Specifications

7–14 EPSON
SED1234/35 Series
● Voltage Regulation Circuit Using Electronic Contrast
Control Register
The contrast control register controls the liquid crystal
driving voltage (V
5). This is accomplished by an elec-
tronic volume control register set command that adjusts
the contrast of the liquid crystal display (see section 1-
22).
The commands provide 4-bits of voltage level data to the
electronic volume control register. This provides for the
selection of 16 different voltage levels for the liquid
crystal driving voltage. When using the electronic vol-
ume control function, it is necessary to close the voltage
regulation circuit using electronic control commands.
For reference information, when the electronic volume
control registor value is at (1, 1, 1, 1), the constant current
value becomes: I
REF
=
3.65µA.
[An exemplary constant setting when the electronic volume control function is being used]
(1) Determining the V
5 voltage setting range by the electronic volume control
Liquid crystal driving voltage V5: max. -6v ~ min. -8V
V
5 variable voltage range: 2V
(2) Determinig the R
b
Rb = V5 variable voltage range/ IREF
= 2V/3.65µA
= 548KΩ
(3) Determining the R
a
Ra =
V
REG
(Use absolute values for VREG and V5 voltage settings.)
(V5 voltage setting max - VREG) / Rb
=
3.1V
(6V - 3.1V) / 548KΩ
= 585KΩ
(4) Regulating the Ra
Set the electronic volume control register to (D3, D2, D1, D0) = (1, 0, 0, 0) or (0, 1, 1, 1) before matching the Ra
value to the optimum contrast.
Since I
REF is a simplified constant voltage source, fluctuations upto ± 40% must be taken into consideration, as a
dispersion range during manufacture. Meanwhile, the temperature dependency of I
REF is : ∆ IREF
=
-0.037µA/°C.
Determine the R
a and Rb for the using LCD panel in consideration of the above dispersion and the variation by the
temperature.
When using the electronic volume control function, in order to compensate the V
5 voltage for dispersion of VREG
and IREF, use a variable registor as Ra and perform optimum contrast adjustment according to the above item (4)
with each IC chip.
When the electronic volume control function is not being used, set the electronic volume control register to (0, 0,
0, 0) using the RES signal or the electronic volume control register setting command.
V5 = (1 +
R
b
) • VREG
·····················
2
Rc
Rc =
Ra × RI
Ra + RI
RI =
V
R
IREF
Fig. 9
Ra
Rb
VR
V0
VDD
VREG
+
-
V5
IREF