LM8207
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SNOSAL5A – SEPTEMBER 2005 – REVISED MARCH 2013
LM8207 TFT 18 Gamma Buffer + VCOM Driver + Voltage Reference
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FEATURES
DESCRIPTION
•
•
•
The LM8207 is a combination of 18-channel gamma
buffers, a VCOM driver and a temperature
compensated internal voltage 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 VCOM 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.
1
2
•
•
Gamma Buffers 1-2 Swing to VDD
Gamma Buffers 17-18 Swing to VSS
Large Output Current VCOM Driver
(ISC = 300 mA)
Stable (1%) Internal 1.295V Reference, to
Improve Picture Quality and Reduce Variations
48-pin TSSOP Package
APPLICATIONS
•
TFT Gamma Curve Connection and VCOM
Voltage Buffering
TFT Panel Block Diagram
VCOM
GAMMA CORRECTION CURVE
LM8342
REFERENCE
LM8207
VREF
18 GAMMA BUFFER
TIMING
CONTROL
MULTI
SOURCE
DC/DC
CONVERTER
& LDO
VCOM
BUFFER
COLUMN DRIVER
ROW
DRIVER
DISPLAY
1
2
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
All trademarks are the property of their respective owners.
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
Copyright © 2005–2013, Texas Instruments Incorporated
LM8207
SNOSAL5A – SEPTEMBER 2005 – REVISED MARCH 2013
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These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
Absolute Maximum Ratings (1) (2)
ESD Tolerance (3)
Human Body
Machine Model
2.5 kV
250V
Supply Voltage (VDD - VSS)
18V
−65°C to +150°C
Storage Temperature Range
Junction Temperature (4)
+150°C
Soldering Information
Infrared or Convection (20 sec.)
230°C
Wave Soldering (10 sec.)
260°C
(1)
(2)
(3)
(4)
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 ensured. For ensured specifications, see the Electrical
Characteristics tables.
When the output of the VCOM buffer exceeds the supply rails, while sinking or sourcing 100 mA, the VCOM output is susceptible to latch.
Human body model, 1.5 kΩ in series with 100 pF. Machine model, 0Ω in series with 200 pF
The maximum power dissipation is a function of TJ(MAX), θJA and TA. The maximum allowable power dissipation at any ambient
temperature is PD = (TJ(MAX) – TA)/θJA. All numbers apply for packages soldered directly onto a PC board.
Operating Ratings (1)
−40°C to +105°C
Operating Temperature Range
Operating Voltage Range
Package Thermal Resistance, θJA (2)
(1)
(2)
2
6V to 16V
48-Pin TSSOP
84°C/W
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 ensured. For ensured specifications, see the Electrical
Characteristics tables.
The maximum power dissipation is a function of TJ(MAX), θJA and TA. The maximum allowable power dissipation at any ambient
temperature is PD = (TJ(MAX) – TA)/θJA. All numbers apply for packages soldered directly onto a PC board.
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16V Electrical Characteristics (1)
Unless otherwise specified, all limits ensured for TJ = 25°C, VDD = 16V, VSS = 0V, & CLOAD = 100 pF (Gamma & VCOM Buffers).
Boldface limits apply at the temperature extremes.
Symbol
Parameter
Min (2)
Conditions
Typ (3)
Max (2)
Units
Gamma Buffers
BW_Gamma
−3 dB Bandwidth
2
MHz
SR_Gamma
Slew Rate (4)
1
V/µs
TREC_Gamma
Output Recovery Time (5)
400
ns
VIN_Gamma
Input Voltage Range
Buffer 1-2
Buffer 3-8 & 11-16
Buffer 9
Buffer 10
Buffer 17-18
Positive
VDD
Negative
VSS +0.6
Positive
VDD-0.6
Negative
VSS +0.6
Positive
VDD −0.6
Negative
VSS
Positive
VDD-0.6
Negative
VSS +0.6
Positive
VDD −0.6
Negative
VOUT_Gamma
Output Voltage Range
Buffer 1-2,
No Load
Positive
Buffer 3-8 & 11-16
No Load
Positive
Buffer 9,
No Load
Positive
Buffer 10,
No Load
Positive
Buffer 17-18,
No Load
Positive
VSS
VDD -0.25
Negative
VDD -0.1
VSS +1.5
VDD −1.2
Negative
VDD −1.0
VSS +0.7
VDD -0.8
VSS +0.8
VDD –1.2
VDD –1.1
VDD –1.6
VDD –1.5
Negative
VSS +1.6
VDD -1.1
VSS +0.6
Negative
Negative
V
VSS +0.6
VSS +0.1
VSS +0.9
VSS +0.7
VSS +0.25
IBIAS_Gamma
Absolute, Input Bias Current
Within Gamma Buffer Output
Voltage Range
30
VOS_Gamma
Input Offset Voltage
Buffer 1-2, VIN = 8V
5
10
Buffer 3-8, 11-16, VIN = 8V
1
5
Buffer 9, VIN = 8V
1
5
Buffer 10, VIN = 8V
1
5
Buffer 17-18, VIN = 8V
5
10
(1)
(2)
(3)
(4)
(5)
V
nA
mV
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 = TA. No specification of parametric performance is indicated in the electrical table under
conditions of internal self-heating where TJ > TA.
