NSC LM8207MTX Tft 18 gamma buffer vcom driver voltage reference Datasheet

LM8207
TFT 18 Gamma Buffer + VCOM Driver + Voltage Reference
General Description
Features
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.
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
n 48-pin TSSOP package
n
n
n
n
Applications
n TFT gamma curve connection and VCOM voltage
buffering
TFT Panel Block Diagram
20137926
© 2005 National Semiconductor Corporation
DS201379
www.national.com
LM8207 TFT 18 Gamma Buffer + VCOM Driver + Voltage Reference
September 2005
LM8207
Absolute Maximum Ratings (Notes 1, 2)
Soldering Information
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
Storage Temperature Range
−40˚C to +105˚C
Operating Voltage Range
18V
6V to 16V
Package Thermal Resistance, θJA (Note 4)
−65˚C to +150˚C
Junction Temperature (Note 4)
260˚C
Operating Temperature Range
250V
Supply Voltage (VDD - VSS)
230˚C
Wave Soldering (10 sec.)
Operating Ratings (Note 1)
2.5 kV
Machine Model
Infrared or Convection (20 sec.)
48-Pin TSSOP
+150˚C
84˚C/W
16V Electrical Characteristics
Unless otherwise specified, all limits guaranteed for TJ = 25˚C, VDD = 16V, VSS = 0V, & CLOAD = 100 pF (Gamma & VCOM
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
TREC_Gamma
Output Recovery Time (Note 9)
400
ns
VIN_Gamma
Input Voltage Range
VOUT_Gamma Output Voltage Range
Buffer 1-2
Positive
VDD
Negative
VSS +0.6
Buffer 3-8 & 11-16
Positive
VDD-0.6
Negative
VSS +0.6
Buffer 9
Positive
VDD −0.6
Negative
VSS
Buffer 10
Positive
VDD-0.6
Negative
VSS +0.6
Buffer 17-18
Positive
VDD −0.6
Negative
VSS
Buffer 1-2,
No Load
Positive
Buffer 3-8 & 11-16
No Load
Positive
Negative
Buffer 9,
No Load
Negative
VDD -0.25
Negative
Positive
Buffer 10,
No Load
Positive
VDD −1.0
Positive
Negative
VSS +1.6
VDD -1.1
VSS +0.6
VSS +0.7
VDD -0.8
VSS +0.8
VDD –1.2
VDD –1.1
VDD –1.6
VDD –1.5
Negative
Buffer 17-18,
No Load
VDD -0.1
VSS +1.5
VDD −1.2
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
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
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2
V
30
nA
mV
LM8207
16V Electrical Characteristics
(Continued)
Unless otherwise specified, all limits guaranteed for TJ = 25˚C, VDD = 16V, VSS = 0V, & CLOAD = 100 pF (Gamma & VCOM
Buffers). Boldface limits apply at the temperature extremes. (Note 5)
Symbol
IOUT_Gamma
Parameter
Linear Output Current
(Note 10)
Conditions
46
Sourcing
20
Sinking
0.2
0.33
Buffer 3-8 & 11-16
Sourcing
10
24.5
Sinking
3.5
5.5
Sourcing
4.5
9.4
Sinking
15
27
Buffer 10
Buffer 17-18
Power Supply Rejection Ratio
Typ
(Note 7)
Buffer 1-2
Buffer 9
PSRR
Min
(Note 6)
Max
(Note 6)
Units
mA
Sourcing
23
34.8
Sinking
3.5
5.5
Sourcing
0.2
0.33
Sinking
20
50
75
88
dB
VDD - VSS = 6V to 16V
VCOM Driver
BW_VCOM
Bandwidth
10
MHz
SR_ VCOM
Slew Rate (Note 8)
4.5
V/µs
T_REC_VCOM
Output Recovery Time (Note 9)
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
50
VOS_VCOM
Input Offset Voltage
VIN = 8 V
1
IOUT_LIN_VCOM Linear Output Current
(Notes 10, 11)
Sourcing
160
Sinking
150
IOUT_SC_VCOM Short Circuit Output Current
(Notes 11, 12)
Sourcing
220
300
Sinking
220
300
PSRR
VDD - VSS = 6V to 16V
75
88
1.28
1.295
Power Supply Rejection Ratio
V
VSS +1.2
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
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 kΩ in series with 100 pF. Machine model, 0Ω in 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 = (TJ(MAX) – TA)/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 = TA. No guarantee of parametric performance is indicated in the electrical table under conditions of internal self-heating where TJ > TA.
