PCA85176 All information provided in this document is subject to legal disclaimers. © NXP Semiconductors N.V. 2015. All rights reserved.
Product data sheet Rev. 6 — 7 April 2015 14 of 60
NXP Semiconductors
PCA85176
40 x 4 automotive 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 .
The advantage of these LCD drive modes is a reduction of the LCD full scale voltage V
LCD
as follows:
• 1:3 multiplex (
1
⁄
2
bias):
• 1:4 multiplex (
1
⁄
2
bias):
These compare with when
1
⁄
3
bias is used.
It should be noted that V
LCD
is sometimes referred as the LCD operating voltage.
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+
---------------------------==
V
LCD
6V
off RMS
2.449V
off RMS
==
V
LCD
43
3
----------------------
2.309V
off RMS
==