All limits are specified by design or statistical analysis.
Typical values represent the parametric norm at the time of characterization.
Slew Rate is measured for VIN = 4 VPP. 10% -90% values are used. Slew rate is the average of the rising and falling slew rates
4 VPP 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.
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16V Electrical Characteristics(1) (continued)
Unless otherwise specified, all limits ensured for TJ = 25°C, VDD = 16V, VSS = 0V, & CLOAD = 100 pF (Gamma & VCOM Buffers).
Boldface limits apply at the temperature extremes.
Symbol
IOUT_Gamma
Parameter
Linear Output Current
(6)
Buffer 1-2
Buffer 3-8 & 11-16
Buffer 9
Buffer 10
Buffer 17-18
PSRR
Power Supply Rejection Ratio
Min (2)
Typ (3)
Sourcing
20
46
Sinking
0.2
0.33
Sourcing
10
24.5
Sinking
3.5
5.5
Sourcing
4.5
9.4
Sinking
15
27
Sourcing
23
34.8
Sinking
3.5
5.5
Sourcing
0.2
0.33
Sinking
20
50
75
88
dB
Conditions
VDD - VSS = 6V to 16V
Max (2)
Units
mA
VCOM Driver
BW_VCOM
Bandwidth
10
MHz
SR_ VCOM
Slew Rate (4)
4.5
V/μs
T_REC_VCOM
Output Recovery Time (5)
200
ns
VIN_VCOM
Input Voltage Range
VOUT_VCOM
Output Voltage Range
Positive
VDD
Negative
VSS +0.6
No Load
Positive
VDD –1.0
Negative
VDD –0.7
VSS +0.9
IBIAS_VCOM
Input Bias Current
Within VCOM Buffer Output
Voltage Range
VOS_VCOM
Input Offset Voltage
VIN = 8 V
1
IOUT_LIN_VCOM
Linear Output Current (6) (7)
Sourcing
160
Sinking
150
IOUT_SC_VCOM
PSRR
Short Circuit Output Current (7) (8)
Power Supply Rejection Ratio
V
VSS +1.2
50
Sourcing
220
300
Sinking
220
300
VDD - VSS = 6V to 16V
75
88
1.28
1.295
V
nA
10
mV
mA
mA
dB
Voltage Reference Section
VREF
Voltage
No Load
RegLOAD
Load Regulation
IOUT = 0 to 10 mA
VREF_ACC
Voltage Accuracy
No Load, VREF = 1.295V
VREF_MAX
Max Programming Range
IOUT = 4 mA
Input Bias Current
Within VREF Output Voltage
Range
IIN_VREF
TC_VREF
Temperature Stability
IOUT_VREF
Max Output Current
PSRR
Power Supply Rejection Ratio
(Line Regulation)
Supply Current
1.31
V
0.14
mV/mA
1
%
VDD −0.3
V
10
50
nA
70
ppm/°C
71
mA
70
80
dB
4.5
6.5
Sourcing, VOUT = 1.295 V
Miscellaneous
IS
(6)
(7)
(8)
4
8.5
9.5
mA
Linear output current measured at |VOUT - VIN| = 0.1V.
This is a momentary test. Continuous large output currents may result in exceeding the maximum power dissipation and damage the
device.
Short circuit current measured at |VOUT - VIN| = 1V.