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LM8207
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 = 4 VPP. 10% -90% values are used. Slew rate is the average of the rising and falling slew rates
Note 9: 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.
Note 10: Linear output current measured at |VOUT - VIN| = 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 - VIN| = 1V.
Connection Diagram
48-Pin TSSOP
20137902
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
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LM8207
Pin Descriptions
(Continued)
22,23
NC
No connection
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
Ordering Information
Package
48-Pin TSSOP
Part Number
LM8207MT
LM8207MTX
Package Marking
LM8207MT
Transport Media
38 Units/Rail
1k Units Tape and Reel
NSC Drawing
MTD48
Block Diagram
20137901
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LM8207
Typical Performance Characteristics
At TJ = 25˚C, VDD = 16V, VSS = 0V. Unless otherwise
specified.
Output Voltage Swing (Negative rail)
Output Voltage Swing (Positive rail)
20137907
20137908
Voltage Drop vs. Output Current (VCOM Buffer)
Voltage Drop vs. Output Current (VCOM Buffer)
20137942
20137944
Voltage Drop vs. Output Current (Gamma Buffer)
Voltage Drop vs. Output Current (Gamma Buffer)
20137941
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20137943
6
Offset Voltage vs. Supply Voltage (Gamma Buffer)
Offset Voltage vs. Supply Voltage (VCOM Buffer)
20137937
20137938
Recovery Time (VCOM Buffer) Positive Slope
(CL = 100 pF)
Recovery Time (VCOM Buffer) Negative Slope
(CL = 100 pF)
20137911
20137913
Large Signal Transient Response (VCOM Buffer)
Positive Slope (CL = 100 pF)
Large Signal Transient Response (VCOM Buffer)
Negative Slope (CL = 100 pF)
20137915
20137916
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LM8207
Typical Performance Characteristics At TJ = 25˚C, VDD = 16V, VSS = 0V. Unless otherwise
specified. (Continued)
LM8207
Typical Performance Characteristics At TJ = 25˚C, VDD = 16V, VSS = 0V. Unless otherwise
specified. (Continued)
Frequency Response for Various Temperature
(VCOM Buffer)
Frequency Response for Various Load
(VCOM Buffer)
20137934
20137932
Gain/Phase (Gamma Buffer)
(CL = 100 pF)
PSRR (VCOM Buffer)
20137903
20137919
Recovery Time (Gamma Buffer 3-16) Positive Slope
(CL = 100 pF)
Recovery Time (Gamma Buffer 3-16) Negative Slope
(CL = 100 pF)
20137912
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20137914
8
Large Signal Transient Response (Gamma Buffer 3-16)
Negative slope (CL = 100 pF)
Large Signal Transient Response (Gamma Buffer 3-16)
Positive slope (CL = 100 pF)
20137917
20137918
Frequency Response for Various Load
(Gamma Buffer)
Frequency Response for Various Temperature
(Gamma Buffer)
20137933
20137935
Gain/Phase (Gamma Buffer)
(CL = 100 pF)
PSRR (Gamma Buffer)
20137920
20137904
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LM8207
Typical Performance Characteristics At TJ = 25˚C, VDD = 16V, VSS = 0V. Unless otherwise
specified. (Continued)
LM8207
Typical Performance Characteristics At TJ = 25˚C, VDD = 16V, VSS = 0V. Unless otherwise
specified. (Continued)
Supply Current vs. Supply Voltage
Voltage Reference vs. Output Current
20137940
20137939
Voltage Reference PSRR
(Line Regulation)
Voltage Reference vs. Supply Voltage
20137945
20137936
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10
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.
