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PC8532-1 Datasheet PDF : 44 Pages
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NXP Semiconductors
PCF8532
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
mode-set commands from the command decoder. The biasing configurations that apply to
the preferred modes of operation, together with the biasing characteristics as functions of
VLCD and the resulting discrimination ratios (D), are given in Table 5.
Table 5. Preferred LCD drive modes: summary of characteristics
LCD drive mode Number of:
Backplanes Bias levels
LCD bias
configuration
V------oV--f--f-L-(--RC----MD----S----)
static
1
2
static
0
1:2 multiplex
2
3
1⁄2
0.354
1:2 multiplex
2
4
1⁄3
1:3 multiplex
3
4
1⁄3
0.333
0.333
1:4 multiplex
4
4
1⁄3
0.333
V------oV--n---L(--R-C--M-D----S---)
1
0.791
0.745
0.638
0.577
D = V-V-----oo--f-n-f--((-R-R---M-M---S-S---)-)
∞
2.236
2.236
1.915
1.732
A practical value for VLCD is determined by equating Voff(RMS) with a defined LCD
threshold voltage (Vth), typically when the LCD exhibits approximately 10 % contrast. In
the static drive mode a suitable choice is VLCD > 3Vth.
Multiplex drive modes of 1:3 and 1:4 with 1⁄2 bias are possible but the discrimination and
hence the contrast ratios are smaller.
Bias is calculated by 1-----+1-----a-- , where the values for a are
a = 1 for 1⁄2 bias
a = 2 for 1⁄3 bias
The RMS on-state voltage (Von(RMS)) for the LCD is calculated with the equation
V on(RMS)
=
V LCD
-1n- + (n – 1) × 1-----+1-----a-- 2
------------------------------------------------------------
n
(1)
where VLCD is the resultant voltage at the LCD segment and 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 (Voff(RMS)) for the LCD is calculated with the equation:
V off (RMS) = V LCD a--n--2---×-–---(--(-1-2----a+-----+a---)-n--2--)
(2)
Discrimination is the ratio of Von(RMS) to Voff(RMS) and is determined from the equation:
PCF8532_1
Product data sheet
Rev. 1 — 10 February 2009
© NXP B.V. 2009. All rights reserved.
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