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Connection Diagram
OUT_VREF
1
48
IN_VREF
NC
2
47
NC
IN_1
3
46
OUT_1
IN_2
4
45
OUT_2
IN_3
5
44
OUT_3
IN_4
6
43
OUT_4
IN_5
7
42
OUT_5
IN_6
8
41
OUT_6
LM8207
IN_7
9
40
OUT_7
IN_8
10
39
OUT_8
IN_9
11
38
OUT_9
VDD
12
37
VSS
IN_10
13
36
OUT_10
IN_11
14
35
OUT_11
IN_12
15
34
OUT_12
IN_13
16
33
OUT_13
IN_14
17
32
OUT_14
IN_15
18
31
OUT_15
IN_16
19
30
OUT_16
IN_17
20
29
OUT_17
IN_18
21
28
OUT_18
NC
22
27
NC
NC
23
26
NC
IN_VCOM
24
25
OUT_VCOM
Figure 1. 48-Pin TSSOP
Top View
PIN DESCRIPTIONS
Pin #
Description
Remark
1
OUT_VREF
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
VDD
Positive supply voltage (VDD)
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
22,23
NC
No connection
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PIN DESCRIPTIONS (continued)
24
IN_VCOM
Input VCOM
25
OUT_VCOM
Output VCOM
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
VSS
Negative supply voltage (VSS)
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_VREF
Reference voltage amplifier feedback input
Block Diagram
VDD
VSS
IN_1
OUT_1
IN_2
OUT_2
IN_17
OUT_17
IN_18
OUT_18
IN_VCOM
OUT_VCOM
VREF
1.295V
+
-
IN_VREF
6
OUT_VREF
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Typical Performance Characteristics
At TJ = 25°C, VDD = 16V, VSS = 0V. Unless otherwise specified.
Output Voltage Swing (Positive rail)
16
1.8
15.8
1.6
15.6
1.4
15.4
1.2
VCOM BUFFER
1
0.8
GAMMA BUFFER 17-18
VCOM BUFFER
15
14.8
14.6
0.6
GAMMA BUFFER 3-16
14.4
0.4
GAMMA BUFFER 17-18
0.2
GAMMA BUFFER 3-16
14.2
GAMMA BUFFER 1-2
14
0
0
0.4
0.8
1.2
14
2
1.6
14.4
14.8
15.2
16
15.6
VIN (V)
VIN (V)
Figure 2.
Figure 3.
Voltage Drop vs. Output Current (VCOM Buffer)
Voltage Drop vs. Output Current (VCOM Buffer)
10
10
1
1
125°C
0.1
25°C
85°C
-40°C
0.01
VOUT FROM VIN (V)
VOUT FROM VIN (V)
GAMMA BUFFER 1-2
15.2
VOUT (V)
VOUT (V)
Output Voltage Swing (Negative rail)
2
0.1
125°C
-40°C
0.01
85°C
25°C
0.001
0.1
0.001
1
10
100
1000
0.1
ISINKING (mA)
1
10
100
1000
ISOURCING (mA)
Figure 4.
Figure 5.
Voltage Drop vs. Output Current (Gamma Buffer)
Voltage Drop vs. Output Current (Gamma Buffer)
10
10
-40°C
-40°C
0.1
25°C
0.01
1
VOUT FROM VIN (V)
VOUT FROM VIN (V)
125°C
1
25°C
0.1
85°C
0.01
125°C
125°C
85°C
0.001
0.1
1
10
100
0.001
0.1
ISINKING (mA)
1
10
ISOURCING (mA)
Figure 6.
Figure 7.
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Typical Performance Characteristics (continued)
At TJ = 25°C, VDD = 16V, VSS = 0V. Unless otherwise specified.
Offset Voltage vs. Supply Voltage (Gamma Buffer)
Offset Voltage vs. Supply Voltage (VCOM Buffer)
3
3
2
25°C
-40°C
85°C
VOS (mV)
VOS (mV)
2
125°C
1
1
0
85°C
25°C
-40°C
125°C
0
-1
6
8
12
10
14
16
6
12
14
16
Figure 8.
Figure 9.
Recovery Time (VCOM Buffer) Negative Slope
(CL = 100 pF)
Recovery Time (VCOM Buffer) Positive Slope
(CL = 100 pF)
8.8
VPULSE = 4 VPP
VPULSE = 4 VPP
8.6
8.6
8.4
8.4
8.2
8.2
OUT_VCOM
OUT_VCOM
10
VDD (V)
8.8
8.0
7.8
8.0
7.8
7.6
7.6
7.4
7.4
7.2
7.2
TIME 200 ns/DIV
TIME 200 ns/DIV
Figure 10.
Figure 11.
