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 www.national.com 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. 3 www.national.com 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 www.national.com 4 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 5 www.national.com 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 www.national.com 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 7 www.national.com 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 www.national.com 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 9 www.national.com 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 www.national.com 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. 11 www.national.com 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. www.national.com 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. 13 www.national.com 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 www.national.com 14 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. 15 www.national.com 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. www.national.com 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 www.national.com LM8207 Application Section LM8207 Application Section (Continued) 20137929 FIGURE 12. LM8207 Configuration www.national.com 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 www.national.com LM8207 Application Section LM8207 Application Section (Continued) 20137950 FIGURE 13. Schematic LM8207 Evaluation Board www.national.com 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 www.national.com 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. LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. 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