LM8207
TFT 18 Gamma Buffer + V
COM
Driver + Voltage Reference
General Description
The LM8207 is a combination of 18-channel gamma buffers,
aV
COM
driver and a temperature compensated internal volt-
age reference. It is designed for buffering voltage levels and
driving high capacitive loads in large TFT panels. The
gamma buffers are individually optimized to the input/output
requirements of their respective gamma position to cover the
whole voltage range from rail to rail. Any desired gamma
correction curve can be obtained by combining the gamma
buffers with external resistors. The V
COM
driver has a high
output current capability and is stable with large capacitive
loads, typical for large panel sizes. This will result in a fast
recovery time for large voltage variations at the output. The
internal band gap reference can be used to form a highly
stable voltage to generate the gamma correction voltages. In
combination with the internal amplifier, the reference voltage
can be programmed to voltages up to the positive rail. The
LM8207 is offered in a 48-pin TSSOP package.
Features
nGamma buffers 1-2 swing to V
DD
nGamma buffers 17-18 swing to V
SS
nLarge output current V
COM
driver (I
SC
= 300 mA)
nStable (1%) internal 1.295V reference, to improve
picture quality and reduce variations
n48-pin TSSOP package
Applications
nTFT gamma curve connection and V
COM
voltage
buffering
TFT Panel Block Diagram
20137926
September 2005
LM8207 TFT 18 Gamma Buffer + V
COM
Driver + Voltage Reference
© 2005 National Semiconductor Corporation DS201379 www.national.com
Absolute Maximum Ratings (Notes 1, 2)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
ESD Tolerance (Note 3)
Human Body 2.5 kV
Machine Model 250V
Supply Voltage (V
DD
-V
SS
) 18V
Storage Temperature Range −65˚C to +150˚C
Junction Temperature (Note 4) +150˚C
Soldering Information
Infrared or Convection (20 sec.) 230˚C
Wave Soldering (10 sec.) 260˚C
Operating Ratings (Note 1)
Operating Temperature Range −40˚C to +105˚C
Operating Voltage Range 6V to 16V
Package Thermal Resistance, θ
JA
(Note 4)
48-Pin TSSOP 84˚C/W
16V Electrical Characteristics
Unless otherwise specified, all limits guaranteed for T
J
= 25˚C, V
DD
= 16V, V
SS
=0V,&C
LOAD
= 100 pF (Gamma & V
COM
Buffers). Boldface limits apply at the temperature extremes. (Note 5)
Symbol Parameter Conditions Min
(Note 6)
Typ
(Note 7)
Max
(Note 6)
Units
Gamma Buffers
BW_Gamma −3 dB Bandwidth 2 MHz
SR_Gamma Slew Rate (Note 8) 1 V/µs
T
REC
_Gamma Output Recovery Time (Note 9) 400 ns
V
IN
_Gamma Input Voltage Range Buffer 1-2 Positive V
DD
V
Negative V
SS
+0.6
Buffer 3-8 & 11-16 Positive V
DD
-0.6
Negative V
SS
+0.6
Buffer 9 Positive V
DD
−0.6
Negative V
SS
Buffer 10 Positive V
DD
-0.6
Negative V
SS
+0.6
Buffer 17-18 Positive V
DD
−0.6
Negative V
SS
V
OUT
_Gamma Output Voltage Range Buffer 1-2,
No Load
Positive V
DD
-0.25 V
DD
-0.1
V
Negative V
SS
+1.5 V
SS
+1.6
Buffer 3-8 & 11-16
No Load
Positive V
DD
−1.2 V
DD
-1.1
Negative V
SS
+0.6 V
SS
+0.7
Buffer 9,
No Load
Positive V
DD
−1.0 V
