NATIONAL SEMICONDUCTOR LM6588 TFT-LCD Quad 16V RRIO High Output Current Operational Amplifier handbook
Contents
1. 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 Veixgi 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 TFT Display Application Continueo 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 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 VPIXEL TRANSMITTED LIGHT 1111111 J PORER f pi KORR ITO KORR GLASS SUBSTRATE LIQUID CRYSTAL Tl A na BOTTOM MATERIAL VES ITO Arm GLASS POLARITY SUBSTRATE gt J POLARIZER y LIGHT SOURCE 20073426 FIGURE 1 Individual L2. 5V GAIN dB PHASE 10k 100k 1M 10M 100M FREQUENCY Hz 20073405 PSRR Vs 5V PSRR dB w oe a ___ 1 Es 10 1k 100k 10M FREQUENCY Hz 20073407 Gain Phase vs Temperature Vs 16V mese EET ENT LET zo LLLI ELE e E LT ac 100 s 28 50 S 2 lt DN NL Aom E 20 20 AM XN CN io o LLL 10 Fy 16V 10 20 20 10k 100k 1M 10M 100M FREQUENCY Hz 20073404 Gain Phase vs Capacitive Loading Vs 16V 100 100 o o rse A II To LU UU N p s UP UH Ul X 20 8 2 TN 40 GAIN 10pF 40 V ESANI TRUN S so L N Me sore Hh les ro ELT Sa 4o AH Ie LUE SQ 10 10 20 20 10 100k 1M 10M 100M FREQUENCY Hz PSRR Vs 16V PSRR dB 10 1k 1M 10M FREQUENCY Hz 20073408 www national com 88991011 LM6588 Typical Performance Characteristics Unless otherwise specified all limits guaranteed for T 25 C Vom 1 2Vg and R 2kQ Continued CMRR Vs 5V CMRR Vs 16V CIN UT ENUM IN S O TNI KJ KJ E oL LLL IN gt gt S o AL LI ENT LA ATT LN IE NI iub rd hun 10 1k 1M 10M 10 1k 1M 10M FREQUENCY Hz FREQUENCY Hz 20073409 20073410 Settling Time vs Input Step
3. TIT T LI LL COLUMN LINE 20073428 FIGURE 3 Model of Impedance between Vcom and Column Lines A Vcom 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 Vcom driver output resulting in an additional pole in the op amp s feed back loop However the op amp remains stable because C oAp and Resp create zero that cancels the effect of this pole The range of C is to 200nF and Resp is 200 to 1000 so this zero will have a frequency in the range of www national com 88991017 LM6588 TFT Display Application continued 8KHz to 160KHz which is much lower than the gain band width of most op amps As a result the Vcom load adds very little phase lag when op amp loop gain is unity and this allows the Vcom 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 20Q 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 20Q the unity gain frequency is virtually unchanged 6 05MHz with 42 9 C of phase margin Voom LOAD CLOAD APPROX 50 200nF Vpp 2 8 e 8 r 8 a 8 e Res
4. ALL b Li PLANE A 0 008 0 010 0014 0 050 0 014 0 020 0 356 E ee e a 0 203 0 254 0 076 0 050 0 356 0 356 0 508 TYP ALL LEADS 0 004 0 406 1 270 TYP 0 008 0 102 TYP ALL LEADS EC a 2 ALL LEAD TIPS M314A REV Hj 14 Pin SOIC NS Package Number M14A SYMM S YMM ee 5 94 14X 1 18 f ee NN 14 _ GAGE PLANE T ge ug 0 25 ESTO 2TCTSTA 12X 0 65 0 8 7 d ALL LEAD TIPS RECOMMENDED LAND PATTERN c SEATING PLANE DETAIL A TYPICAL SEE DETAIL A 0 9 y f 9 09 0 20 09 0 B al CO 14X 0 19 0 30 ALL LEAD TIPS 0 130 A BO Ea B DIMENSIONS ARE IN MILLIMETERS 12X 0 65 DIMENSIONS IN FOR REFERENCE ONLY MTC14 Rev D 14 Pin TSSOP NS Package Number MTC14 15 www national com 88991011 LM6588 TFT LCD Quad 16V RRIO High Output Current Operational Amplifier 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 LIFE SUPPORT POLICY NATIONAL S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRES
5. C W Storage Temperature Range 65 C to 150 C 16V DC Electrical Characteristics note 13 Unless otherwise specified all limits guaranteed for at T 25 C Voy Vg and 2kQ Boldface limits apply at the tem perature extremes Symbol Parameter Conditions Min Typ Max Units Note 6 Note 5 Note 6 6 TC Vos npu Offset Voltage Average 5 Drift lB Input Bias Current 0 3 0 3 1 UA 7 los Input Offset Current 16 150 300 Breeman 08 Ratio 70 dB Vom 0 to 14 5V 78 103 eo ee _ PSRR Power Supply Rejection Ratio Voy 1V 80 103 dB CMVR Input Common Mode Voltage CMRR gt 50dB 0 0 2 V Range 16 2 16 80 Ay Large Signal Voltage Gain 2kQ Vo 0 5 to 15 5V 108 dB Vo Output Swing High 2kQ 15 8 15 9 15 6 V Output Swing Low 2kQ Z 04100 0 200 Output Short Circuit Current 170 230 5 g 10 20 mA Note 11 NEN NN Note 12 ls Supply Current per Amp 1200 1500 16V AC Electrical Characteristics Note 13 Unless otherwise specified all limits guaranteed for at T 25 C Vem 12 and R 2kQ Boldface limits apply at the tem perature extremes Symbol Parameter Conditions Min Typ Max Units Note 6 Note 5 Note 6 SR Slew Rate Note 9 1 Vy 10V pp i vus mA A pes www national com 2 16V AC Electrical Characteristics Note 13 Continued Unless otherwise specified all lim
