LM6588
LM6588 TFT-LCD Quad, 16V RRIO High Output Current Operational Amplifier
Literature Number: SNOSA77C
LM6588
TFT-LCD Quad, 16V RRIO High Output Current
Operational Amplifier
General Description
The LM6588 is a low power, high voltage, rail-to-rail input-
output amplifier ideally suited for LCD panel V
COM
driver and
gamma buffer applications. The LM6588 contains four unity
gain stable amplifiers in one package. It provides a common
mode input ability of 0.5V beyond the supply rails, as well as
an output voltage range that extends to within 50mV of either
supply rail. With these capabilities, the LM6588 provides
maximum dynamic range at any supply voltage. Operating
on supplies ranging from 5V to 16V, while consuming only
750µA per amplifier, the LM6588 has a bandwidth of 24MHz
(−3dB).
The LM6588 also features fast slewing and settling times,
along with a high continuous output capability of 75mA. This
output stage is capable of delivering approximately 200mA
peak currents in order to charge or discharge capacitive
loads. These features are ideal for use in TFT-LCDs.
The LM6588 is available in the industry standard 14-pin SO
package and in the space-saving 14-pin TSSOP package.
The amplifiers are specified for operation over the full −40˚C
to +85˚C temperature range.
Features
(V
S
=5V,T
A
= 25˚C typical values unless specified)
nInput common mode voltage 0.5V beyond rails
nOutput voltage swing (R
L
=2kΩ) 50mV from rails
nOutput short circuit current ±200mA
nContinuous output current 75mA
nSupply current (per amp, no load) 750µA
nSupply voltage range 5V to 16V
nUnity gain stable
n−3dB bandwidth (A
V
= +1) 24MHz
nSlew rate 11V/µSec
nSettling time 270ns
nSO-14 and TSSOP-14 package
nManufactured in National Semiconductor’s
state-of-the-art bonded wafer, trench isolated
complementary bipolar VIP10™technology for high
performance at low power levels
Applications
nLCD panel V
COM
driver
nLCD panel gamma buffer
nLCD panel repair amp
Test Circuit Diagram
20073401
July 2005
LM6588 TFT-LCD Quad, 16V RRIO High Output Current Operational Amplifier
© 2005 National Semiconductor Corporation DS200734 www.national.com
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
ESD Tolerance (Note 2)
Human Body Model 2.5KV
Machine Model 250V
Supply Voltage (V
+
-V
−
) 18V
Differential Input Voltage ±5.5V
Output Short Circuit to Ground (Note 3) Continuous
Storage Temperature Range −65˚C to 150˚C
Input Common Mode Voltage V
−
to V
+
Junction Temperature (Note 4) 150˚C
Operating Ratings (Note 1)
Supply Voltage 4V ≤V
S
≤16V
Temperature Range −40˚C to +85˚C
Thermal Resistance (θ
JA
)
SOIC-14 145˚C/W
TSSOP-14 155˚C/W
16V DC Electrical Characteristics (Note 13)
Unless otherwise specified, all limits guaranteed for at T
J
= 25˚C, V
CM
=
1
⁄
2
V
S
and R
L
=2kΩ.Boldface limits apply at the tem-
perature extremes.
Symbol Parameter Conditions Min
(Note 6)
Typ
(Note 5)
Max
(Note 6)
Units
V
OS
Input Offset Voltage 0.7 4
6mV
TC V
OS
Input Offset Voltage Average
Drift
5µV/˚C
I
B
Input Bias Current −0.3/+0.3 ±1
±7
µA
I
OS
Input Offset Current 16 150
300 nA
R
IN
Input Resistance Common Mode 20 MΩ
Differential Mode 0.5
CMRR Common Mode Rejection
Ratio
V
CM
= 0 to +16V 75
70
103
dB
V
CM
= 0 to 14.5V 78
72
103
PSRR Power Supply Rejection Ratio V
CM
=±1V 80
75
103 dB
CMVR Input Common-Mode Voltage
Range
CMRR >50dB
16.2
0
16
−0.2 V
A
V
Large Signal Voltage Gain
(Note 7)
R
L
=2kΩ,V
O
= 0.5 to +15.5V 80
75
108 dB
V
O
Output Swing High R
L
=2kΩ15.8
15.6
15.9
V
Output Swing Low R
L
=2kΩ0.100 0.200
I
SC
Output Short Circuit Current
(Note 11)
Sourcing 170 230 mA
Sinking 170 230
I
CONT
Continuous Output Current
(Note 12)
Sourcing 40 mA
Sinking 40
I
S
Supply Current (per Amp) 800 1200
1500 µA
16V AC Electrical Characteristics (Note 13)
Unless otherwise specified, all limits guaranteed for at T
J
= 25˚C, V
CM
=
1
⁄
2
V
S
and R
L
=2kΩ.Boldface limits apply at the tem-
perature extremes.
