PCA8551 All information provided in this document is subject to legal disclaimers. © NXP Semiconductors N.V. 2015. All rights reserved.
Product data sheet Rev. 2 — 24 March 2015 17 of 57
NXP Semiconductors
PCA8551
Automotive 36 × 4 LCD segment driver
9.1 LCD bias generator
Fractional LCD biasing voltages are obtained from an internal voltage divider of three
impedances connected between V
LCD
and V
SS
. These intermediate levels are tapped off
at positions of
1
⁄
3
and
2
⁄
3
, or
1
⁄
2
, depending on the bias mode chosen. To keep current
consumption to a minimum, on-chip low-power buffers provide these levels to the display.
9.2 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
Display_ctrl_1 command (see Table 7
). 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 12.
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 with Equation 2
(2)
The values for a
bias
are:
a
bias
= 1 for
1
⁄
2
bias
a
bias
= 2 for
1
⁄
3
bias
The RMS on-state voltage (V
on(RMS)
) for the LCD is calculated with Equation 3:
(3)
Table 12. 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
-------------------------=
1
1a
bias
+
---------------------
V
on RMS
a
bias
2
2a
bias
n
MUX
++
n
MUX
1a
bias
+
2
-----------------------------------------------------
V
LCD
=