6. C typical values unless specified Input common mode voltage 0 5V beyond rails B Output voltage swing RH 2kQ 50mV from rails SO 14 and TSSOP 14 package Manufactured in National Semiconductor s state of the art bonded wafer trench isolated complementary bipolar VIP10 technology for high performance at low power levels B Output short circuit current 200mA Continuous output current 75mA Supply current per amp no load 750 Supply voltage range 5V to 16V Unity gain stable 3dB bandwidth Ay 1 24MHz Slew rate 11V uSec B Settling time 270ns m m Applications m LCD panel Vcom driver m LCD panel gamma buffer m CD panel repair amp 100 100 2 5V 20073401 www national com Jeyiduy 1ndino uBiH A94 GO1 L4AL 88691 LM6588 Absolute Maximum Ratings Note 1 Input Common Mode Voltage V to V If Military Aerospace specified devices are required Junction Temperature Note 4 150 C please contact the National Semiconductor Sales Office Distributors for availability and specifications Operating Ratings note 1 ESD Tolerance Note 2 g gS Note 1 Human Body Model 2 5KV Supply Voltage 4V lt Vs lt 16V Machine Model 250V Temperature Range 40 C to 85 C Supply Voltage V 18V Thermal Resistance 0 4 Differential Input Voltage 5 SOIC 14 145 C W Output Short Circuit to Ground Note 3 Continuous TSSOP 14 155
7. 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
8. 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 Ty No guarantee of parametric performance is indicated in the electrical tables under conditions of internal self heating where Ty gt See applications section for information on temperature de rating of this device Connection Diagram 14 Pin SOIC TSSOP OUT A OUT D IN A 13 IN D IN A 12 IN D M e IN B 10 IN C IN B IN C OUT B OUT C 20073402 Top View Ordering Information Package Package Marking Transport Media NSC Drawing LM6588MA 95 Units Rail 14 Pin SOIC LM6588MA M14A LM6588MAX 2 5k Units Tape and Reel LM6588MT 95 Units Rail 14 Pin TSSOP LM6588MT MTC14 LM6588MTX 2 5k Units Tape and Reel www national com 4 Typical Performance Characteristics Unless otherwise specified all limits guaranteed for T 25 C Vom 1 2Vs and RL 2kQ Gain Phase vs Temperature Vs 5V PHASE GAIN dB PHASE 10k 100k 1M 10M 100M FREQUENCY Hz 20073403 Gain Phase vs Capacitive Loading Vs
9. 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 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
10. 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 e 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 Vos vs Vey chart in the Typical Performance Characteristics section of this datasheet e The input bias currents can be either positive or negative Do not expect a consistent flow in or out of the pins e The part will have different specifications depending on whether the NPN or PNP stage is operating e There is a little more input capacitance then a single stage input although the ESD diodes often swamp out the added base capacitance e 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 no