Symbol Parameter Conditions Min
(Note 6)
Typ
(Note 5)
Max
(Note 6)
Units
SR Slew Rate (Note 9) A
V
= +1, V
IN
= 10V
PP
8 15 V/µs
LM6588
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16V AC Electrical Characteristics (Note 13) (Continued)
Unless otherwise specified, all limits guaranteed for at T
J
= 25˚C, V
CM
=
1
⁄
2
V
S
and R
L
=2kΩ.Boldface limits apply at the tem-
perature extremes.
Symbol Parameter Conditions Min
(Note 6)
Typ
(Note 5)
Max
(Note 6)
Units
Unity Gain Bandwidth Product 15.4 MHz
−3dB Frequency A
V
= +1 10 24 MHz
Φ
m
Phase Margin 61 deg
t
s
Settling Time (0.1%) A
V
= −1, A
O
=±5V, R
L
= 500Ω780 ns
t
p
Propagation Delay A
V
= −2, V
IN
=±5V, R
L
= 500Ω20 ns
HD2 2
nd
Harmonic Distortion
F
IN
= 1MHz (Note 10)
V
OUT
=2V
PP
−53 dBc
HD3 3rd Harmonic Distortion
F
IN
= 1MHz (Note 10)
V
OUT
=2V
PP
−40 dBc
e
n
Input-Referred Voltage Noise f = 10kHz 23 nV/
5V DC Electrical Characteristics (Note 13)
Unless otherwise specified, all limits guaranteed for at T
J
= 25˚C, V
CM
=
1
⁄
2
V
S
and R
L
=2kΩ.Boldface limits apply at the tem-
perature extremes.
Symbol Parameter Conditions Min
(Note 6)
Typ
(Note 5)
Max
(Note 6)
Units
V
OS
Input Offset Voltage 0.7 4
6mV
TC V
OS
Input Offset Voltage Average
Drift
10 µV/˚C
I
B
Input Bias Current −0.3/+0.3 ±1
±7µA
I
OS
Input Offset Current 20 150
300 nA
R
IN
Input Resistance Common Mode 20 MΩ
Differential Mode 0.5
CMRR Common Mode Rejection
Ratio
V
CM
Stepped from 0 to 5V 70
66
105
dB
V
CM
Stepped from 0 to 3.5V 75
70
105
PSRR Power Supply Rejection Ratio V
S
=V
CC
= 3.5V to 5.5V 80
75
92 dB
CMVR Input Common-Mode Voltage
Range
CMRR >50dB
5.2
0.0
5.0
−0.2 V
A
V
Large Signal Voltage Gain
(Note 7)
R
L
=2kΩ,V
O
= 0 to 5V 80
75
106 dB
V
O
Output Swing High R
L
=2kΩ4.85
4.7
4.95
V
Output Swing Low R
L
=2kΩ0.05 0.15
I
SC
Output Short Circuit Current
(Note 11)
Sourcing 160 200 mA
Sinking 160 200
I
CONT
Continuous Output Current
(Note 12)
Sourcing 75 mA
Sinking 75
I
S
Supply Current (per Amp) 750 1000
1250 µA
LM6588
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5V AC Electrical Characteristics (Note 13)
Unless otherwise specified, all limits guaranteed for at T
J
= 25˚C, V
CM
=
1
⁄
2
V
S
and R
L
=2kΩ.Boldface limits apply at the tem-
perature extremes.
Symbol Parameter Conditions Min
(Note 6)
Typ
(Note 5)
Max
(Note 6)
Units
SR Slew Rate (Note 9) A
V
= +1, V
IN
= 3.5V
PP
11 V/µs
Unity Gain Bandwidth Product 15.3 MHz
−3dB Frequency A
V
= +1 10 24 MHz
Φ
m
Phase Margin 56 deg
t
s
Settling Time (0.1%) A
V
= −1, V
O
=±1V, R
L
= 500Ω270 ns
t
p
Propagation Delay A
V
= −2, V
IN
=±1V, R
L
= 500Ω21 ns
HD2 2
nd
Harmonic Distortion
F
IN
= 1MHz (Note 10)
V
OUT
=2V
PP
−53 dBc
HD3 3rd Harmonic Distortion
F
IN
= 1MHz (Note 10)
V
OUT
=2V
PP
−40 dBc
e
n
Input-Referred Voltage Noise f = 10kHz 23 nV/
Note 1: 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 and the test conditions, see the Electrical
Characteristics.
Note 2: For testing purposes, ESD was applied using human body model, 1.5kΩin series with 100pF.
Note 3: Applies to both single-supply and split-supply operation. Continuous short circuit operation at elevated ambient temperature can result in exceeding the
maximum allowed junction temperature of 150˚C
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: Typical values represent the most likely parametric norm.
