Datasheet

V
COM
DRIVER
+
-
COLUMN DRIVERS
C
STRAY
C
STRAY
C
STRAY
V
COM
V
COM
V
COM
APPROX
V
DD
/2
PIXEL
TFT-LCD PANEL
PIXEL
PIXEL
V
COM
ROW DRIVERS
LM6588
www.ti.com SNOSA77D –MAY 2003–REVISED MARCH 2013
Figure 26. TFT Display
Figure 26 is a simplified block diagram of a TFT display, showing how individual pixels are connected to the row,
column, and V
COM
lines. Each pixel is represented by capacitor with an NMOS transistor connected to its top
plate. Pixels in a TFT panel are arranged in rows and columns. Row lines are connected to the NMOS gates,
and column lines to the NMOS sources. The back plate of every pixel is connected to a common voltage called
V
COM
. Pixel brightness is controlled by voltage applied to the top plates, and the Column Drivers supply this
voltage via the column lines. Column Drivers ‘write’ this voltage to the pixels one row at a time, and this is
accomplished by having the Row Drivers select an individual row of pixels when their voltage levels are
transmitted by the Column Drivers. The Row Drivers sequentially apply a large positive pulse (typically 25V to
35V) to each row line. This turns-on NMOS transistors connected to an individual row, allowing voltages from the
column lines to be transmitted to the pixels.
V
COM
DRIVER
The V
COM
driver supplies a common voltage (V
COM
) to all the pixels in a TFT panel. V
COM
is a constant DC
voltage that lies in the middle of the column drivers’ output voltage range. As a result, when the column drivers
write to a row of pixels, they apply voltages that are either positive or negative with respect to V
COM
. In fact, the
polarity of a pixel is reversed each time its row is selected. This allows the column drivers to apply an alternating
voltage to the pixels rather than a DC signal, which can ‘burn’ a pattern into an LCD display.
When column drivers write to the pixels, current pulses are injected onto the V
COM
line. These pulses result from
charging stray capacitance between V
COM
and the column lines (see Figure 26), which ranges typically from
16pF to 33pF per column. Pixel capacitance contributes very little to these pulses because only one pixel at a
time is connected to a column, and the capacitance of a single pixel is on the order of only 0.5pF. Each column
line has a significant amount of series resistance (typically 2kΩ to 40kΩ), so the stray capacitance is distributed
along the entire length of a column. This can be modeled by the multi-segment RC network shown in Figure 27.
The total capacitance between V
COM
and the column lines can range from 25nF to 100nF, and charging this
capacitance can result in positive or negative current pulses of 100mA, or more. In addition, a similar distributed
capacitance of approximately the same value exists between V
COM
and the row lines. Therefore, the V
COM
driver’s load is the sum of these distributed RC networks with a total capacitance of 50nF to 200nF, and this load
can modeled like the circuit in Figure 27.
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