PCA8546 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2013. All rights reserved.
Product data sheet Rev. 1 — 13 November 2013 22 of 63
NXP Semiconductors
PCA8546
4 x 44 automotive LCD driver
8.4 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
set-bias-mode command (see Table 15
).
Fractional LCD biasing voltages are obtained from an internal voltage divider. The biasing
configurations, the biasing characteristics as functions of V
LCD
and the resulting
discrimination ratios (D), are given in Table 22
.
Discrimination is a term which is defined as the ratio of the on and off RMS voltage across
a segment. It can be thought of as a measurement of contrast.
[1] Determined from Equation 5.
[2] Determined from Equation 4
.
[3] In these examples, the discrimination factor and hence the contrast ratios are smaller. The advantage of
these LCD drive modes is a reduction of the LCD voltage V
LCD
.
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.
Bias is calculated by , where the values for a are
a = 1 for
1
⁄
2
bias
a = 2 for
1
⁄
3
bias
a = 3 for
1
⁄
4
bias
The RMS on-state voltage (V
on(RMS)
) for the LCD is calculated with Equation 3
(3)
where V
LCD
is the resultant voltage at the LCD segment and where the value for n is
n = 4 for 1:4 multiplex drive
The RMS off-state voltage (V
off(RMS)
) for the LCD is calculated with Equation 4:
(4)
Discrimination is the ratio of V
on(RMS)
to V
off(RMS)
and is determined from Equation 5:
Table 22. Preferred LCD drive modes: summary of characteristics
LCD multiplex
drive mode
Number of: LCD bias
configuration
[1]
V
LCD
[2]
Backplanes Levels
1:4
[3]
43
1
⁄
2
0.433 0.661 1.527 2.309V
off(RMS)
1:4 4 4
1
⁄
3
0.333 0.577 1.732 3.0V
off(RMS)
1:4
[3]
45
1
⁄
4
0.331 0.545 1.646 3.024V
off(RMS)
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
=