Note 6: All limits are guaranteed by testing or statistical analysis.
Note 7: Large signal voltage gain is the total output swing divided by the input signal required to produce that swing.
Note 8: The open loop output current is guaranteed, by the measurement of the open loop output voltage swing.
Note 9: Slew rate is the average of the raising and falling slew rates.
Note 10: Harmonics are measured with AV= +2 and RL= 100Ωand VIN =1V
PP to give VOUT =2V
PP.
Note 11: Continuous operation at these output currents will exceed the power dissipation ability of the device
Note 12: Power dissipation limits may be exceeded if all four amplifiers source or sink 40mA. Voltage across the output transistors and their output currents must
be taken into account to determine the power dissipation of the device
Note 13: Electrical table values apply only for factory testing conditions at the temperature indicated. Factory testing conditions result in very limited self-heating of
the device such that TJ=T
A. No guarantee of parametric performance is indicated in the electrical tables under conditions of internal self heating where TJ>TA.
See applications section for information on temperature de-rating of this device.
Connection Diagram
14-Pin SOIC/TSSOP
20073402
Top View
Ordering Information
Package Part Number Package Marking Transport Media NSC Drawing
14-Pin SOIC LM6588MA LM6588MA 95 Units/Rail M14A
LM6588MAX 2.5k Units Tape and Reel
14-Pin TSSOP LM6588MT LM6588MT 95 Units/Rail MTC14
LM6588MTX 2.5k Units Tape and Reel
LM6588
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Typical Performance Characteristics Unless otherwise specified, all limits guaranteed for T
J
= 25˚C,
V
CM
= 1/2V
S
and R
L
=2kΩ.
Gain Phase vs. Temperature (V
S
= 5V) Gain Phase vs. Temperature (V
S
= 16V)
20073403 20073404
Gain Phase vs. Capacitive Loading (V
S
= 5V) Gain Phase vs. Capacitive Loading (V
S
= 16V)
20073405 20073406
PSRR (V
S
= 5V) PSRR (V
S
= 16V)
20073407 20073408
LM6588
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Typical Performance Characteristics Unless otherwise specified, all limits guaranteed for T
J
= 25˚C,
VCM = 1/2V
S
and R
L
=2kΩ. (Continued)
CMRR (V
S
= 5V) CMRR (V
S
= 16V)
20073409 20073410
Settling Time vs. Input Step Amplitude
(Output Slew and Settle Time)
Settling Time vs. Capacitive Loading
(Output Slew and Settle Time)
20073411
20073412
Crosstalk Rejection vs. Frequency
(Output to Output) Input Voltage Noise vs. Frequency
20073413 20073414
LM6588
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Typical Performance Characteristics Unless otherwise specified, all limits guaranteed for T
J
= 25˚C,
VCM = 1/2V
S
and R
L
=2kΩ. (Continued)
Stability vs. Capacitive Load Unity Gain (V
S
= 16V) Large Signal Step Response
20073415
20073416
Small Signal Step Response Small Signal Step Response
20073417 20073418
Closed Loop Output Impedance vs. Frequency (A
V
= +1) I
SUPPLY
vs. Common Mode Voltage (V
S
=±5V)
20073419 20073420
LM6588
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Typical Performance Characteristics Unless otherwise specified, all limits guaranteed for T
J
= 25˚C,
VCM = 1/2V
S
and R
L
=2kΩ. (Continued)
V
OS
vs. Common Mode Voltage (V
S
= 16V) V
OS
vs. V
OUT
(Typical Unit), (V
S
= 10V)
20073421 20073422
V
OUT
from V
+
vs. I
SOURCE
V
OUT
from V
−
vs. I
SINK
20073423 20073424
I
SUPPLY
vs. Supply Voltage
20073425
LM6588
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Application Notes
CIRCUIT DESCRIPTION
GENERAL & SPEC
The LM6588 is a bipolar process operational amplifier. It has
an exceptional output current capability of 200mA. The part
has both rail to rail inputs and outputs. It has a −3dB band-
width of 24MHz. The part has input voltage noise of 23nV/
, and 2
nd
and 3
rd
harmonic distortion of −53dB and
−40dB respectively.
INPUT SECTION
The LM6588 has rail to rail inputs and thus has an input
range over which the device may be biased of V
−
minus
0.5V, and V
+
plus 0.5V. The ultimate limit on input voltage
excursion is the ESD protection diodes on the input pins.
The most important consideration in Rail-to-Rail input op
amps is to understand the input structure. Most Rail-to-Rail
input amps use two differential input pairs to achieve this
function. This is how the LM6588 works. A conventional PNP
differential transistor pair provides the input gain from 0.5V
below the negative rail to about one volt below the positive
rail. At this point internal circuitry activates a differential NPN
transistor pair that allows the part to function from 1 volt
below the positive rail to 0.5V above the positive rail. The
effect on the inputs pins is as if there were two different op
amps connected to the inputs. This has several unique
implications.