11. Amplitude Settling Time vs Capacitive Loading Output Slew and Settle Time Output Slew and Settle Time 2 2 gt gt I 5 5 S ae 0 75 1 125 15 175 2 0 10 20 30 40 50 60 70 80 CAPACITIVE LOADING pF INPUT STEP AMPLITUDE Vpp 20073411 Crosstalk Rejection vs Frequency Output to Output Input Voltage Noise vs Frequency 1000 a ab cil eo 8 j B cun P eo M 2 ott LEN 2 X H 10 111111 s oer 111111 CERTE EHE ETE ELTE LU REO 10 1k 100k 10M 1 10 100 1k 10k 1M FREQUENCY Hz FREQUENCY Hz 20073413 20073414 www national com 6 Typical Performance Characteristics Unless otherwise specified all limits guaranteed for T 25 C Vom 1 2Vg and R 2kQ Continued Stability vs Capacitive Load Unity Gain Vs 16V Large Signal Step Response ptt LLL LLL 10k 1 L a gt UNSTABLE 5 1k A oam 5 4 82 012 3 4 5 bd 100ns DIV 20073416 Vout V 20073415 Small Signal Step Response Small Signal Step Response 50mV DIV 100ns DIV 50mV DIV 100ns DIV 20073417 20073418 Closed Loop Output Impedance vs Frequency Ay 1 lsuppLy V5 Common Mode Voltage Vs 5V 1000 100 pz 10 E Zour Mi Isuppy MA 0 01 0 001 lt lt FREQUENCY Hz COMMON MODE VOLTAGE V 20073419 20073420 7 www national com 88991011 LM6588 Typical Performance Characteristics Unless o
12. CD Pixel DRIVERS CSTRAY CSTRAY CSTRAY VcoM Vcom PIXEL ROW DRIVERS APPROX Vcom PRIVER 20073427 FIGURE 2 TFT Display Figure 2 is a simplified block diagram of a TFT display showing how individual pixels are connected to the row column and Vcom 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 Vcom Pixel bright ness is controlled by voltage applied to the top plates and 11 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 Vcom DRIVER The Vcom driver supplies a common voltage Vcom to all the pixels in a TFT panel Vcom 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 a
13. MA LEVEL INPUTS LOW POLARITY GAMMA LEVEL INPUTS VGMA1 VGMA9 VGMA10 VGMA3 VGMA5 VGMA4 VGMA7 VGMA6 VGMA8 VGMA2 RESISTIVE RESISTIVE DAC a DAC TRANSMISSION GATES COLUMN DRIVER BUFFERS TRANSMISSION GATES COLUMN DRIVER BUFFERS V gt 20073433 FIGURE 8 Simplified Schematic of Column Driver IC 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 GMAS is typically 10kO to 15kQ 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 1kQ The voltage between VGMA1 and VGMAS5 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 1kQ which is a relatively light load for most op
14. National Semiconductor LM6588 July 2005 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 Vcom 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 5OmV 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 75 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 Test Circuit Diagram 3000 MEASURE CURRENT 8V VOLTAGE 2 5V SQUARE WAVE 2005 National Semiconductor Corporation DS200734 10nF 10nF 10nF b UT JUL Features Vs 5V 25
15. S WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION As used herein 1 Life support devices or systems are devices or systems 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 BANNED SUBSTANCE COMPLIANCE 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 system or to affect its safety or effectiveness National Semiconductor manufactures products and uses packing materials that meet the provisions of the Customer Products Stewardship Specification CSP 9 111C2 and the Banned Substances and Materials of Interest Specification CSP 9 111S2 and contain no Banned Substances as defined in CSP 9 111S2 Leadfree products are ROHS compliant National Semiconductor National Semiconductor Americas Customer Europe Customer Support Center Support Center Fax 49 0 180 530 85 86 Email new feedback nsc com Email europe support 9 nsc com Tel 1 800 272 9959 Deutsch Tel 49 0 69 9508 6208 English Tel 44 0 870 24 0 2171 www national com Frangais Tel 33 0 1 41 91 8790 National Semiconductor National Semiconductor Asia Pacific Custo
16. Vcom node and both transients settle out in approximately 2 As men tioned before the speed at which these transients settle out is a function of the op amp s peak output current The lout trace in Figure 7 shows that the LM6588 can sink and source peak currents of 200mA and 200 This ability to supply large values of output current makes the LM6588 extremely well suited for Vcom Driver applications BVIDIV 2VIDIV 2uSec DIV 20073431 FIGURE 6 Vsw and Vcom Waveforms from Vcom 5 VIDIV 200 mA DIV 2 uSec DIV 20073432 FIGURE 7 Vsw and lour Waveforms from Vcom Test Circuit 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 VGMAGB set gray scale voltage levels that are positive with respect to Vcom high polarity gamma levels VGMA6 to VGMA10 set gray scale voltages negative with respect to Vcom low polarity gamma levels HIGH POLARITY GAM