DD
-0.8
Negative V
SS
+0.8 V
SS
+0.9
Buffer 10,
No Load
Positive V
DD
–1.2 V
DD
–1.1
Negative V
SS
+0.6 V
SS
+0.7
Buffer 17-18,
No Load
Positive V
DD
–1.6 V
DD
–1.5
Negative V
SS
+0.1 V
SS
+0.25
I
BIAS
_Gamma Absolute, Input Bias Current Within Gamma Buffer Output
Voltage Range 30 nA
V
OS
_Gamma Input Offset Voltage Buffer 1-2, V
IN
=8V 5 10
mV
Buffer 3-8, 11-16, V
IN
=8V 1 5
Buffer 9, V
IN
=8V 1 5
Buffer 10, V
IN
=8V 1 5
Buffer 17-18, V
IN
=8V 5 10
LM8207
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16V Electrical Characteristics (Continued)
Unless otherwise specified, all limits guaranteed for T
J
= 25˚C, V
DD
= 16V, V
SS
=0V,&C
LOAD
= 100 pF (Gamma & V
COM
Buffers). Boldface limits apply at the temperature extremes. (Note 5)
Symbol Parameter Conditions Min
(Note 6)
Typ
(Note 7)
Max
(Note 6)
Units
I
OUT
_Gamma Linear Output Current
(Note 10)
Buffer 1-2 Sourcing 20 46
mA
Sinking 0.2 0.33
Buffer 3-8 & 11-16 Sourcing 10 24.5
Sinking 3.5 5.5
Buffer 9 Sourcing 4.5 9.4
Sinking 15 27
Buffer 10 Sourcing 23 34.8
Sinking 3.5 5.5
Buffer 17-18 Sourcing 0.2 0.33
Sinking 20 50
PSRR Power Supply Rejection Ratio V
DD
-V
SS
= 6V to 16V 75 88 dB
V
COM
Driver
BW_V
COM
Bandwidth 10 MHz
SR_ V
COM
Slew Rate (Note 8) 4.5 V/µs
T_
REC
_V
COM
Output Recovery Time (Note 9) 200 ns
V
IN
_V
COM
Input Voltage Range Positive V
DD
V
Negative V
SS
+0.6
V
OUT
_V
COM
Output Voltage Range No Load Positive V
DD
–1.0 V
DD
–0.7 V
Negative V
SS
+0.9 V
SS
+1.2
I
BIAS
_V
COM
Input Bias Current Within V
COM
Buffer Output
Voltage Range 50 nA
V
OS
_V
COM
Input Offset Voltage V
IN
=8V 1 10 mV
I
OUT
_
LIN
_V
COM
Linear Output Current
(Notes 10, 11)
Sourcing 160 mA
Sinking 150
I
OUT
_SC_V
COM
Short Circuit Output Current
(Notes 11, 12)
Sourcing 220 300 mA
Sinking 220 300
PSRR Power Supply Rejection Ratio V
DD
-V
SS
= 6V to 16V 75 88 dB
Voltage Reference Section
V
REF
Voltage No Load 1.28 1.295 1.31 V
Reg
LOAD
Load Regulation I
OUT
= 0 to 10 mA 0.14 mV/mA
V
REF_ACC
Voltage Accuracy No Load, V
REF
= 1.295V 1 %
V
REF_MAX
Max Programming Range I
OUT
=4mA V
DD
−0.3 V
I
IN
_V
REF
Input Bias Current Within V
REF
Output Voltage
Range 10 50 nA
TC_V
REF
Temperature Stability 70 ppm/˚C
I
OUT
_V
REF
Max Output Current Sourcing, V
OUT
= 1.295 V 71 mA
PSRR Power Supply Rejection Ratio
(Line Regulation) 70 80 dB
Miscellaneous
I
S
Supply Current 4.5 6.5 8.5
9.5 mA
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is
intended to be functional, but specific performance is not guaranteed. For guaranteed specifications, see the Electrical Characteristics tables.
Note 2: When the output of the VCOM buffer exceeds the supply rails, while sinking or sourcing 100 mA, the VCOM output is susceptible to latch.
Note 3: Human body model, 1.5 kΩin series with 100 pF. Machine model, 0Ωin series with 200 pF
Note 4: The maximum power dissipation is a function of TJ(MAX),θJA and TA. The maximum allowable power dissipation at any ambient temperature
is PD=(T
J(MAX) –T
A)/θJA. All numbers apply for packages soldered directly onto a PC board.
