LM8207MTX/NOPB National Semiconductor, LM8207MTX/NOPB Datasheet - Page 11

• Manufacturer Part Number: LM8207MTX/NOPB
• Description: IC BUFFER/DVR/VREF 48-TSSOP
• Manufacturer: National Semiconductor

Specifications of LM8207MTX/NOPB

• Applications: TFT-LCD Panels: Gamma Buffer, VCOM Driver
• Output Type: Rail-to-Rail
• Number Of Circuits: 18
• -3db Bandwidth: 2MHz
• Slew Rate: 1 V/µs
• Current - Supply: 6.5mA
• Current - Output / Channel: 50mA
• Voltage - Supply, Single/dual (±): 6 V ~ 16 V
• Mounting Type: Surface Mount
• Package / Case: 48-TFSOP (0.240", 6.10mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: LM8207MTX

Application Section
INTRODUCTION
The performance capabilities of TFT-LCD’s increase rapidly,
with constant improvements such as larger sizes, higher
resolution, and greater brightness. Today’s LCD’s have
screen resolutions of over 1 Mega pixel and higher. The
LM8207 can be used to improve the performance of an LCD.
It is designed for buffering 18 gamma voltage levels and
driving the V
from a highly stable Voltage Reference, which is included in
the LM8207. The LM8207 meets the design requirements
that combine technical improvement with the demand for
cost effective solutions.
The following sections discuss the principle operation of a
TFT-LCD and the principle operation of the LM8207 which
includes sections on each of the following: the Voltage Ref-
erence, the Gamma Buffers, and the V
the next sections present a typical LM8207 configuration and
consider the maximum power dissipation. The end of this
application section introduces the evaluation board and pre-
sents layout recommendations.
PRINCIPLE OPERATION OF A TFT-LCD
This section offers a brief overview of the principle operating
of TFT-LCD’s. There is a detailed description of how infor-
mation is presented on the display. An explanation of how
data is written to the screen pixels and how the pixels are
selected is also included.
Figure 1 shows a simplified illustration of an individual LCD
pixel. The top and bottom plates of a pixel consist of Indium-
Tin Oxide (ITO), which is a transparent, electrically conduc-
tive material. ITO lies on the inner surfaces of two glass
substrates that are the front and back glass panels of a TFT
display. Sandwiched between two ITO plates is an insulating
material (liquid crystal). This alters the polarization of light,
depending on how much voltage (V
the two plates. Polarizer’s are placed on the outer surfaces
of the two glass substrates. In combination with the liquid
COM
FIGURE 1. Individual LCD Pixel
level. These voltage levels can be derived
PIXEL
COM
) is applied across
Buffer. After this,
20137930
11
crystal, the polarizer’s create a variable light filter that modu-
lates light transmitted from the back to the front of a display.
A pixel’s bottom plate lies on the backside of a display where
a light source is applied, and the top plate lies on the front,
facing the viewer. For most TFT displays, a pixel transmits
the greatest amount of light when V
becomes less transparent as the voltage increases with
either a positive or negative polarity.
For color displays, each pixel is built with three individual sub
pixels. Each sub pixel represents a primary color. These
colors are Red, Green and Blue (RGB). Combining these
three primary colors every user-defined color can be cre-
ated.
Figure 2 shows a simplified diagram of a TFT display, show-
ing how individual pixels are connected to the row, column
and V
with a 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
top plates (also called gamma voltage) controls the pixel
brightness. The column drivers supply this gamma voltage
via the column lines, and ‘write’ this voltage to the pixels one
row at a time. This is accomplished by having the row drivers
selecting an individual row of pixels when the column driver
writes the gamma voltage levels. The row drivers sequen-
tially apply a large positive pulse (typically 25V to 35V) to
each row line. This turns on the NMOS transistors connected
to an individual row, allowing voltage from the column lines
to be written to the pixels.
The V
to all the pixels in a TFT panel. V
voltage that is in the middle of the gamma voltage range. As
a result, when a column driver writes to a row of pixels, the
applied voltages are either positive or negative with respect
to V
a row is selected, preventing a pattern from being ‘burned’
into the LCD.
COM
COM
COM
. In fact, the polarity of a pixel is reversed each time
driver (buffer) supplies a common voltage (V
driver. Each pixel is represented by a capacitor
FIGURE 2. TFT Display
COM
. The voltage applied to the
COM
PIXEL
is a constant DC
≤
±
0.5 V, and it
www.national.com
20137931
COM
)