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 2 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 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.
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.
20137930
20137931
FIGURE 1. Individual LCD Pixel
FIGURE 2. TFT Display
Figure 1 shows a simplified illustration of an individual LCD
pixel. The top and bottom plates of a pixel consist of IndiumTin 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
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.
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LM8207
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.
Application Section
LM8207
Application Section
(Continued)
20137926
FIGURE 3. Block Diagram of a Typical TFT-LCD
•
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 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.
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.
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.
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 4 a typical application circuit for VREF is given.
• 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.
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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.
12
LM8207
Application Section
(Continued)
20137921
20137925
FIGURE 5. Operating Output Voltage Range
FIGURE 4. Typical Application Circuit for VREF
The minimum headroom (OUT_VREF with respect to the
positive supply rail VDD) can be measured using the test
circuit shown in Figure 6.
To calculate the output voltage of the Voltage Reference
Amplifier (OUT_VREF) use the following equation:
(1)
As can be seen in the Electrical Characteristics Table on
page 3, 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:
(2)
Example:
VDD = 16V
OUT_VREF = 14.4V
Choose R2 = 5 kΩ. Using Equation (2), this will result in
R1 = 50.6 kΩ
20137924
FIGURE 6. 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 7, the minimum
headroom slightly increases for a constant VDD when the
load current increases.
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 5 shows the operating output voltage
range.
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LM8207
Application Section
(Continued)
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.
•
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
20137922
FIGURE 7. 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 8.
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 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.”
20137923
FIGURE 8. Typical Gamma Correction Curve
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LM8207
Application Section
(Continued)
20137927
FIGURE 9. Typical TFT Display Application Diagram Using the LM8207
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.
The values of the resistors in the gamma correction curve
are calculated such that a current of 1 mA flows in the
resistor chain.
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LM8207
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
20137946
FIGURE 10. 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.
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 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 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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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_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.
16
(Continued)
The typical application in Figure 11 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.
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.
Another, more flexible, solution is to use National Semiconductor’s programmable VCOM calibrator, the LM8342. The
VCOM level can be adjusted using an I2C 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 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.
20137928
FIGURE 11. VCOM Buffer
17
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LM8207
Application Section
LM8207
Application Section
(Continued)
20137929
FIGURE 12. LM8207 Configuration
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18
Two issues are not considered in the calculation:
(Continued)
• 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.
Example:
When the LM8207 is in steady state biasing, the V buffer is
considered at three various load conditions:
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 on page 2 of the datasheet.)
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:
(3)
TJ = TA + θJA x PDISSIPATION
Where
IOUTRMS
(mA)
VCOM
Level (V)
Dissipation
(mW)
Temp
Rise
TJ
TA
= Ambient temperature
10
8
80
7
56
θJA
= Thermal resistance of package (See
Operating Ratings table on page 2)
(84˚C/W)
50
8
400
35
83
100
8
800
67
107
When IOUTRMS = 100 mA, the package (TJ) will exceed the Operating
Temperature!
PDISSIPATION = Total power dissipation of the LM8207
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
National Semiconductor’s 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.
Example:
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.
19
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LM8207
Application Section
LM8207
Application Section
(Continued)
20137950
FIGURE 13. Schematic LM8207 Evaluation Board
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20
LM8207
Application Section
(Continued)
Bottom View
20137951
FIGURE 14. Layout of LM8207 Evaluation Board (Actual Size)
TOP View
20137952
FIGURE 15. Layout of LM8207 Evaluation Board (Actual Size)
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.
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.
21
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LM8207 TFT 18 Gamma Buffer + VCOM Driver + Voltage Reference
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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