Note 5: Electrical Table values apply only for factory testing conditions at the temperature indicated. Factory testing condition result in very limited self-heating of
the device such that TJ=T
A. No guarantee of parametric performance is indicated in the electrical table under conditions of internal self-heating where TJ>TA.
LM8207
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16V Electrical Characteristics (Continued)
Note 6: All limits are guaranteed by design or statistical analysis.
Note 7: Typical values represent the parametric norm at the time of characterization.
Note 8: Slew Rate is measured for VIN =4V
PP. 10% -90% values are used. Slew rate is the average of the rising and falling slew rates
Note 9: 4V
PP pulse (50 ns rise time) applied to one side of 100 pF series output capacitance, other side connected to output of buffer. Output to within 0.1% of
input voltage.
Note 10: Linear output current measured at |VOUT -V
IN| = 0.1V.
Note 11: This is a momentary test. Continuous large output currents may result in exceeding the maximum power dissipation and damage the device.
Note 12: Short circuit current measured at |VOUT -V
IN|=1V.
Connection Diagram
48-Pin TSSOP
20137902
Top View
Pin Descriptions
Pin # Description Remark
1 OUT_V
REF
Reference voltage amplifier output
2 NC No connection
3 IN_1 Input gamma buffer 1
4 IN_2 Input gamma buffer 2
5 IN_3 Input gamma buffer 3
6 IN_4 Input gamma buffer 4
7 IN_5 Input gamma buffer 5
8 IN_6 Input gamma buffer 6
9 IN_7 Input gamma buffer 7
10 IN_8 Input gamma buffer 8
11 IN_9 Input gamma buffer 9
12 V
DD
Positive supply voltage (V
DD
)
13 IN_10 Input gamma buffer 10
14 IN_11 Input gamma buffer 11
15 IN_12 Input gamma buffer 12
16 IN_13 Input gamma buffer 13
17 IN_14 Input gamma buffer 14
18 IN_15 Input gamma buffer 15
19 IN_16 Input gamma buffer 16
20 IN_17 Input gamma buffer 17
21 IN_18 Input gamma buffer 18
LM8207
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Pin Descriptions (Continued)
22,23 NC No connection
24 IN_V
COM
Input V
COM
25 OUT_V
COM
Output V
COM
26,27 NC No connection
28 OUT_18 Output gamma buffer 18
29 OUT_17 Output gamma buffer 17
30 OUT_16 Output gamma buffer 16
31 OUT_15 Output gamma buffer 15
32 OUT_14 Output gamma buffer 14
33 OUT_13 Output gamma buffer 13
34 OUT_12 Output gamma buffer 12
35 OUT_11 Output gamma buffer 11
36 OUT_10 Output gamma buffer 10
37 V
SS
Negative supply voltage (V
SS
)
38 OUT_9 Output gamma buffer 9
39 OUT_8 Output gamma buffer 8
40 OUT_7 Output gamma buffer 7
41 OUT_6 Output gamma buffer 6
42 OUT_5 Output gamma buffer 5
43 OUT_4 Output gamma buffer 4
44 OUT_3 Output gamma buffer 3
45 OUT_2 Output gamma buffer 2
46 OUT_1 Output gamma buffer 1
47 NC No connection
48 IN_V
REF
Reference voltage amplifier feedback input
Ordering Information
Package Part Number Package Marking Transport Media NSC Drawing
48-Pin TSSOP LM8207MT LM8207MT 38 Units/Rail MTD48
LM8207MTX 1k Units Tape and Reel
Block Diagram
20137901
LM8207
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Typical Performance Characteristics At T
J
= 25˚C, V
DD
= 16V, V
SS
= 0V. Unless otherwise
specified.
