NXP Semiconductors
PCF8566
Universal LCD driver for low multiplex rates
Table 5.
Preferred LCD drive modes: summary of characteristics
Backplanes
Bias levels
2
3
4
4
4
LCD bias
configuration
static
1
⁄
2
1
⁄
3
1
⁄
3
1
⁄
3
LCD drive mode Number of:
V
off
(
RMS
)
--------------------------
V
LCD
0
0.354
0.333
0.333
0.333
V
on
(
RMS
)
------------------------
-
V
LCD
1
0.791
0.745
0.638
0.577
V
on
(
RMS
)
D
= --------------------------
V
off
(
RMS
)
∞
2.236
2.236
1.915
1.732
static
1:2 multiplex
1:2 multiplex
1:3 multiplex
1:4 multiplex
1
2
2
3
4
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
1
⁄
2
bias are possible but the discrimination and
hence the contrast ratios are smaller.
1
-
Bias is calculated by ------------ , where the values for a are
1
+
a
a = 1 for
1
⁄
2
bias
a = 2 for
1
⁄
3
bias
The RMS on-state voltage (V
on(RMS)
) for the LCD is calculated with the equation
2
1
1
-- +
(
n
–
1
) ×
------------
-
-
1
+
a
n
-----------------------------------------------------------
-
n
V
on
(
RMS
)
=
V
LCD
(1)
where V
LCD
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 (V
off(RMS)
) for the LCD is calculated with the equation:
V
off
(
RMS
)
=
V
LCD
a
–
(
2a
+
n
)
--------------------------------
-
2
n
× (
1
+
a
)
2
(2)
Discrimination is the ratio of V
on(RMS)
to V
off(RMS)
and is determined from the equation:
V
on
(
RMS
)
-----------------------
=
-
V
off
(
RMS
)
(
a
+
1
)
+
(
n
–
1
)
-------------------------------------------
-
2
(
a
–
1
)
+
(
n
–
1
)
2
(3)
Using
the discrimination for an LCD drive mode of
•
1:3 multiplex with
1
⁄
2
bias is
3
=
1.732
PCF8566_7
© NXP B.V. 2009. All rights reserved.
Product data sheet
Rev. 07 — 25 February 2009
8 of 48
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