PCF8533 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2012. All rights reserved.
Product data sheet Rev. 6 — 1 October 2012 10 of 53
NXP Semiconductors
PCF8533
Universal LCD driver for low multiplex rates
A practical value for V
LCD
is determined by equating V
off(RMS)
with a defined LCD
threshold voltage (V
th(off)
), typically when the LCD exhibits approximately 10 % contrast. In
the static drive mode, a suitable choice is V
LCD
>3V
th(off)
.
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 , where the values for a are
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 Equation 1:
(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:
(2)
Discrimination is the ratio of V
on(RMS)
to V
off(RMS)
and is determined from Equation 3:
(3)
Using Equation 3
, the discrimination for an LCD drive mode of 1:3 multiplex with
1
⁄
2
bias is and the discrimination for an LCD drive mode of 1:4 multiplex with
1
⁄
2
bias is .
Table 13. Biasing characteristics
LCD drive
mode
Number of: LCD bias
configuration
Backplanes Levels
static 1 2 static 0 1
1:2 multiplex 2 3
1
⁄
2
0.354 0.791 2.236
1:2 multiplex 2 4
1
⁄
3
0.333 0.745 2.236
1:3 multiplex 3 4
1
⁄
3
0.333 0.638 1.915
1:4 multiplex 4 4
1
⁄
3
0.333 0.577 1.732
V
off RMS
V
LCD
-------------------------
V
on RMS
V
LCD
------------------------
D
V
on RMS
V
off RMS
-------------------------=
V
on RMS
a
2
2a n++
n 1a+
2
------------------------------
V
LCD
=
V
off RMS
a
2
2a– n+
n 1a+
2
------------------------------
V
LCD
=
D
V
on RMS
V
off RMS
-----------------------
a
2
2a n++
a
2
2a– n+
---------------------------==