•The input offset voltage will change, sometimes from
positive to negative as the inputs transition between the
two stages at about a volt below the positive rail. this
effect is seen in the V
OS
vs. V
CM
chart in the Typical
Performance Characteristics section of this datasheet.
•The input bias currents can be either positive or negative.
Do not expect a consistent flow in or out of the pins.
•The part will have different specifications depending on
whether the NPN or PNP stage is operating.
•There is a little more input capacitance then a single
stage input although the ESD diodes often swamp out the
added base capacitance.
•Since the input offset voltages can change from positive
to negative the output may not be monotonic when the
inputs are transitioning between the two stages and the
part is in a high gain configuration.
It should be remembered that swinging the inputs across the
input stage transition may cause output distortion and accu-
racy anomalies. It is also worth noting that anytime any amps
inputs are swung near the rails THD and other specs are
sure to suffer.
OUTPUT SECTION
Current Rating
The LM6588 has an output current rating, sinking or sourc-
ing, of 200mA. The LM6588 is ideally suited to loads that
require a high value of peak current but only a reduced value
of average current. This condition is typical of driving the
gate of a MOSFET. While the output drive rating is 200mA
peak, and the output structure supports rail-to-rail operation,
the attainable output current is reduced when the gain and
drive conditions are such that the output voltage approaches
either rail.
Output Power
Because of the increased output drive capability, internal
heat dissipation must be held to a level that does not in-
crease the junction temperature above its maximum rated
value of 150˚ C.
Power Requirements
The LM6588 operates from a voltage supply, of V
+
and
ground, or from a V
−
and V
+
split supply. Single-ended
voltage range is +5V to +16V and split supply range is ±2.5V
to ±8.0V.
APPLICATION HINTS
POWER SUPPLIES
Sequencing
Best practice design technique for operational amplifiers
includes careful attention to power sequencing. Although the
LM6588 is a bipolar op amp, recommended op amp turn on
power sequencing of ground (or V
−
), followed by V
+
, fol-
lowed by input signal should be observed. Turn off power
sequence should be the reverse of the turn-on sequence.
Depending on how the amp is biased the outputs may swing
to the rails on power-on or power-off. Due to the high output
currents and rail to rail output stage in the LM6588 the output
may oscillate very slightly if the power is slowly raised be-
tween 2V and 4V The part is unconditionally stable at 5V.
Quick turn-off and turn-on times will eliminate oscillation
problems.
PSRR and Noise
Care should be taken to minimize the noise in the power
supply rails. The figure of merit for an op amp’s ability to
keep power supply noise out of the signal is called Power
Supply Rejection Ratio (PSRR). Observe from the PSRR
charts in the Typical Performance Characteristics section
that the PSRR falls of dramatically as the frequency of the
noise on the power supply line goes up. This is one of the
reasons switching power supplies can cause problems. It
should also be noticed from the charts that the negative
supply pin is far more susceptible to power noise. The de-
sign engineer should determine the switching frequencies
and ripple voltages of the power supplies in the system. If
required, a series resistor or in the case of a high current op
amp like the LM6588, a series inductor can be used to filter
out the noise.
Transients
In addition to the ripple and noise on the power supplies
there are also transient voltage changes. This can be
caused by another device on the same power supply sud-
denly drawing current or suddenly stopping a current draw.
The design engineer should insure that there are no damag-
ing transients induced on the power supply lines when the op
amp suddenly changes current delivery.
LAYOUT
Ground Planes
Do not assume the ground (or more properly, the common or
return) of the power supply is an ocean of zero impedance.
The thinner the trace, the higher the resistance. Thin traces
cause tiny inductances in the power lines. These can react
against the large currents the LM6588 is capable of deliver-
ing to cause oscillations, instability, overshoot and distortion.
A ground plane is the most effective way of insuring the
LM6588
www.national.com9
Application Notes (Continued)
ground is a uniform low impedance. If a four layer board
cannot be used, consider pouring a plane on one side of a
two layer board. If this cannot be done be sure to use as
wide a trace as practicable and use extra decoupling capaci-
tors to minimize the AC variations on the ground rail.
Decoupling
A high-speed, high-current amp like the LM6588 must have
generous decoupling capacitors. They should be as close to
the power pins as possible. Putting them on the back side
opposite the power pins may give the tightest layout. If
ground and power planes are available, the placement of the
decoupling caps are not as critical.
Breadboards
The high currents and high frequencies the LM6588 oper-
ates at, as well as thermal considerations, require that pro-
totyping of the design be done on a circuit board as opposed
to a “Proto-Board” style breadboard.