Output Voltage Swing (Negative rail) Output Voltage Swing (Positive rail)
20137907 20137908
Voltage Drop vs. Output Current (V
COM
Buffer) Voltage Drop vs. Output Current (V
COM
Buffer)
20137942 20137944
Voltage Drop vs. Output Current (Gamma Buffer) Voltage Drop vs. Output Current (Gamma Buffer)
20137941 20137943
LM8207
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Typical Performance Characteristics At T
J
= 25˚C, V
DD
= 16V, V
SS
= 0V. Unless otherwise
specified. (Continued)
Offset Voltage vs. Supply Voltage (Gamma Buffer) Offset Voltage vs. Supply Voltage (V
COM
Buffer)
20137937 20137938
Recovery Time (V
COM
Buffer) Negative Slope
(C
L
= 100 pF)
Recovery Time (V
COM
Buffer) Positive Slope
(C
L
= 100 pF)
20137911 20137913
Large Signal Transient Response (V
COM
Buffer)
Negative Slope (C
L
= 100 pF)
Large Signal Transient Response (V
COM
Buffer)
Positive Slope (C
L
= 100 pF)
20137915 20137916
LM8207
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Typical Performance Characteristics At T
J
= 25˚C, V
DD
= 16V, V
SS
= 0V. Unless otherwise
specified. (Continued)
Frequency Response for Various Temperature
(V
COM
Buffer)
Frequency Response for Various Load
(V
COM
Buffer)
20137932 20137934
Gain/Phase (Gamma Buffer)
(C
L
= 100 pF) PSRR (V
COM
Buffer)
20137903 20137919
Recovery Time (Gamma Buffer 3-16) Negative Slope
(C
L
= 100 pF)
Recovery Time (Gamma Buffer 3-16) Positive Slope
(C
L
= 100 pF)
20137912 20137914
LM8207
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Typical Performance Characteristics At T
J
= 25˚C, V
DD
= 16V, V
SS
= 0V. Unless otherwise
specified. (Continued)
Large Signal Transient Response (Gamma Buffer 3-16)
Negative slope (C
L
= 100 pF)
Large Signal Transient Response (Gamma Buffer 3-16)
Positive slope (C
L
= 100 pF)
20137917 20137918
Frequency Response for Various Load
(Gamma Buffer)
Frequency Response for Various Temperature
(Gamma Buffer)
20137933 20137935
Gain/Phase (Gamma Buffer)
(C
L
= 100 pF) PSRR (Gamma Buffer)
20137904 20137920
LM8207
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Typical Performance Characteristics At T
J
= 25˚C, V
DD
= 16V, V
SS
= 0V. Unless otherwise
specified. (Continued)
Supply Current vs. Supply Voltage Voltage Reference vs. Output Current
20137939 20137940
Voltage Reference vs. Supply Voltage
Voltage Reference PSRR
(Line Regulation)
20137936 20137945
LM8207
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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
COM
level. These voltage levels can be derived
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
COM
Buffer. After this,
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
PIXEL
) is applied across
the two plates. Polarizer’s are placed on the outer surfaces
of the two glass substrates. In combination with the liquid
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
PIXEL
≤±0.5 V, and it
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
COM
driver. Each pixel is represented by a capacitor
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
COM
. The voltage applied to the
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
COM
driver (buffer) supplies a common voltage (V
COM
)
to all the pixels in a TFT panel. V
COM
is a constant DC
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
COM
. In fact, the polarity of a pixel is reversed each time
a row is selected, preventing a pattern from being ‘burned’
into the LCD.
20137930
FIGURE 1. Individual LCD Pixel 20137931
FIGURE 2. TFT Display
LM8207
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Application Section (Continued)
Figure 3 shows how the display information is refreshed.
Using the row and column drivers, one pixel is addressed at
the display. The column driver receives the digital color data
from the timing controller. The corresponding gamma volt-
age will be determined, using the gamma correction curve.
In fact, the gamma correction curve is just a voltage refer-
ence with 18 output tabs, which presets the color intensity
settings. This gamma voltage is written to the pixel. The
column driver selects one column at the time; the changing
in the load may affect the ‘tabs’ of the gamma correction
curve. This problem can be solved using ‘gamma buffers’ to
isolate the gamma correction curve from the column driver.
PRINCIPLE OPERATION of the LM8207
The LM8207 combines three basic functions used in TFT
displays:
•Voltage Reference
To improve picture quality and to reduce brightness varia-
tions, a highly stable reference voltage is available. It has
a low drift over the operation temperature range. This
output voltage (OUT_V
REF
) is used as the reference
voltage to define the gamma correction values.
