PCA8576C Datasheet, PDF (7/44 Pages) NXP Semiconductors – Universal LCD driver for low multiplex rates

NXP Semiconductors
PCA8576C
Universal LCD driver for low multiplex rates
7.3 LCD voltage selector
The LCD voltage selector coordinates the multiplexing of the LCD in accordance with the
selected LCD drive configuration. The operation of the voltage selector is controlled by the
mode-set command from the command decoder. The biasing configurations that apply to
the preferred modes of operation, together with the biasing characteristics as functions of
V_LCD and the resulting discrimination ratios (D) are given in Table 5.
Table 5. Biasing characteristics
LCD drive
mode
Number of:
LCD bias
Backplanes Levels ^{configuration}
static
1
1:2 multiplex 2
1:2 multiplex 2
1:3 multiplex 3
1:4 multiplex 4
2
static
3
1_⁄2
4
1_⁄3
4
1_⁄3
4
1_⁄3
V_{-----o--f--f--(--R---M-----S---)}
V_LCD
0
0.354
0.333
0.333
0.333
-^V----^o--ⁿ---⁽--^R----^M----^S---⁾
V_LCD
1
0.791
0.745
0.638
0.577
D = -^V----^o---ⁿ--⁽--^R----^M----^S----⁾
V_off(RMS)
∞
2.236
2.236
1.915
1.732
A practical value for V_LCD is determined by equating V_off(RMS) with a defined LCD
threshold voltage (V_th), typically when the LCD exhibits approximately 10 % contrast. In
the static drive mode a suitable choice is V_LCD > 3V_th.
Multiplex drive modes of 1:3 and 1:4 with ¹â„₂ bias are possible but the discrimination and
hence the contrast ratios are smaller.
Bias is calculated by -----¹------- , where the values for a are
1+a
a = 1 for ¹â„₂ bias
a = 2 for ¹â„₃ bias
The RMS on-state voltage (V_on(RMS)) for the LCD is calculated with Equation 1:
V_on(RMS)
=
V_LCD
--^a---²----⁺-----²---^a----⁺-----ⁿ---
n × (1 + a)²
(1)
where the values for n are
n = 1 for static drive mode
n = 2 for 1:2 multiplex drive mode
n = 3 for 1:3 multiplex drive mode
n = 4 for 1:4 multiplex drive mode
The RMS off-state voltage (V_off(RMS)) for the LCD is calculated with Equation 2:
PCA8576C
Product data sheet
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---⁾
V_off(RMS)
=
(_---a_-----+-----1_----)_--²_----+_-----(_--n_-----–_----1_----)
(a – 1)² + (n – 1)
All information provided in this document is subject to legal disclaimers.
Rev. 1 — 22 July 2010
(3)
© NXP B.V. 2010. All rights reserved.
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