STABILITY
General:
High speed parts with large output current capability require
special care to insure lack of oscillations. Keep the ”+” pin
isolated from the output to insure stability. As noted above
care should be take to insure the large output currents do not
appear in the ground or ground plane and then get coupled
into the “+” pin. As always, good tight layout is essential as is
adequate use of decoupling capacitors on the power sup-
plies.
Unity Gain
The unity gain or voltage-follower configuration is the most
subject to oscillation. If a part is stable at unity gain it is
almost certain to work in other configurations. In certain
applications where the part is setting a reference voltage or
is being used as a buffer greater stability can be achieved by
configuring the part as a gain of −1 or −2 or +2.
Phase Margin
The phase margin of an op amps gain-phase plot is an
indication of the stability of the amp. It is desirable to have at
least 45˚C of phase margin to insure stability in all cases.
The LM6588 has 60˚C of phase margin even with it’s large
output currents and Rail-to-Rail output stage, which are
generally more prone to stability issues.
Capacitive Load
The LM6588 can withstand 30pF of capacitive load in a unity
gain configuration before stability issues arise. At very large
capacitances, the load capacitor will attenuate the gain like
any other heavy load and the part becomes stable again.
The LM6588 will be stable at 330nF and higher load capaci-
tance. Refer to the chart in the Typical Performance Char-
acteristics section.
OUTPUT
Swing vs. Current
The LM6588 will get to about 25mV or 30mV of either rail
when there is no load. The LM6588 can sink or source
hundreds of milliamperes while remaining less then 0.5V
away from the rail. It should be noted that if the outputs are
driven to the rail and the part can no longer maintain the
feedback loop, the internal circuitry will deliver large base
currents into the huge output transistors, trying to get the
outputs to get past the saturation voltage. The base currents
will approach 16 milliamperes and this will appear as an
increase in power supply current. Operating at this power
dissipation level for extended periods will damage the part,
especially in the higher thermal resistance TSSOP package.
Because of this phenomenon, unused parts should not have
the inputs strapped to either rail, but should have the inputs
biased at the midpoint or at least a diode drop (0.6V) within
the rails.
Self Heating
As discussed above the LM6588 is capable of significant
power by virtue of its 200mA current handling capability. A
TSSOP package cannot sustain these power levels for more
then a brief period.
TFT Display Application
INTRODUCTION
In today’s high-resolution TFT displays, op amps are used
for the following three functions:
1. V
COM
Driver
2. Gamma Buffer
3. Panel Repair Buffer
All of these functions utilize op amps as non-inverting, unity-
gain buffers. The V
COM
Driver and Gamma Buffer are buffers
that supply a well regulated DC voltage. A Panel Repair
Buffer, on the other hand, provides a high frequency signal
that contains part of the display’s visual image.
In an effort to reduce production costs, display manufactur-
ers use a minimum variety of different parts in their TFT
displays. As a result, the same type of op amp will be used
for the V
COM
Driver, Gamma Buffer, and Panel Repair Buffer.
To perform all these functions, such an op amp must have
the following characteristics:
1. Large output current drive
2. Rail to rail input common mode range
3. Rail to rail output swing
4. Medium speed gain bandwidth and slew rate
The LM6588 meets these requirements. It has a rail-to-rail
input and output, typical gain bandwidth and slew rate of
15MHz and 15V/µs, and it can supply up to 200mA of output
current. The following sections will describe the operation of
V
COM
Drivers, Gamma Buffers, and Panel Repair Buffers,
showing how the LM6588 is well suited for each of these
functions.
BRIEF OVERVIEW OF TFT DISPLAY
To better understand these op amp applications, let’s first
review a few basic concepts of how a TFT display operates.
Figure 1 is a simplified illustration of an LCD pixel. The top
and bottom plates of each pixel consist of Indium-Tin oxide
(ITO), which is a transparent, electrically conductive mate-
rial. ITO lies on the inner surfaces of two glass substrates
that are the front and back glass panels of a TFT display.
Sandwiched between the two ITO plates is an insulating
material (liquid crystal) that alters the polarization of light to a
lesser a greater amount, depending on how much voltage
(V
PIXEL
) is applied across the two plates. Polarizers are
placed on the outer surfaces of the two glass substrates,
which in combination with the liquid crystal create a variable
LM6588
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TFT Display Application (Continued)
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. On a Twisted
Neumatic (TN) display, which is typical of most TFT displays,
a pixel transmits the greatest amount of light when V
PIXEL
is
less ±0.5V, and it becomes less transparent as this voltage
increases with either a positive or negative polarity. In short,
an LCD pixel can be thought of as a capacitor, through
which, a controlled amount of light is transmitted by varying
V
PIXEL
.
Figure 2 is a simplified block diagram of a TFT display,
showing how individual pixels are connected to the row,
column, and V
COM
lines. Each pixel is represented by ca-
pacitor with an NMOS transistor connected to its top plate.
Pixels in a TFT panel are arranged in rows and columns.
