UC1682
80x104RGB CSTN Controller-Driver
Revision 0.6
29
LCD VOLTAGE SETTING
MULTIPLEX RATES
Multiplex Rate is completely software
programmable in UC1682 via registers CEN,
DST, DEN, and partial display control LC[9:8].
Combined with low power partial display mode
and a low bias ratio of 5, UC1682 can support
wide variety of display control options. For
example, when a system goes into stand-by
mode, a large portion of LCD screen can be
turned off to conserve power.
BIAS RATIO SELECTION
Bias Ratio (BR) is defined as the ratio between
VLCD and VBIAS, i.e.
BR = VLCD /VBIAS,
where VBIAS = VB1+ – VB1– = VB0+ – VB0–.
The theoretical optimum Bias Ratio can be
estimated by
1
+
Mux
. BR of value 15~20%
lower/higher than the optimum value calculated
above will not cause significant visible change in
image quality.
Due to the nature of STN operation, an LCD
designed for good gray-shade performance at
high Mux Rate (e.g. MR=80), can generally
perform very well as a black and white display, at
lower Mux Rate. However, it is also true that such
technique generally can not maintain LCD’s
quality of gray shade performance, since the
contrast of the LCD will increase as Mux Rate
decreases, and the shades near the two ends of
the spectrum will start to lose visibility.
UC1682 supports four BR as listed below. BR
can be selected by software program.
BR
0
1
2
3
Bias Ratio
5
7
8
9
Table 1: Bias Ratios
TEMPERATURE COMPENSATION
Four (4) different temperature compensation
coefficients can be selected via software. The
four coefficients are given below:
TC
0
1
2
3
% per
o
C
-0.05
-0.10
-0.15
-0.20
Table 2: Temperature Compensation
VLCD GENERATION
VLCD may be supplied either by internal charge
pump or by external power supply. The source of
VLCD is controlled by PC[3:2]. For good product
reliability, it is recommended to keep VLCD under
12V over the entire operating range.
When VLCD is generated internally, the voltage
level of VLCD is determined by three control
registers: BR (Bias Ratio), PM (Potentiometer),
and TC (Temperature Compensation), with the
following relationship:
%)
)
25
(
1
(
)
(
0
T
PM
V
LCD
C
T
PM
C
C
V
×
−
+
×
×
+
=
where
CV0 and CPM are two constants, whose value
depends on the setting of BR register, as
illustrated in the table on the next page,
PM is the numerical value of PM register,
T
is the ambient temperature in
OC, and
CT is the temperature compensation
coefficient as selected by TC register.
VLCD FINE TUNING
Gray shade and color STN LCD is sensitive to
even a 1% mismatch between IC driving voltage
and the VOP of LCD. However, it is difficult for
LCD makers to guarantee such high precision
matching of parts from different venders. It is
therefore necessary to adjust VLCD to match the
actual VOP of the LCD.
For the best results, software or OTP based VLCD
adjustment is the recommended method for VLCD
fine tuning.
For applications where mechanical manual fine
tuning of VLCD becomes necessary, then VBIAS pin
may be used with an external trim pot to fine tune
the VLCD.
LOAD DRIVING STRENGTH
The power supply circuit of UC1682 is designed
to handle LCD panels with load capacitance up to
~20nF when VDD2 = 2.5V. For larger LCD panels
use higher VDD and COF packaging.
20nF is also the recommended limit for LCD
panel size for COG applications.
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