Large Signal Transient Response (VCOM Buffer)
Negative Slope (CL = 100 pF)
Large Signal Transient Response (VCOM Buffer)
Positive Slope (CL = 100 pF)
10.5
10.5
10.0
10.0
9.5
9.5
9.0
OUT_VCOM
8.5
8.0
7.5
IN_VCOM
VOLTAGE (V)
VOLTAGE (V)
9.0
IN_VCOM
8.5
8.0
OUT_VCOM
7.5
7.0
7.0
6.5
6.5
6.0
6.0
5.5
8
8
VDD (V)
5.5
1 Ps/DIV
1 Ps/DIV
Figure 12.
Figure 13.
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Typical Performance Characteristics (continued)
At TJ = 25°C, VDD = 16V, VSS = 0V. Unless otherwise specified.
Frequency Response for Various Temperature
(VCOM Buffer)
Frequency Response for Various Load
(VCOM Buffer)
3
1
0
CL = 100 pF
-40°C
0
-1
CL = 68 pF
GAIN (dB)
GAIN (dB)
25°C
-2
85°C
105°C
-3
-4
CL = 15 pF
-3
NO LOAD
-6
-5
-6
100k
1M
10M
-9
100k
100M
1M
10M
FREQUENCY (Hz)
FREQUENCY (Hz)
Figure 14.
Figure 15.
Gain/Phase (Gamma Buffer)
(CL = 100 pF)
PSRR (VCOM Buffer)
0
3
100M
100
90
PHASE
80
-90
0
-3
PHASE (°)
GAIN (dB)
-180
PSRR (dB)
70
GAIN
50
40
30
-270
-6
60
20
10
-9
10k
100k
1M
10M
0
100
-360
100M
1k
10k
100k
FREQUENCY (Hz)
FREQUENCY (Hz)
Figure 16.
Figure 17.
Recovery Time (Gamma Buffer 3-16) Negative Slope
(CL = 100 pF)
Recovery Time (Gamma Buffer 3-16) Positive Slope
(CL = 100 pF)
10.0
10.0
VPULSE = 4 VPP
9.5
9.5
9.0
9.0
OUT_GAMMA
OUT_GAMMA
VPULSE = 4 VPP
8.5
8.0
7.5
8.5
8.0
7.5
7.0
7.0
6.5
6.5
6.0
6.0
TIME 200 ns/DIV
TIME 200 ns/DIV
Figure 18.
Figure 19.
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Typical Performance Characteristics (continued)
At TJ = 25°C, VDD = 16V, VSS = 0V. Unless otherwise specified.
Large Signal Transient Response (Gamma Buffer 3-16)
Positive slope (CL = 100 pF)
10.5
10.5
10.0
10.0
9.5
9.5
9.0
9.0
VOLTAGE (V)
VOLTAGE (V)
Large Signal Transient Response (Gamma Buffer 3-16)
Negative slope (CL = 100 pF)
8.5
8.0
7.5
OUT_GAMMA
7.0
IN_GAMMA
OUT_GAMMA
8.5
8.0
7.5
7.0
IN_GAMMA
6.5
6.5
6.0
6.0
5.5
5.5
1 Ps/DIV
1 Ps/DIV
Figure 20.
Figure 21.
Frequency Response for Various Load
(Gamma Buffer)
Frequency Response for Various Temperature
(Gamma Buffer)
3
1
-40°C
CL = 100 pF
0
CL = 15 pF
25°C
-1
CL = 68 pF
GAIN (dB)
GAIN (dB)
0
-3
NO LOAD
85°C
-2
105°C
-3
-4
-6
-5
-9
100k
1M
-6
100k
10M
1M
10M
FREQUENCY (Hz)
FREQUENCY (Hz)
Figure 22.
Figure 23.
Gain/Phase (Gamma Buffer)
(CL = 100 pF)
PSRR (Gamma Buffer)
100
0
3
90
PHASE
80
-90
-3
-180
70
PSRR (dB)
GAIN (dB)
GAIN
PHASE (°)
0
60
50
40
30
-6
-270
20
10
-9
-360
10k
100k
1M
10M
1k
10k
100k
FREQUENCY (Hz)
FREQUENCY (Hz)
Figure 24.
10
0
100
Figure 25.
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Typical Performance Characteristics (continued)
At TJ = 25°C, VDD = 16V, VSS = 0V. Unless otherwise specified.
Supply Current vs. Supply Voltage
Voltage Reference vs. Output Current
7
1.36
6.8
1.35
85°C
25°C
6.6
125°C
1.34
6.2
VREF (V)
IDD (mA)
6.4
125°C
6
-40°C
5.8
1.33
1.32
25°C
1.31
5.6
1.30
85°C
1.29
-40°C
5.4
5.2
5
1.28
6
8
10
12
14
16
0
5 10 15 20 25 30 35 40 45 50 55 60
ILOAD (mA)
VDD (V)
Figure 26.