17. amps In many displays VGMA1 can be less than 500mV below Vpp 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 13 TFT LCD PANEL 20073434 FIGURE 9 Basic Gamma Buffer Configuration 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 1ys 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 1us or less T PD SET 20073435 FIGURE 10 Large Signal Transient Response of an Operational Amplifier Figure 10 illustrates how an op amp responds to a large signal transient When such a transient occurs at t O the output does not start changing until Tpp 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 Tpp to Tsp From t Tgp to TseT the output settles to its final value Vg at a speed determined by the op amp s small signal frequency response Although propagation de www nat
18. 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 www national com 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 funct
19. ional com 88991011 LM6588 TFT Display Application continued lay and slew limited response time t 0 to Tsg can be calculated from data sheet specifications the small signal settling time Tsp to Taser 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 2V od OR 0 AMPLIFIER UNDER TEST TEK 7633 STORAGE HP 8082 SCOPE PULSE GENERATOR TEK 7814 6 5V SAMPLING INPUT 20073436 FIGURE 11 Gamma Buffer Settling Time Test Circuit The test circuit in Figure 11 was used to measure LM6588 settling time for a 2V pulse and 1kQ load 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 for a O to 2V pulse was slightly less 150ns These values are much smaller than the desired response time of 1us 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 o
20. ions 1 Voom Driver 2 Gamma Buffer 3 Panel Repair Buffer All of these functions utilize op amps as non inverting unity gain buffers The Vcom 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 Vcom 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 hail 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 us and it can supply up to 200mA of output current The following sections will describe the operation of Vcom 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
21. its guaranteed for at T 25 C Voy 12 and 2kQ Boldface limits apply at the tem perature extremes Symbol Parameter Conditions Units Na 6 NS 5 hes 6 Unity Gain Bandwidth Product MHz 5 meses LI deg HD2 2 9 Harmonic Distortion Vow dBc HD3 3rd mE epe Vout 2Vpp dBc 7 5V DC Electrical Characteristics note 13 Unless otherwise specified all limits guaranteed for at T 25 C Vem 12 and R 2kQ Boldface limits apply at the tem perature extremes Symbol Parameter Conditions Min Typ Max Units Note 6 Note 5 Note 6 Vos Input Offset Voltage 0 7 4 Drift 7 300 Differential Mode D Ratio dB Vom Stepped from 0 to 3 5V 75 CMVR Input Common Mode Voltage CMRR gt 50dB 0 0 0 2 V Range 5 2 5 0 Ay Large Signal Voltage Gain Ry 2kQ Vo 0 to 5V 80 106 dB Note 7 75 _ 4 7 V Note 11 m ls Current per Amp 1250 3 www national com 88991017 LM6588 5V AC Electrical Characteristics note 13 Unless otherwise specified all limits guaranteed for at T 25 C Voy Vg R 2kQ Boldface limits apply at the tem perature extremes Symbol Parameter Conditions Max Units ial SR Slew Rate Note 9 c rae _ M ves on PmseMagn se HD2 2 9 Harmonic Distortion 2Vee 53 dBc HD3 3rd Harmonic Di
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23. oximately 1 1us These observed values are very close to the desired 1us specification demonstrat ing that the LM6588 has the bandwidth and slew rate re quired for repair buffers in high resolution TFT displays PANEL COLUMN DRIVER REPAIR BUFFER 20 TO 1000 PANEL REPAIR LINE TFT LCD PANEL 20073437 FIGURE 12 Panel Repair Buffer 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 Vcom Driver Gamma Buffer and Panel Repair Buffer and the LM6588 can be used for all of them As a Vcom Driver the LM6588 can supply large values of output current to regulate Vcom 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 Physical Dimensions inches millimeters unless otherwise noted 0 335 0 344 8 509 8 738 0 228 0 244 5 791 6 198 LEAD NO 1 IDENT Y 0 010 0 254 0 150 0 157 3 810 3 988 0 010 0 020 0 053 0 069 0 254 0 508 9 1 345 1 753 MAX TYP 0 004 0 010 0 102 0 254
