Application Note Voltage Generating Circuits for LCD Contrast Control

APPLICATION NOTE
LIQUID CRYSTAL DISPLAY MODULE
Voltage Generating Circuits for LCD Contrast Control
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2511 Technology Drive, Suite 101
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Ph: 847-844-8795
Fax: 847-844-8796
April 02, 2013
Application Note
Newhaven Display International, LLC
Voltage Generating Circuits for LCD Contrast Control
Most Liquid Crystal Display modules require a Positive or Negative voltage that is higher than the logic
voltage used to power the LCD. This voltage, called Vl, Vee or Bias voltage, requires a second power
supply. If this power source is not available, the LCD Bias voltage must be generated from an existing
voltage, either the logic voltage (+3.0-+5v) or a battery. This application note illustrates circuits for
generating either a Positivor Negative LCD Bias voltage from such a voltage source.
The LCD Bias voltage is used to directly power the circuits that drive the LCD glass. This voltage sets the
contrast level of the LCD. Since any changes in this voltage will cause a visible change in the contrast of
the LCD, it must be regulated to more than about 200mV. Any noise or ripple on this signal may cause
visible artifacts on the LCD so they must be kept below about 100mV.
Charge pump circuits:
The simplest and least expensive way of generating an LCD bias voltage is with a charge pump circuit. A
charge pump generates a voltage that is some multiple of the peak to peak (P-P) voltage of the input
square wave. The output can be either positive or negative.
These simple circuits can be used to generate the bias voltage for character type displays and small
graphics types. They have the advantage of being very low in cost but are not regulated and cannot
deliver much current. They are also sensitive to variations in the source voltage (Vdd) , so it cannot be
driven directly from a battery.
The driving signal is usually derived from an existing clock signal or generated directly by an I/O pin on a
microprocessor. The frequency of the signal can be anywhere from about 1kHz to 50kHz or higher. If the
signal is above about 5kHz the simple 1N4148 diodes should be replaced by Schottky diodes such as
the 1N5817. The capacitors should also be upgraded to low ESR types. The device generating the signal
must be capable of delivering the load current times the multiplication value. In the circuit in Figure 1
driving a small character display the input signal should be able
+5v
*
47uF
to sink and source at least 4mA.
0v
Fig 1 shows a simple charge pump circuit that generates a
Negative 4v from a Positive 5v square wave. It is suitable for
driving the Vl line on an extended temperature LCD character
module.
10k
+
* 1N4148
1N5817
*
22uF
CONTRAST
CONTROL
Fig 1
Adjustable Voltage Inverter:
It might be desirable to allow the end user of a product to have access to the contrast adjustment. The
circuit in Fig 2 utilizes a pot to adjust the contrast voltage from 0v to -12v The voltage output can be
set to any range of voltages within the 0 to -12v limits by adding one or two resistors in series with the
pot. The total resistance of the pot and any added resistors should not exceed 50k. If the end user is not
to have control of the contrast the pot can be substituted with a fixed resistor to set the voltage to the
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LCD to give the best
contrast, which would
also eliminate the need
to adjust a pot during
production. The
efficiency is high enough
to be used with battery
operated equipment and
the output can drive
most small graphics
displays, up to about
240 x 64 pixels resolution.
+5v
+
10µF
1 14
13
+VS +VS +VS
2
ON/OFF
LX
VLCD
0 to-12V
12
+ 47µF*
30v
50k
CONTRAST addition.
33µH**
MAX759
CC 7
9
DRV-
5.1k
Digitally adjustable
inverter.
VREF
GND
10
Some applications
require the user to have
control of the contrast
but do not lend
themselves to using a
pot to make the
adjustment. The circuit
in Fig 3 allows a
micro controller to adjust
the VL voltage in a very
simple manner. It also
provides an input to shut
off the negative voltage
so the display can be
shut down by the micro
controller if desired. This
shutdown signal can
also be used to properly
sequence the power to
the display during
power-up and powerdown sequences.
1N5819
11
LX
SHDN
3
.047 µF
Fig 2
* Tantalum or low ESR aluminum electrolytic
** Less than 0.25 ohms DC resistance.
+5v
+
10µF
0.25Ω
1
+VS
2
ADJUST
3
ON/OFF
ADJ
CS
CTRL
DHI
8
7
ZTX750
1N5819
MAX749
4
RVMAX
VLCD
FB
470Ω
DLOW
220pF
47µH**
6
+ 22µF*
30v
GND
5
Fig 3
ON/OFF signal. A logic 0
on this pin will turn the
display off by removing
the VL voltage. If this signal is not needed, tie pin 3 to +5v.