Row lines are connected to the NMOS gates, and column
lines to the NMOS sources. The back plate of every pixel is
connected to a common voltage called V
COM
. Pixel bright-
ness is controlled by voltage applied to the top plates, and
the Column Drivers supply this voltage via the column lines.
Column Drivers ‘write’ this voltage to the pixels one row at a
time, and this is accomplished by having the Row Drivers
select an individual row of pixels when their voltage levels
are transmitted by the Column Drivers. The Row Drivers
sequentially apply a large positive pulse (typically 25V to
35V) to each row line. This turns-on NMOS transistors con-
nected to an individual row, allowing voltages from the col-
umn lines to be transmitted to the pixels.
V
COM
DRIVER
The V
COM
driver supplies a common voltage (V
COM
)toall
the pixels in a TFT panel. V
COM
is a constant DC voltage that
lies in the middle of the column drivers’ output voltage range.
As a result, when the column drivers write to a row of pixels,
they apply voltages that are either positive or negative with
respect to V
COM
. In fact, the polarity of a pixel is reversed
each time its row is selected. This allows the column drivers
to apply an alternating voltage to the pixels rather than a DC
signal, which can ‘burn’ a pattern into an LCD display.
When column drivers write to the pixels, current pulses are
injected onto the V
COM
line. These pulses result from charg-
ing stray capacitance between V
COM
and the column lines
(see Figure 2), which ranges typically from 16pF to 33pF per
column. Pixel capacitance contributes very little to these
pulses because only one pixel at a time is connected to a
column, and the capacitance of a single pixel is on the order
of only 0.5pF. Each column line has a significant amount of
series resistance (typically 2kΩto 40kΩ), so the stray ca-
pacitance is distributed along the entire length of a column.
This can be modeled by the multi-segment RC network
shown in Figure 3. The total capacitance between V
COM
and
the column lines can range from 25nF to 100nF, and charg-
ing this capacitance can result in positive or negative current
pulses of 100mA, or more. In addition, a similar distributed
capacitance of approximately the same value exists be-
tween V
COM
and the row lines. Therefore, the V
COM
driver’s
load is the sum of these distributed RC networks with a total
capacitance of 50nF to 200nF, and this load can modeled
like the circuit in Figure 3.
AV
COM
driver is essentially a voltage regulator that can
source and sink current into a large capacitive load. To
simplify the analysis of this driver, the distributed RC network
of Figure 3 has been reduced to a single RC load in Figure
4. This load places a large capacitance on the V
COM
driver
output, resulting in an additional pole in the op amp’s feed-
back loop. However, the op amp remains stable because
C
LOAD
and R
ESR
create a zero that cancels the effect of this
pole. The range of C
LOAD
is 50nF to 200nF and R
ESR
is 20Ω
to 100Ω, so this zero will have a frequency in the range of
20073426
FIGURE 1. Individual LCD Pixel
20073427
FIGURE 2. TFT Display
20073428
FIGURE 3. Model of Impedance between V
COM
and
Column Lines
LM6588
www.national.com11
TFT Display Application (Continued)
8KHz to 160KHz, which is much lower than the gain band-
width of most op amps. As a result, the V
COM
load adds very
little phase lag when op amp loop gain is unity, and this
allows the V
COM
Driver to remain stable. This was verified by
measuring the small-signal bandwidth of the LM6588 with
the RC load of Figure 4. When driving an RC load of 50nF
and 20Ω, the LM6588 has a unity gain frequency of 6.12MHz
with 41.5˚C of phase margin. If the load capacitor is in-
creased to 200nF and the resistance remains 20Ω, the unity
gain frequency is virtually unchanged: 6.05MHz with 42.9˚C
of phase margin.
AV
COM
Driver’s large-signal response time is determined by
the op amp’s maximum output current, not by its slew rate.
This is easily shown by calculating how much output current
is required to slew a 50nF load capacitance at the LM6588
slew rate of 14V/µs:
I
OUT
= 14V/µs x 50nF
= 700mA
700mA exceeds the maximum current specification for the
LM6588 and almost all other op amps, confirming that a
V
COM
driver’s speed is limited by its peak output current. In
order to minimize V
COM
transients, the op amp used as a
V
COM
Driver must supply large values of output current.
Figure 5 is a common test circuit used for measuring V
COM
driver response time. The RC network of R
L1
to R
L3
and C
1
to C
4
models the distributed RC load of a V
COM
line. This RC
network is a gross simplification of what the actual imped-
ance is on a TFT panel. However, it does provide a useful
test for measuring the op amp’s transient response when
driving a large capacitive load. A low impedance MOSFET
driver applies a 5V square wave to V
SW
, generating large
current pulses in the RC network. Scope photos from this
circuit are shown in Figure 6 and Figure 7.Figure 6 shows
the test circuit generates positive and negative voltage
spikes with an amplitude of ±3.2V at the V
COM
node, and
both transients settle-out in approximately 2µs. As men-
tioned before, the speed at which these transients settle-out
is a function of the op amp’s peak output current. The I
OUT
trace in Figure 7 shows that the LM6588 can sink and source
peak currents of −200mA and 200mA. This ability to supply
large values of output current makes the LM6588 extremely
well suited for V
COM
Driver applications.
