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Basics of EL Panel
Drive Techniques
Thin film electroluminescent (EL) panels operate on the principle of successive pulses of opposite polarity. These pulses
must exceed a threshold of approximately 200V for the panel to emit light.
A flat panel display is a sandwich of phosphor material with
dielectric coating on either side; transparent ITO (Indium Tin
Oxide) row electrodes on one side and column electrodes
on the opposite side. These layers are built up on a sheet of
glass to form a very thin, lightweight display panel.
Since the drive electrodes are dielectrically isolated from
the phosphor material, and each other, the display panel exhibits a capacitive load to the drive electronics. On larger
panels this capacitance can be quite high. Surge currents
can be large; therefore, coupling from the row to the column
electrodes should be considered.
Bottom Data Input
Column Clock
Column Enable
Column Latch
Electroluminescent display
(512 x 256 pixels)
Right Row
Because the phosphor requires successive pulses of opposite polarity to operate, an opposite polarity refresh pulse is
applied to all row electrodes simultaneously while the column drivers are kept at ground. The sequence then begins
Left Row
Depending on the data to be displayed in each column, the
column electrode electronics supply a voltage of opposite
polarity to the row scan pulse. This combination of row and
column voltage across the phosphor will exceed the threshold and cause the phosphor in areas between the energized
row electrodes and the energized column electrodes to glow.
This sequence, applied to successive rows, causes certain
portions of the display to be illuminated.
Generally, the row electrode electronics supply the major
portion of the threshold voltage, called the scan pulse, and
the opposite polarity “refresh” pulse , which is necessary for
the panel to emit light. The refresh pulse is usually applied
to all rows at one time while the scan pulse is applied to one
row at a time (starting with row #1), similar to a television
raster scan.
The drive electronics used to operate the panel are organized in a manner to surround the display panel with contacts as shown in Figure 1.
Application Note
Figure 1: Block diagram of the driver system for a TFEL (Thin Film Electroluminescent) panel.
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Note that the column drivers have two data lines with interleaved pixel data.
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Row 1
Row 2
Row 3
Row 4
Column 1
Pixel On
V Pixel
(Row 1, Col. 1)
V Scan
Pixel Off
Pixel Off
V Scan
V Ref
Figure 2: Simplified diagram illustrating row and column timing to operate an EL Panel. VREF only lights pixels that were turned on by VSCAN and VPP pulses in the previous frame of information.
Since these MOSFET source connections are connected to
chip ground, the entire device needs to be isolated or “floated” from the system ground. The control signals to the row
driver chips therefore must be opto-isolated from the system
ground. Figure 3 shows a simplified way to accomplish this.
again at row #1 with the next frame of data. Figure 2 is a representative timing diagram of the signals applied to a TFEL
panel showing the first four rows and the first column.
Due to the fact that the phosphor illumination threshold has
a slope of illumination versus applied voltage within a short
range, the column drive electronics can be made to vary the
applied voltage within this range, dictated by the intensity of
light desired for a particular element on the display. By this
means, a gray shade image can be created using the EL
The two high voltage supplies are switched to the row substrate (driver chip ground) using MOSFET switches. Application of the voltages to the panel is as follows:
The refresh pulse is applied to the entire panel at
the same time by pulsing on “C,” forward biasing the
body-drain diodes on all row outputs. The panel is
returned to ground by pulsing “D” while having all
the row driver outputs on. The scan pulse is applied,
one row at a time, by pulsing on “A” while the selected row output is on. The selected row is returned
Row Drivers (HV51, HV52, HV70)
To allow the open drain outputs to provide the opposite polarity pulses to the panel, the sources of the output MOSFETs
must be switched between the different voltages required for
the panel.
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+190V Refresh
Row Driver
System Ground
Q1,Q4 = 450V MOSFET
Q2,Q3 = 350V MOSFET
-190V Scan
Figure 2: Simplified diagram illustrating row and column timing to operate an EL Panel. VREF only lights pixels that were turned on by VSCAN and VPP pulses in the previous frame of information.
During the time that the data for one row is being displayed,
the data for the next row is being loaded into the shift registers, awaiting the display of the next row. When a row is
completed, the column driver VPP is brought low and the data
waiting in the shift register is loaded into the output latches.
The cycle then begins again for each successive row.
The column drivers are designed with a serial shift register
output for use in cascading the column drivers together. This
allows the data for one row to be loaded serially, using one
serial input at the first column driver device.
to ground by turning on “B.” The next row to be
scanned is then selected, and the scan is repeated;
first “A,” then “B.” When the entire panel has been
scanned, the refresh sequence is executed; first “C,”
then “D.” The scan cycle then begins again. In this
way the proper voltages and sequences are applied
to the panel for operation.
Monolevel Column Driver (HV577)
The column drivers are used to apply the data to be displayed onto the panel. The data for each row of picture elements (pixels) is loaded into all the column drivers serially
and latched into the output latches. The outputs are thus
turned to their desired state, and then the high voltage (VPP)
is applied. Columns selected for data display are connected
to VPP through the CMOS output and are pulled up to VPP. The
combination of the column VPP and the selected row voltage
will cause selected pixels to light in that particular row.
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Gray Scale Column Driver (HV623)
This device is designed to take seven data inputs in parallel into seven shift registers. The data is then taken from
equivalent stages of each shift register and converted to an
analog level, 1 of 128 between ground and VPP. This is done
by a digital counter using seven bits of input data. The counter starts with zero and increments up to turn on an output.
This transistor allows the output to ramp at the same rate as
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the ramp voltage (VR) using a holding capacitor (CH) for coupling. At the end of the gray scale cycle, the outputs will be
at their respective levels based on the seven bit input value.
The output voltage is applied to the column of the panel and
is combined with the row scan voltage to vary the light output
from each pixel in the selected row.
operate at this rate. This is fine for office and home use but
does not provide enough brightness to accommodate most
military applications. By increasing the refresh rate up to tenfold, a dramatic increase in brightness can be achieved.
This increase in refresh rate requires some changes in the
column driver configuration. Instead of cascading all the column drivers together, each column driver shift register input
is driven in parallel by the controlling system at the same
time. This increases the number of data lines required but
allows the data to be loaded much faster, enabling the faster
frame rates desired. The row drivers are used at a much
slower rate, so no changes are required to achieve faster
Panel Brightness
The varying brightness of an EL panel by voltage variation
can only achieve a limited range. Dramatically increased
panel output such as required by panels to be operated in direct sunlight, requires another method of increasing output.
This is done by increasing the panel frame rate, or refresh
rate. Normal CRT based systems work on a 60Hz frame rate.
Most applications of EL panels replacing CRTs, then, also
Supertex inc. does not recommend the use of its products in life support applications, and will not knowingly sell them for use in such applications unless it receives
an adequate “product liability indemnification insurance agreement.” Supertex inc. does not assume responsibility for use of devices described, and limits its liability
to the replacement of the devices determined defective due to workmanship. No responsibility is assumed for possible omissions and inaccuracies. Circuitry and
specifications are subject to change without notice. For the latest product specifications refer to the Supertex inc. (website: http//
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