LCD Type MCU
LCD Voltage Source and Biasing
The time and amplitude varying signals generated by the Holtek LCD Type microcontrollers re-
quire the generation of several voltage levels for their operation. The number of voltage levels
used by the signal depends upon device and the chosen bias configuration options.
LCD Biasing
The HT49C30L, HT49C50L and HT49C70L devices have a fixed bias value of 1/2 whereas all
other devices have a configuration option to select either 1/2 or 1/3 bias. For the 1/2 bias configura-
tion option, three voltage levels VSS, VA and VB are utilized. With the exception of the HT49C30L,
HT49C50L and HT49C70L devices, for R type biasing VA is equal to VLCD, which is an externally
supplied voltage source. This could be the microcontroller power supply or some other voltage
source. VB is generated internally by the microcontroller and will have a value equal to VLCD/2.
For the 1/3 bias option, four voltage levels VSS, VA, VB and VC are utilized. An external LCD volt-
age source is also provided on pin VLCD to generate these voltages. The actual value of the VA,
VB and VC voltages depends on whether the R or C type option is selected. Note that because the
C type bias option uses a charge pump circuit, higher voltages than what is provided externally on
VLCD can be generated. This feature is useful in applications where the microcontroller supply
voltage is less than the supply voltage required by the LCD.
For the R type 1/3 bias option, the voltage level, VA, is the same as the externally supplied VLCD
voltage while voltage levels VB and VC, are determined by an internal resistor divider. For the R
type 1/2 bias option, VC is not required. Note that no external resistors are required. It is however
recommended that external capacitors are still connected to pins V1 and V2 to stabilize these inter-
nally generated voltage levels. In the case of the C type bias option, a charge-pump capacitor be-
tween pins C1 and C2 and filter capacitors on pins V1 and V2 are required to generate the
necessary voltage levels. In the case of HT49C30L, HT49C50L and HT49C70L devices, pin V2 is
used as the LCD voltage source input, therefore the capacitor is omitted from this pin, however, an
additional capacitor should be connected to pin VLCD.
For the HT49C30L, HT49C50L and HT49C70L devices, which always utilize a low voltage supply,
perhaps operating from a single cell battery, the higher supply voltages required by the LCD is gen-
erated by an internal charge pump voltage-doubler circuit. For these devices an external voltage
supply is connected to pin V2, normally this is the microcontroller power supply but could be some
other voltage source. The voltage supplied to pin V2 will be doubled in value by the internal charge
pump circuit creating the VA value. The actual value of external voltage supplied to pin V2 de-
pends upon the voltage requirements of the application LCD.
V M A X
(= V L C D )
( H T 4 9 R U 8 0 /H T 4 9 C U 8 0 o n ly )
L C D
P o w e r S u p p ly
V M A X
( H T 4 9 R U 8 0 /H T 4 9 C U 8 0 o n ly )
L C D
P o w e r S u p p ly
V
V
A
V L C D
R
R
R
(= V L C D )
V
A
V L C D
(= V L C D
´
2 /3 )
(= V L C D
´
1 /3 )
B
R
V
C
(= V L C D
´
1 /2 )
V
B
R
L C D
O n /O ff
L C D
O n /O ff
R
ty p e 1 /3 B ia s
E x c e p t H T 4 9 C 3 0 L , H T 4 9 C 5 0 L , H T 4 9 C 7 0 L
R
ty p e 1 /2 B ia s
E x c e p t H T 4 9 C 3 0 L , H T 4 9 C 5 0 L , H T 4 9 C 7 0 L
R Type Bias Voltage Levels
56
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