20073429
FIGURE 4. V
COM
Driver with Simplified Load
20073430
FIGURE 5. V
COM
Driver Test Circuit
20073431
FIGURE 6. V
SW
and V
COM
Waveforms from V
COM
20073432
FIGURE 7. V
SW
and I
OUT
Waveforms from V
COM
Test
Circuit
LM6588
www.national.com 12
TFT Display Application (Continued)
GAMMA BUFFER
Illumination in a TFT display, also referred to as grayscale, is
set by a series of discrete voltage levels that are applied to
each LCD pixel. These voltage levels are generated by
resistive DAC networks that reside inside each of the column
driver ICs. For example, a column driver with 64 Grayscale
levels has a two 6 bit resistive DACs. Typically, the two
DACs will have their 64 resistors grouped into four seg-
ments, as shown in Figure 8. Each of these segments is
connected to external voltage lines, VGMA1 to VGMA10,
which are the Gamma Levels. VGMA1 to VGMA5 set gray-
scale voltage levels that are positive with respect to V
COM
(high polarity gamma levels). VGMA6 to VGMA10 set gray-
scale voltages negative with respect to V
COM
(low polarity
gamma levels).
Figure 9 shows how column drivers in a TFT display are
connected to the gamma levels. VGMA1, VGMA5, VGMA6,
and VGMA10 are driven by the Gamma Buffers. These
buffers serve as low impedance voltage sources that gener-
ate the display’s gamma levels. The Gamma Buffers’ outputs
are set by a simple resistive ladder, as shown in Figure 9.
Note that VGMA2 to VGMA4 and VGMA7 to VGMA9 are
usually connected to the column drivers even though they
are not driven by external buffers. Doing so, forces the
gamma levels in all the column drivers to be identical, mini-
mizing grayscale mismatch between column drivers. Refer-
ring again to Figure 9, the resistive load of a column driver
DAC (i.e. resistance between GMA1 to GMA5) is typically
10kΩto 15kΩ. On a typical display such as XGA, there can
be up to 10 column drivers, so the total resistive load on a
Gamma Buffer output can be as low as 1kΩ. The voltage
between VGMA1 and VGMA5 can range from 3V to 6V,
depending on the type of TFT panel. Therefore, maximum
load current supplied by a Gamma Buffer is approximately
6V/1kΩ= 6mA, which is a relatively light load for most op
amps. In many displays, VGMA1 can be less than 500mV
below V
DD
, and VGMA10 can be less than 500mV above
ground. Under these conditions, an op amp used for the
Gamma Buffer must have rail-to-rail inputs and outputs, like
the LM6588.
Another important specification for Gamma Buffers is small
signal bandwidth and slew rate. When column drivers select
which voltage levels are written to a row of pixels, their
internal DACs inject current spikes into the Gamma Lines.
This generates voltage transients at the Gamma Buffer out-
puts, and they should settle-out in less than 1µs to insure a
steady output voltage from the column drivers. Typically,
these transients have a maximum amplitude of 2V, so a
gamma buffer must have sufficient bandwidth and slew rate
to recover from a 2V transient in 1µs or less.
Figure 10 illustrates how an op amp responds to a large-
signal transient. When such a transient occurs att=0,the
output does not start changing until T
PD
, which is the op
amp’s propagation delay time (typically 20ns for the
LM6588). The output then changes at the op amp’s slew rate
from t = T
PD
to T
SR
.Fromt=T
SR
to T
SE
T, the output settles
to its final value (V
F
) at a speed determined by the op amp’s
small-signal frequency response. Although propagation de-
20073433
FIGURE 8. Simplified Schematic of Column Driver IC
20073434
FIGURE 9. Basic Gamma Buffer Configuration
20073435
FIGURE 10. Large Signal Transient Response of an
Operational Amplifier
LM6588
www.national.com13
TFT Display Application (Continued)
lay and slew limited response time (t=0toT
SR
) can be
calculated from data sheet specifications, the small signal
settling time (T
SR
to T
SET
) cannot. This is because an op
amp’s gain vs. frequency has multiple poles, and as a result,
small-signal settling time can not be calculated as a simple
function of the op amp’s gain bandwidth. Therefore, the only
accurate method for determining op amp settling time is to
measure it directly.