•Gamma Buffers
The gamma correction curve can be defined easily using
an external chain of precision resistors. To ensure load
independent gamma correction levels, 18 gamma buff-
ers, each having a low output resistance, can be used to
drive the TFT display column drivers.
•V
COM
Buffer
The V
COM
buffer supplies a common voltage, which is
applied to the back plate of all the pixels. Writing color
information to all the pixels will cause high current varia-
tions at the V
COM
level so this V
COM
buffer is designed for
driving large output currents.
These three functions are discussed in detail in the following
sections.
VOLTAGE REFERENCE
The internal Voltage Reference of the LM8207 can be used
to improve picture stability. This accurate reference is highly
stable over the operation temperature range. The output
voltage (OUT_V
REF
) of the Voltage Reference can be set
using two external resistors. In the next two sections, the
possibilities for setting the output voltage of the Voltage
Reference and the operating range of the Voltage Reference
are discussed.
SETTING THE OUTPUT VOLTAGE OF THE VOLTAGE
REFERENCE
The output voltage of the Voltage Reference Amplifier
(OUT_V
REF
) can be set using the internal reference in com-
bination with the internal amplifier and two external resistors.
In Figure 4 a typical application circuit for V
REF
is given.
20137926
FIGURE 3. Block Diagram of a Typical TFT-LCD
LM8207
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Application Section (Continued)
To calculate the output voltage of the Voltage Reference
Amplifier (OUT_V
REF
) use the following equation:
(1)
As can be seen in the Electrical Characteristics Table on
page 3, I
IN_V
REF has a typical value of −10 nA. Using resistor
values for R
1
=9kΩand R
2
=1kΩthis results in a gain of
10 and OUT_V
REF
= 12.95 V an error will be introduced of
−10 nA*9 kΩ= −90 µV. This error can be neglected. The
simplified formula for calculating the OUT_V
REF
is:
(2)
Example:
V
DD
= 16V
OUT_V
REF
= 14.4V
Choose R
2
=5kΩ. Using Equation (2), this will result in
R
1
= 50.6 kΩ
THE OPERATING RANGE OF THE VOLTAGE
REFERENCE
The output of the Voltage Reference Amplifier has a mini-
mum of 1.295V (R
1
= 0). This is determined by the value of
the internal reference. The maximum output voltage
(OUT_V
REFMAX
) can approach the positive supply rail V
DD
.
The voltage is limited by the output resistance (R
OUT
)ofthe
output stage of the internal amplifier and depends on the
load current. Figure 5 shows the operating output voltage
range.
The minimum headroom (OUT_V
REF
with respect to the
positive supply rail V
DD
) can be measured using the test
circuit shown in Figure 6.
The headroom is measured by varying both the supply volt-
age and the output current (I
LOAD
) for a fixed programmed
value of OUT_V
REF
. As shown in Figure 7, the minimum
headroom slightly increases for a constant V
DD
when the
load current increases.
20137925
FIGURE 4. Typical Application Circuit for V
REF
20137921
FIGURE 5. Operating Output Voltage Range
20137924
FIGURE 6. Headroom Test Circuit with Variable Output
Current Load
LM8207
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Application Section (Continued)
GAMMA BUFFERS
This section gives an overview for the applications of the
gamma buffers and also defines the gamma correction
curve. Specifications for the buffers are derived from their
operation range. Also included are the formulas for the real-
ization of the gamma correction curve using external resis-
tors. An overview is given for the gamma voltage accuracy,
using the LM8207 in combination with external resistors.
As discussed in the section entitled “Principle Operation of a
TFT-LCD,” the basic function of the gamma buffers is to
make the gamma correction curve independent of the be-
havior of the column driver. Writing data to each subsequent
pixel will cause load variations. The gamma buffers have a
low impedance output and can handle these variations with-
out changing the gamma correction curve. A typical gamma
correction curve is given in Figure 8.