Figure 27.
Voltage Reference vs. Supply Voltage
Voltage Reference PSRR
(Line Regulation)
80
1.33
70
1.32
125°C
60
PSRR (dB)
VREF (V)
1.31
25°C
1.30
50
40
30
1.29
20
85°C
1.28
-40°C
10
0
1.27
6
8
10
12
14
16
10
100
1k
10k
10k
FREQUENCY (Hz)
VDD (V)
Figure 28.
Figure 29.
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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 VCOM 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 Reference, the Gamma Buffers, and the VCOM
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 presents 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 information 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.
TRANSMITTED LIGHT
POLARIZER
GLASS
SUBSTRATE
LIQUID CRYSTAL
MATERIAL
GLASS
SUBSTRATE
TOP ITO
PLATE
BOTTOM ITO
PLATE
VPIXEL
± POLARITY
POLARIZER
LIGHT SOURCE
Figure 30. Individual LCD Pixel
Figure 30 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 conductive 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 (VPIXEL) 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 modulates 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 VPIXEL ≤ ±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 created.
Figure 31 shows a simplified diagram of a TFT display, showing how individual pixels are connected to the row,
column and VCOM 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
VCOM. The voltage applied to the top plates (also called gamma voltage) controls the pixel brightness. The
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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 sequentially 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.
ROW DRIVERS
COLUMN DRIVERS
CSTRAY
CSTRAY
CSTRAY
VCOM
VCOM
VCOM
PIXEL
PIXEL
PIXEL
APPROX
TFT-LCD PANEL
VDD/2
VCOM BUFFER
Figure 31. TFT Display
The VCOM driver (buffer) supplies a common voltage (VCOM) to all the pixels in a TFT panel. VCOM 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 VCOM. In fact, the polarity of a pixel is
reversed each time a row is selected, preventing a pattern from being ‘burned’ into the LCD.
VCOM
GAMMA CORRECTION CURVE
LM8342
REFERENCE
LM8207
VREF
18 GAMMA BUFFER
TIMING
CONTROL
MULTI
SOURCE
DC/DC
CONVERTER
& LDO
VCOM
BUFFER
COLUMN DRIVER
ROW
DRIVER
DISPLAY
Figure 32. Block Diagram of a Typical TFT-LCD
Figure 32 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 voltage will be determined, using the gamma correction curve. In fact, the gamma
correction curve is just a voltage reference 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.
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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 variations, a highly stable reference voltage is
available. It has a low drift over the operation temperature range. This output voltage (OUT_VREF) 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 buffers, each having a low output
resistance, can be used to drive the TFT display column drivers.
• VCOM Buffer
– The VCOM 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 variations at the VCOM level so this VCOM 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_VREF) 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_VREF) can be set using the internal reference in
combination with the internal amplifier and two external resistors. In Figure 33 a typical application circuit for VREF
is given.
+
1.295V
VREF
+
-
OUT_VREF
R1
IN_VREF
IIN_V
REF
R2
LM8207
Figure 33. Typical Application Circuit for VREF
To calculate the output voltage of the Voltage Reference Amplifier (OUT_VREF) use the following equation:
§R + R
2
¨ 1
¨
R2
©
§
¨
¨
©
OUT_VREF = 1.295 x
+ R1 x IIN_VREF
(1)
As can be seen in the 16V Electrical Characteristics table, IIN_VREF has a typical value of −10 nA. Using resistor
values for R1 = 9 kΩ and R2 = 1 kΩ this results in a gain of 10 and OUT_VREF = 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_VREF is:
14
§R + R
2
¨ 1
¨
R2
©
§
¨
¨
©
OUT_VREF = 1.295 x
(2)
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Example:
VDD = 16V
OUT_VREF = 14.4V
Choose R2 = 5 kΩ. Using Equation 2, this will result in R1 = 50.6 kΩ
THE OPERATING RANGE OF THE VOLTAGE REFERENCE
The output of the Voltage Reference Amplifier has a minimum of 1.295V (R1 = 0). This is determined by the
value of the internal reference. The maximum output voltage (OUT_VREFMAX) can approach the positive supply
rail VDD. The voltage is limited by the output resistance (ROUT) of the output stage of the internal amplifier and
depends on the load current. Figure 34 shows the operating output voltage range.
