on or off, determine the transmissibility of
the pixel.
For any given liquid, there are two threshold values defined. One point is at 10 % relative
transmission (at V_low) and the other at 90 % relative transmission (at V_high), see Figure 4.
For a good contrast performance, the following rules should be followed:
PCA8576D
Product data sheet
V_{onRMS}  V_high
(4)
V_{offRMS}  V_low
(5)
V_on(RMS) and V_off(RMS) are properties of the display driver and are affected by the selection
of a, n (see Equation 1 to Equation 3) and the V_LCD voltage.
All information provided in this document is subject to legal disclaimers.
Rev. 1 — 4 April 2011
© NXP B.V. 2011. All rights reserved.
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