Each buffer covers a part of the correction curve and, there-
fore, has its own specifications. All buffers require that the
output should recover quickly from disturbances caused by
the switching of the column driver. The gamma voltage level
of each buffer (VGMA1…VGMA18) depends on its position
for the levels decrease sequentially. To best utilize the
LM8207, each buffer is optimized for its position in the
gamma correction curve.
•Gamma Buffers 1-2
Operating voltage range: V
DD
to V
SS
+2V. Due to the
operating voltage, only negative transitions at the output
are possible. Positive transitions will exceed the supply
voltage V
DD
. These buffers are able to source current to
bias the resistive load of the column driver having an
open collector structure. To meet the operating voltage
range, these outputs need a resistive load connected to a
lower potential sourcing an output current of at least
1 mA.
•Gamma Buffers 3-16
Operating voltage range: V
DD
–1VtoV
SS
+ 1V. Due to
the operating range, both positive and negative transi-
tions at the outputs are possible.
•Gamma Buffers 17-18
Operating voltage range: V
DD
-2toV
SS
. Due to the
operating voltage, only positive transitions at the output
are possible. Negative transitions will exceed the nega-
tive supply voltage V
SS
. These buffers are able to sink
current from the resistive load of the column driver having
an open collector structure. To meet the operating volt-
age range, these outputs need a resistive load connected
to a higher potential sinking an output current of at least
1mA
Example:
A typical application using the LM8207 is given in Figure 9.
The corresponding gamma correction curve
(VGMA1...VGMA18) is defined in Table 1. The Voltage Ref-
erence supplies the 14.4V to the resistor network. The cal-
culations for the resistor values and for setting the Voltage
Reference are shown in the section “Voltage Reference.”
20137922
FIGURE 7. Voltage Reference Headroom vs. Load
Current
20137923
FIGURE 8. Typical Gamma Correction Curve
LM8207
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Application Section (Continued)
The values of the resistors in the gamma correction curve
are calculated such that a current of 1 mA flows in the
resistor chain.
Where x is the index for the corresponding gamma voltage
and has a range of 1 to 18.
Using these formulas the resistor values in Table 1 are
calculated. High accuracy resistors values can be realized
using 0.1% resistors. A method for fine-tuning the resistor
value is to combine two resistors in series.
20137927
FIGURE 9. Typical TFT Display Application Diagram Using the LM8207
LM8207
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Application Section (Continued)
TABLE 1. Resistor Values for Defining the Gamma
Correction Curve
Gamma Curve Definition
VGMA
Node
VGMA Voltage Calculated Resistance (Ω)
1 11.59 210
2 11.38 2200
3 9.18 670
4 8.51 670
5 7.84 430
6 7.41 280
7 7.13 980
8 6.15 80
9 6.07 170
10 5.90 80
11 5.82 980
12 4.84 280
13 4.56 430
14 4.13 670
15 3.46 730
16 2.73 2190
17 0.54 210
18 0.33 330
Changing the gamma correction curve, in combination with
the load of the column drivers can impact the behavior of the
gamma buffers. Gamma buffers 1 and 2 are designed for
operating voltages near V
DD
, and will source the current into
the column drivers. Gamma buffers 17 and 18 are designed
for operating voltages near V
SS
and will sink this current.
Buffers 3 to 16 are designed to operate in the mid-voltage
range and can sink or source current. Under special circum-
stances, by increasing the voltage gap between gamma
buffer 1 and gamma buffer 2, in combination with a low
impedance load of the column driver between these outputs,
the output of buffer 2 has to sink more current than possible,
and can saturate. This will result in a setting error of the
inputs of the column driver.
For buffer 17 and 18 an identical situation can occur, by
increasing the operating voltage range of buffer 17 with
respect to buffer 18.
A simple and cost effective solution is to lower the resistance
between buffer 2 and 3 or buffer 16 and 17, using an
additional by-pass resistor R
S
. This method is presented in
Figure 10. This will not affect the desired voltage levels, and
buffer 3 which has a larger linear output current spec will sink
the current instead of buffer 2. The resistor value R
S
can be
calculated by the voltage drop divided by the current. The
resistor value should be low enough to sink this current,
otherwise buffer 2 and/or buffer 17 will still saturate.