16V
OUT_VREF
HEADROOM
OPERATING OUTPUT
VOLTAGE RANGE
|
1.295V
0
0
IOUT_V
REF
ILOAD
Figure 34. Operating Output Voltage Range
The minimum headroom (OUT_VREF with respect to the positive supply rail VDD) can be measured using the test
circuit shown in Figure 35.
+
1.295V
VREF
+
-
OUT_VREF
R1
IN_VREF
ILOAD
R2
LM8207
Figure 35. Headroom Test Circuit with Variable Output Current Load
The headroom is measured by varying both the supply voltage and the output current (ILOAD) for a fixed
programmed value of OUT_VREF. As shown in Figure 36, the minimum headroom slightly increases for a
constant VDD when the load current increases.
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0.9
0.8
HEADROOM (V)
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0
2
4
6
8
10
ILOAD (mA)
Figure 36. Voltage Reference Headroom vs. Load Current
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
realization of the gamma correction curve using external resistors. 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 behavior 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 without changing the gamma correction curve. A typical gamma correction curve is given in
Figure 37.
VGMA1
VGMA3
VGMA5
VGMA7
VCOM
VGMA12
VGMA14
VGMA16
VGMA18
0
32
64
96
128 160 192 224 256
INPUT (DIGITAL VALUE)
Figure 37. Typical Gamma Correction Curve
Each buffer covers a part of the correction curve and, therefore, 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: VDD to VSS +2V. Due to the operating voltage, only negative transitions at the
output are possible. Positive transitions will exceed the supply voltage VDD. 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: VDD – 1V to VSS + 1V. Due to the operating range, both positive and negative
transitions at the outputs are possible.
16
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•
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Gamma Buffers 17-18
– Operating voltage range: VDD - 2 to VSS. Due to the operating voltage, only positive transitions at the
output are possible. Negative transitions will exceed the negative supply voltage VSS. These buffers are
able to sink current from 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 higher potential
sinking an output current of at least 1 mA
Example:
A typical application using the LM8207 is given in Figure 38. The corresponding gamma correction curve
(VGMA1...VGMA18) is defined in Table 1. The Voltage Reference supplies the 14.4V to the resistor network. The
calculations for the resistor values and for setting the Voltage Reference are shown in the section “Voltage
Reference.”
LM8207
14.4 V
GAMMA_1
VGMA1
VGMA2
R1
VGMA3
R2
GAMMA_2
GAMMA_3
GAMMA_4
GAMMA_5
GAMMA_6
GAMMA_7
GAMMA_8
GAMMA_9
GAMMA_10
COLUMN DRIVERS
GAMMA_11
GAMMA_12
GAMMA_13
GAMMA_14
GAMMA_15
VGMA16
GAMMA_16
VGMA17
R16
VGMA18
R17
GAMMA_17
GAMMA_18
R18
Figure 38. Typical TFT Display Application Diagram Using the LM8207
The values of the resistors in the gamma correction curve are calculated such that a current of 1 mA flows in the
resistor chain.
I
§
¨
¨
©
©
and RX =
§
¨
¨
©
VGMAx
I
§
¨
¨
©
§ VGMA18
¨
R18 = ¨
18
¦ Ry
y=x+1
-
where
•
x is the index for the corresponding gamma voltage and has a range of 1 to 18
(3)
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.
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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 VDD, and will
source the current into the column drivers. Gamma buffers 17 and 18 are designed for operating voltages near
VSS 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 circumstances, 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 RS. This method is presented in Figure 39. 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 RS 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.
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LM8207
COLUMN DRIVER
OUT_1
IOUT_1
OUT_2
IOUT_2
OUT_3
IOUT_3
V3
OUT_4
IOUT_4
V4
V1
V2
RS
BY-PASS RESISTOR
Figure 39. Using additional by-pass resistor to increase current sinking capability
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 determine the accuracy of each gamma voltage.
Major effects are:
• Variation of the internal voltage reference. This can be found in the 16V 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_VREF).
Using 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 (VOS) of the buffers. Variations of VOS (output offset voltage) of the buffers, will affect the
gamma correction curve. The contribution of VOS is higher for the buffers driving the lower gamma voltages.
Minor effects are:
• Input current (IBIAS) of the gamma buffers. Variations of the input current (IBIAS) of the gamma buffers caused
by temperature changes, will affect the gamma correction voltages.
VCOM BUFFER
The VCOM 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 VCOM line. These pulses are the result of charging
the capacitance between VCOM 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 positive or negative
current pulses of 100 mA or more, depending on the panel size. The VCOM buffer is designed to handle these
pulses. A VCOM buffer is basically a voltage regulator that can sink or source current in large capacitive loads.