The test circuit in Figure 11 was used to measure LM6588
settling time for a 2V pulse and 1kΩload, which represents
the maximum transient amplitude and output load for a
gamma buffer. With this test system, the LM6588 settled to
within ±30mV of 2V pulse in approximately 170ns. Settling
time fora0to–2Vpulse was slightly less, 150ns. These
values are much smaller than the desired response time of
1µs, so the LM6588 has sufficient bandwidth and slew rate
for regulating gamma line transients.
PANEL REPAIR BUFFER
It is not uncommon for a TFT panel to be manufactured with
an open in one or two of its column or row lines. In order to
repair these opens, TFT panels have uncommitted repair
lines that run along their periphery. When an open line is
identified during a panel’s final assembly, a repair line re-
routes its signal past the open. Figure 12 illustrates how a
column is repaired. The column driver’s output is sent to the
other end of an open column via a repair line, and the repair
line is driven by a panel repair buffer. When a column or row
line is repaired, the capacitance on that line increases sub-
stantially. For instance, a column typically has 50pF to
100pF of line capacitance, but a repaired column can have
up to 200pF. Column drivers are not designed to drive this
extra capacitance, so a panel repair buffer provides addi-
tional output current to the repaired column line. Note that
there is typically a 20Ωto 100Ωresistor in series with the
buffer output. This resistor isolates the output from the
200pF of capacitance on a repaired column line, ensuring
that the buffer remains stable. A pole is created by this
resistor and capacitance, but its frequency will be in the
range of 8MHz to 40MHz, so it will have only a minor effect
on the buffer’s transient response time. Panel repair buffers
transmit a column driver signal, and as mentioned in the
gamma buffer section, this signal is set by the gamma levels.
It was also mentioned that many displays have upper and
lower gamma levels that are within 500mV of the supply
rails. Therefore, op amps used as panel repair buffers should
have rail-to-rail input and stages. Otherwise, they may clip
the column driver signal.
The signal from a panel repair buffer is stored by a pixel
when the pixel’s row is selected. In high-resolution displays,
each row is selected for as little as 11µs. To insure that a
pixel has adequate time to settle-out during this brief period,
a panel repair buffer should settle to within 1% of its final
value approximately 1µs after a row is selected. This is
hardest to achieve when transmitting a column line’s maxi-
mum voltage swing, which is the difference between the
upper and lower gamma levels (i.e. voltage between VGMA1
and VGMA10). For a LM6588, the most demanding applica-
tion occurs in displays that operate from a 16V supply. In
these displays, voltage difference between the top and bot-
tom gamma levels can be as large as 15V, so the LM6588
needs to transmit a ±15V pulse and settle to within 60mV of
its final value in approximately 1µs (60mV is approximately
1% of the dynamic range of the high or low polarity gamma
levels). LM6588 settling times for 15V and –15V pulses were
measured in a test circuit similar to the one in Figure 11.V
+
and V
−
were set to 15.5V and –0.5V, respectively, when
measuring settling time for a 0V to 15V pulse. Likewise, V
+
and V
−
were set to 0.5V and –15.5V when measuring set-
tling time for a 0V to –15V pulse. In both cases, the LM6588
output was connected to a series RC load of 51Ωand 200pF.
When tested this way, the LM6588 settled to within 60mV of
15V or –15V in approximately 1.1µs. These observed values
are very close to the desired 1µs specification, demonstrat-
ing that the LM6588 has the bandwidth and slew rate re-
quired for repair buffers in high-resolution TFT displays.
SUMMARY
This application note provided a basic explanation of how op
amps are used in TFT displays, and it also presented the
specifications required for these op amps. There are three
major op amp applications in a display: V
COM
Driver,
Gamma Buffer, and Panel Repair Buffer, and the LM6588
can be used for all of them. As a V
COM
Driver, the LM6588
can supply large values of output current to regulate V
COM
load transients. It has rail-to-rail input common-mode range
and output swing required for gamma buffers and panel
repair buffers. It also has the necessary gain bandwidth and
slew-rate for regulating gamma levels and driving column
repair lines. All these features make the LM6588 very well
suited for use in TFT displays.
20073436
FIGURE 11. Gamma Buffer Settling Time Test Circuit
20073437
FIGURE 12. Panel Repair Buffer
LM6588
www.national.com 14
Physical Dimensions inches (millimeters) unless otherwise noted
14-Pin SOIC
NS Package Number M14A
14-Pin TSSOP
NS Package Number MTC14
LM6588
www.national.com15
Notes
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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WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR
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which, (a) are intended for surgical implant into the body, or
(b) support or sustain life, and whose failure to perform when
properly used in accordance with instructions for use
provided in the labeling, can be reasonably expected to result
in a significant injury to the user.
2. A critical component is any component of a life support
device or system whose failure to perform can be reasonably
expected to cause the failure of the life support device or
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www.national.com
LM6588 TFT-LCD Quad, 16V RRIO High Output Current Operational Amplifier
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