GAMMA VOLTAGE ACCURACY
Adding buffers to the tabs of the gamma correction resistor
chain will make the values more independent of the load
variations. Unfortunately, there are some other effects that
will influence the gamma values. The following effects deter-
mine the accuracy of each gamma voltage.
Major effects are:
•Variation of the internal voltage reference. This can be
found in the Electrical Characteristics Table. This is the
maximum variation between parts.
•Variation of the feedback resistors used for setting the
output voltage of the voltage reference (OUT_V
REF
). Us-
ing high accuracy resistors will result in a small variation
of the output voltage between different boards
•The accuracy of the resistors obtained from the gamma
correction voltage curve. The gamma correction curve
will be affected by the accuracy of the resistors. This will
vary over different boards. Temperature variations will not
affect this curve.
•Output offset voltage (V
OS
) of the buffers. Variations of
V
OS
(output offset voltage) of the buffers, will affect the
gamma correction curve. The contribution of V
OS
is
higher for the buffers driving the lower gamma voltages.
Minor effects are:
•Input current (I
BIAS
) of the gamma buffers. Variations of
the input current (I
BIAS
) of the gamma buffers caused by
temperature changes, will affect the gamma correction
voltages.
20137946
FIGURE 10. Using additional by-pass resistor to
increase current sinking capability
LM8207
www.national.com 16
Application Section (Continued)
V
COM
BUFFER
The V
COM
buffer supplies a common voltage to the back
plate of all the pixels in a TFT panel. When column drivers
write to the pixels, current pulses will occur onto the V
COM
line. These pulses are the result of charging the capacitance
between V
COM
and the column lines. This capacitance is a
combination of stray capacitance and pixel capacitance. This
stray capacitance varies between panel sizes but typically
ranges from 16 pF to 33 pF per column. Pixel capacitance is
in the order of 0.5 pF and contributes very little to these
pulses because only one pixel at a time is connected to a
column. Charging this capacitance can result in short posi-
tive or negative current pulses of 100 mA or more, depend-
ing on the panel size. The V
COM
buffer is designed to handle
these pulses. A V
COM
buffer is basically a voltage regulator
that can sink or source current in large capacitive loads. The
V
COM
buffer should recover very fast from these distur-
bances. The operating voltage of the V
COM
buffer is in the
middle of the gamma voltage range.
The typical application in Figure 11 shows the V
COM
buffer
supplying a common voltage to the back plate of the display.
This level can be adjusted by changing the value of the
resistors. Increasing the value of R
1
or decreasing the value
of R
2
will decrease the V
COM
level. Increasing the value of
R
2
or decreasing the value of R
1
will increase the V
COM
level.
Another, more flexible, solution is to use National Semicon-
ductor’s programmable V
COM
calibrator, the LM8342. The
V
COM
level can be adjusted using an I
2
C interface. See the
LM8342’s datasheet for more detailed information about this
part.
LM8207 CONFIGURATION
A complete configured typical application of the LM8207 is
given in Figure 12. All three basic functions of the LM8207
are discussed in the previous sections. Details for setting the
Voltage Reference are given in the “Voltage Reference”
section. Calculations for defining a gamma correction curve
are given in the section entitled “Gamma Buffers.” Defining
and adjusting the V
COM
level is discussed in the “V
COM
Buffer” section. The LM8207 is an 18 channel gamma buffer
plus a V
COM
buffer. In certain applications some of the
gamma buffers or the V
COM
buffer may not be used. In such
cases it is recommended that the unused buffer input pins be
tied to the input voltage range value.
20137928
FIGURE 11. VCOM Buffer
LM8207
www.national.com17
Application Section (Continued)
20137929
FIGURE 12. LM8207 Configuration
LM8207
www.national.com 18
Application Section (Continued)
MAXIMUM POWER DISSIPATION
The maximum power dissipation in the LM8207 TSSOP
package depends on the ambient temperature and the in-
crease of the junction temperature of the die. Exceeding the
maximum temperature will damage the part. (See the Abso-
lute Maximum Ratings table on page 2 of the datasheet.)
