PCF8576D All information provided in this document is subject to legal disclaimers. © NXP Semiconductors N.V. 2015. All rights reserved.
Product data sheet Rev. 15 — 12 February 2015 9 of 59
NXP Semiconductors
PCF8576D
40 × 4 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 the
mode-set command from the command decoder. The biasing configurations that apply to
the preferred modes of operation, together with the biasing characteristics as functions of
V
LCD
and the resulting discrimination ratios (D) are given in Table 6.
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.
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:
Table 6. 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
=