* Tantalum or low ESR aluminum electrolytic
** Less than 0.3ohms DC resistance.
Output voltage control. The maximum voltage is set using a single resistor, RVMAX. See Table 1. The
Table 1
MAX OUTPUT VOLTAGE
RVMAX (ohms)
-5V
240k
-8V
360k
-12V
580k
-15V
750k
output can then be adjusted from 33% to 100% of this value using the internal 64-step DAC / counter.
On power-up or after a RESET command the output voltage is set to mid-range which is 67% of the
maximum voltage. Each rising edge of the ADJUST signal increments the DAC output. When the
DAC reaches 100% output, the next pulse will cause it to wrap-around to the 33% value.
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ADJUST
ON/OFF
tRS
tR
ON
RESET
Fig 4
tRH
ON
tR = Minimum pulse width, 25nS typ. 85nS max.
tRS,tRH = Minimum setup/hold, 0nS min., 0nS min.
A RESET is accomplished by setting the ADJUST line high and then setting the ON/OFF line low for
longer than 400nS. See Fig 4.
High Voltage Circuits for Larger Panels:
Modern 1 / 4 VGA to full VGA size panels require Vee voltages above 20v. Most monochrome
panels require a Negative Vee voltage while most color panels require a Positive voltage. Many of
these panels are used in hand held, battery operated, applications and require very efficient
conversion using a supply voltage that changes as the charge on the batteries is gradually depleted.
Several semiconductor manufactures have responded to this need with new devices made especially
for this application. While the
voltage converter can be
1N5819
47µH**
done “in-house” it is usually
V
+V
not economical to do so
because of the complexity of
+
a circuit that has the
2.2mΩ
10µF
1
5
regulation qualities and the
+V
SW
efficiency required.
LCD
IN
IN
+
Fig 5 is a circuit based on
the Linear Technologies
3
LT1615 series chips. The
FB
4
circuit shown here generates
SHDN
a Positive voltage for a small
130kΩ
GND
1 / 4 VGA (320x240) color
2
graphics display that could
* Tantalum or low ESR aluminum electrolytic
Fig 5
** Less than 0.3ohms DC resistance.
be used in a palm sized PC
running Windows CE.
A Negative voltage version LT1617is also available. The device in this example runs from a pair of AA
LT1615
2.2µF*
35v
batteries and must produce a stable output voltage with an input that varies from 2.0v ~ 3.2v.
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General Circuit Considerations:
All components associated with the circuits in Fig 2, 3 and 5 should be placed physically close to the IC.
The decoupling capacitor on the input voltage line should be placed as close to the VIN and GND pins of the IC
as possible.
Power Sequencing Considerations:
The order in which the power supplies are applied to an LCD, power sequencing, must be considered when
designing an LCD bias power supply. The power sequencing requirement can be summarized by stating that
the VEE (VL ) must never be present without VDD also being present. If this condition exists, even for a short
period of time, the display may be permanently damaged. The desired power on sequencing for graphics type
LCDs with an external controller is shown in Figure 6. For graphics type LCDs with a built in controller you can
ignore the “signal” line as this is taken care of in the controller at power on time. For character displays only the
VDD and VEE (V L) lines need be considered.
All of the circuits described here, except the charge pump in Fig 1, have provisions to shut down the voltage
VDD
SIGNALS
0~50ms
0~50ms
>20ms
>0ms
VEE
>20ms
>0ms
DISPOFF
Power On
Power Off
Fig 6
generator with a logic signal. Using this signal the generator IC is kept in the shutdown mode until VDD is stable
and the LCD controller has been initialized and has started to scan the display. At this time VEE can be applied
to the display safely. The turn off procedure is just the reverse of the turn on procedure.
Sources:
LinearTechnology
Maxim Integrated Products, Inc.
1630McCarthyBlvd.
Milpitas,CA95035
(408)432-1900
www.liner-tech.com
120 San Gabriel Dr.
Sunnyvale, CA 94086
(408) 737-7600
www.maxim-ic.com
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