The V
COM
buffer of the LM8207 is designed for use in pulsed
conditions. Driving a continuous current of several hundred
mA to a load will damage the part due to the high power
consumption of the output stage of the V
COM
buffer.
The maximum operating temperature can be calculated us-
ing this formula:
T
J
=T
A
+θ
JA
xP
DISSIPATION
(3)
Where
T
A
= Ambient temperature
θ
JA
= Thermal resistance of package (See
Operating Ratings table on page 2)
(84˚C/W)
P
DISSIPATION
= Total power dissipation of the LM8207
Example:
The estimated power consumption of the LM8207 in a
steady state situation with no load is:
V
DD
= 16V
I
DD
= 6 mA (all buffers within
normal operating range)
OUT_V
REF
= 14.4V
I
LOAD
=3mA
V
DD
xI
DD
=16Vx6mA
(V
DD
- OUT_V
REF
)xI
LOAD
=
(16 V – 14.4 V)x3mA =4.8mW
Total steady state power
dissipation = 100.8 mW
For an ambient temperature T
A
of 40˚C and a dissipated
power of 100.8 mW, the junction temperature T
J
will be 49˚C.
This will not exceed the maximum operating temperature.
Two issues are not considered in the calculation:
•Continuous power dissipation of the gamma buffers. This
is load dependent, and can be calculated using the volt-
age drop over the output stage times the output current:
P=(V
DD
-V
GMAx
)xI
OUT
for current sourcing
P=(V
GMAx
)xI
OUT
for current sinking
•Pulsed power dissipation of the buffers. The RMS value
of this pulsed current depends on the magnitude of the
current fluctuations and the duty cycle. This can majorly
contribute to the total power dissipation.
Example:
When the LM8207 is in steady state biasing, the V buffer is
considered at three various load conditions:
I
OUTRMS
(mA)
V
COM
Level (V)
Dissipation
(mW)
Temp
Rise
T
J
10 8 80 7 56
50 8 400 35 83
100 8 800 67 107
When IOUTRMS = 100 mA, the package (TJ) will exceed the Operating
Temperature!
EVALUATION BOARD
For testing purposes an evaluation board is available. It is
intended to evaluate the following functions:
•The Voltage Reference is fully adjustable within the op-
erating range. For optimal output voltage ranges, user
defined resistors can be trimmed by using two resistors in
series.
•The Gamma correction curve is user defined using exter-
nal resistors. Each optimal value can be achieved by
using two series resistors for fine-tuning.
•The V
COM
node input voltage can be achieved using
National Semiconductor’s LM8342 programmable V
COM
calibrator, or using an external supply.
•For testing, an additional dummy load can be connected
to all outputs of the gamma buffers.
LM8207
www.national.com19
Application Section (Continued)
20137950
FIGURE 13. Schematic LM8207 Evaluation Board
LM8207
www.national.com 20
Application Section (Continued)
LAYOUT RECOMMENDATIONS
A proper layout is necessary for optimum performance of the
LM8207. A low impedance and clean ground plane is rec-
ommended. The traces from the V
SS
pin to the ground plane
should be as short as possible. Decoupling capacitors
should be placed very close to the V
DD
pin. Connections of
these decoupling capacitors to the ground plane should be
very short. An additional decoupling capacitor for OUT_V
REF
is recommended.
Due to the heavy current peaks and short transitions at the
V
COM
node, traces from the output of the V
COM
buffer should
be low impedance and as short as possible, to minimize both
voltage drops over the trace and unwanted EM distur-
bances.
Bottom View
20137951
FIGURE 14. Layout of LM8207 Evaluation Board (Actual Size)
TOP View
20137952
FIGURE 15. Layout of LM8207 Evaluation Board (Actual Size)
LM8207
www.national.com21
Physical Dimensions inches (millimeters) unless otherwise noted
48-Pin TSSOP
NS Package Number MTD48
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves
the right at any time without notice to change said circuitry and specifications.
For the most current product information visit us at www.national.com.
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device or system whose failure to perform can be reasonably
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www.national.com
LM8207 TFT 18 Gamma Buffer + V
COM
Driver + Voltage Reference