The VCOM buffer should recover very fast from these disturbances. The operating voltage of the VCOM buffer is in
the middle of the gamma voltage range.
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VREF
LM8342
VCOM
R1
5 k:
VCOM
BUFFER
DISPLAY
REFERENCE
R2
5 k:
Figure 40. VCOM Buffer
The typical application in Figure 40 shows the VCOM 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 R1 or
decreasing the value of R2 will decrease the VCOM level. Increasing the value of R2 or decreasing the value of R1
will increase the VCOM level.
Another, more flexible, solution is to use Texas Instruments’ programmable VCOM calibrator, the LM8342. The
VCOM level can be adjusted using an I2C interface. See the LM8342 Programmable TFT V COM Calibrator with
Non-Volatile Memory datasheet (literature number: snosam0) for more detailed information about this part.
LM8207 CONFIGURATION
A complete configured typical application of the LM8207 is given in Figure 41. 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 VCOM level is discussed in the “VCOM Buffer” section. The LM8207 is
an 18 channel gamma buffer plus a VCOM buffer. In certain applications some of the gamma buffers or the VCOM
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.
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OUT_VREF
2800:
IN_1
210:
IN_2
2200:
5 k:
IN_VREF
50.6 k:
14.4V
OUT_1
OUT_2
IN_3
OUT_3
IN_4
OUT_4
670:
670:
IN_5
OUT_5
IN_6
OUT_6
IN_7
OUT_7
IN_8
OUT_8
IN_9
OUT_9
430:
280:
980:
80:
170:
LM8207
IN_10
OUT_10
ABC
COLUMN
DRIVERS
80:
IN_11
OUT_11
IN_12
OUT_12
IN_13
OUT_13
IN_14
OUT_14
IN_15
OUT_15
IN_16
OUT_16
IN_17
OUT_17
IN_18
OUT_18
980:
280:
430:
670:
730:
2190:
210:
330:
VREF
PROGRAMMABLE
5 k:
VCOM CALIBRATOR
5 k:
IN_VCOM
TO DISPLAY
VSS
VDD
LM8342
OUT_VCOM
+
100 PF
10 nF
16V
Figure 41. LM8207 Configuration
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MAXIMUM POWER DISSIPATION
The maximum power dissipation in the LM8207 TSSOP package depends on the ambient temperature and the
increase of the junction temperature of the die. Exceeding the maximum temperature will damage the part. (See
the Absolute Maximum Ratings table.) The VCOM 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 VCOM buffer.
The maximum operating temperature can be calculated using this formula:
TJ = TA + θJA x PDISSIPATION
where
•
•
•
TA = Ambient temperature
θJA = Thermal resistance of package (See Operating Ratings table) (84°C/W)
PDISSIPATION = Total power dissipation of the LM8207
(4)
Examples:
The estimated power consumption of the LM8207 in a steady state situation with no load is:
VDD
= 16V
IDD
= 6 mA (all buffers within normal operating range)
OUT_VREF
= 14.4V
ILOAD
= 3 mA
VDD x IDD
= 16 V x 6 mA
(VDD - OUT_VREF) x ILOAD = (16 V – 14.4 V) x 3 mA
= 4.8 mW
Total steady state power dissipation
= 100.8 mW
For an ambient temperature TA of 40°C and a dissipated power of 100.8 mW, the junction temperature TJ 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
voltage drop over the output stage times the output current:
– P = (VDD-VGMAx) x IOUT for current sourcing
– P = (VGMAx) x IOUT 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.
When the LM8207 is in steady state biasing, the V buffer is considered at three various load conditions:
(1)
IOUTRMS (mA)
VCOM Level (V)
Dissipation (mW)
Temp Rise
TJ
10
8
50
8
80
7
56
400
35
100 (1)
8
800
83
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 operating 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 external resistors. Each optimal value can be achieved by
using two series resistors for fine-tuning.
• The VCOM node input voltage can be achieved using Texas Instruments’ LM8342 programmable VCOM
calibrator, or using an external supply.
• For testing, an additional dummy load can be connected to all outputs of the gamma buffers.