24. p 20 1009 8 P 8 6 8 7 8 0 L 20073429 FIGURE 4 Vcom Driver with Simplified Load A Vcom 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 us 700mA 700mA exceeds the maximum current specification for the LM6588 and almost all other op amps confirming that a Vcom Qriver s speed is limited by its peak output current In order to minimize Vcom transients the op amp used as a Vcom Driver must supply large values of output current F2 3000 Voom Vcom LOAD Ra Rio Rie 100 102 100 20073430 FIGURE 5 Vcom Driver Test Circuit www national com Figure 5 is a common test circuit used for measuring Vcom driver response time The RC network of to RH 4 and C to C4 models the distributed RC load of a Vcom 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 Vow 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
25. pen line is identified during a panel s final assembly a repair line re routes its signal past the open Figure 72 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 20Q to 100Q resistor 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 www national com rails Therefore amps used as panel repair buffers should have rail to rail input and stages Otherwise they may clip
26. pply voltages that are either positive or negative with respect to Vcom 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 Vcom line These pulses result from charg ing stray capacitance between Vcom 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 2kQ to 40kQ 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 Vcom 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 Vcom and the row lines Therefore the Vcom 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 R R R R R i
27. stortion Vout 2Vpp 40 dBc 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 5kQ in 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 Tymax and Ta The maximum allowable power dissipation at any ambient temperature is Tumax 8A 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 Ay 2 and R 1000 and Viy 1Vpp to give Voyt 2Vpp Note 11 Continuous operation at
28. 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 11 To insure that a pixel has adequate time to settle out during this brief period a panel repair buffer should settle to within 196 of its final value approximately 1us 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 1us 60mV is approximately 196 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 OV to 15V pulse Likewise V and were set to 0 5V and 15 5V when measuring set tling time for a OV to 15V pulse In both cases the LM6588 output was connected to a series RC load of 51 and 200pF When tested this way the LM6588 settled to within G0mV of 15V or 15V in appr
29. 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 www national com 88991017 LM6588 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
30. therwise specified all limits guaranteed for T 25 Vom 1 2Vg and R 2kQ Continued Vos VS Common Mode Voltage Vs 16V Vos VS Vout Typical Unit Vs 10V S S o o gt gt 7 5 3 4 1 3 7 7 6 5 4 3 2 1012 3456 7 COMMON MODE VOLTAGE V Vout V 20073421 20073422 Vout from vs lsource Vout from vs Isink 10 S 4 EN S gt E E an LE 2 2 0 1 gt gt 0 01 1 10 100 1000 1 10 100 1000 IsouRcE MA mA 20073423 20073424 lsuppLy VS Supply Voltage x E gt l CL ai D _ 0 V 20073425 www national com 8 Application Notes CIRCUIT DESCRIPTION GENERAL amp 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 JHz and 2 and 3 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
31. ting 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
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