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R7
IN_1
J5
Vref
IN_2
IN_3
IN_4
IN_5
OUT_1
1
2
R3
R8
R8*
R3*
OUT_2
R4
R9
R4*
OUT_3
R6
R10
R6*
OUT_4
R11
R14
R9*
R10*
R14*
R11*
OUT_5
R2 R2* R1 R1*
R15
R12
R17
J1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
IN_1
IN_8
IN_2
IN_3
IN_4
IN_5
IN_6
IN_9
IN_7
IN_8
IN_9
IN_10
IN_11
IN_10
IN_12
IN_13
IN_14
IN_15
IN_16
IN_11
IN_17
IN_18
IN_VCOM
IN_12
R17*
R18
R18*
R19
R19*
R20
R20*
R25
R25*
R26
IN_13
R26*
R16*
46
45
44
43
42
41
40
39
38
36
35
34
33
32
31
30
29
28
27
26
25
OUT_7
R21
R29
R29*
OUT_13
R30
R30*
OUT_14
R34
R34*
C2
R31*
OUT_15
R35
R35
J3
R33
IN_17
R33*
J4
BANANA JACK
R32*
BANANA JACK
IN_16
R35*
OUT_16
R36
R36*
OUT_17
R39
R37
IN_18
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
R28*
R31
IN_15
J2
OUT_12
37
C1
R21*
OUT_8
OUT_1
OUT_2
OUT_3
R22
OUT_4
OUT_5
R22*
OUT_6
OUT_9
OUT_7
OUT_8
R23
OUT_9
OUT_10
R23*
OUT_10 OUT_11
OUT_12
OUT_13
R24
OUT_14
OUT_15
R24*
OUT_11 OUT_16
OUT_17
OUT_18
R28
OUT_VCOM
VSS
12
IN_14
R27*
OUT_6
R16
OUT_1
IN_1
OUT_2
IN_2
OUT_3
IN_3
OUT_4
IN_4
OUT_5
IN_5
OUT_6
IN_6
OUT_7
IN_7
OUT_8
IN_8
OUT_9
IN_9
OUT_10
IN_10
OUT_11
IN_11
OUT_12
IN_12
OUT_13
IN_13
OUT_14
IN_14
OUT_15
IN_15
OUT_16
IN_16
OUT_17
IN_17
OUT_18
IN_18
NC
NC
NC
NC
IN_VCOM OUT_VCOM
VDD
R27
R15*
in_Vref
3
4
5
6
7
8
9
10
11
13
14
15
16
17
18
19
20
21
22
23
24
R13*
U1
out_Vref
R13
IN_7
48
1
IN_6
R12*
R39*
R37*
OUT_18
R38
OUT_VCOM
IN_VCOM
Figure 42. Schematic LM8207 Evaluation Board
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Product Folder Links: LM8207
23
LM8207
SNOSAL5A – SEPTEMBER 2005 – REVISED MARCH 2013
www.ti.com
Figure 43. Layout of LM8207 Evaluation Board (Actual Size) — Bottom View
Figure 44. Layout of LM8207 Evaluation Board (Actual Size) — Top View
24
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Product Folder Links: LM8207
LM8207
www.ti.com
SNOSAL5A – SEPTEMBER 2005 – REVISED MARCH 2013
LAYOUT RECOMMENDATIONS
A proper layout is necessary for optimum performance of the LM8207. A low impedance and clean ground plane
is recommended. The traces from the VSS pin to the ground plane should be as short as possible. Decoupling
capacitors should be placed very close to the VDD pin. Connections of these decoupling capacitors to the ground
plane should be very short. An additional decoupling capacitor for OUT_VREF is recommended.
Due to the heavy current peaks and short transitions at the VCOM node, traces from the output of the VCOM buffer
should be low impedance and as short as possible, to minimize both voltage drops over the trace and unwanted
EM disturbances.
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Product Folder Links: LM8207
25
LM8207
SNOSAL5A – SEPTEMBER 2005 – REVISED MARCH 2013
www.ti.com
REVISION HISTORY
Changes from Original (March 2013) to Revision A
•
26
Page
Changed layout of National Data Sheet to TI format .......................................................................................................... 25
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Product Folder Links: LM8207
PACKAGE OPTION ADDENDUM
www.ti.com
10-Dec-2020
PACKAGING INFORMATION
Orderable Device
Status
(1)
Package Type Package Pins Package
Drawing
Qty
Eco Plan
(2)
Lead finish/
Ball material
MSL Peak Temp
Op Temp (°C)
Device Marking
(3)
(4/5)
(6)
LM8207MTX/NOPB
NRND
TSSOP
DGG
48
1000
RoHS & Green
SN
Level-2-260C-1 YEAR
-40 to 105
LM8207MT
(1)
The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2)
RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance
do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may
reference these types of products as "Pb-Free".
RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption.
Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of
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