LM2426

LM2426TE Monolithic Triple Channel 30 MHz DTV Driver May 2003 LM2426TE Monolithic Triple Channel 30 MHz DTV Driver General Description The LM2426TE is an integrated high voltage CRT driver circuit designed for use in HDTV applications. The IC contains three high input impedance, wide band amplifiers which directly drive the RGB cathodes of a CRT. Each channel has its gain internally set to −53 and can drive CRT capacitive loads as well as resistive loads present in other applications, limited only by the package’s power dissipation. The IC is packaged in an industry standard 11-lead TO-220 molded plastic power package designed specifically to meet high voltage spacing requirements. See Thermal Considerations section. Features n 0V to 5V input range n Greater than 130VPP output swing capability n Stable with 0–20 pF capacitive loads and inductive peaking networks n Convenient TO-220 staggered thin lead package style Applications n AC coupled HDTV applications using the 1080i and 720p formats as well as standard NTSC and PAL formats. Connection Diagram Schematic Diagram 20066402 Note: Tab is at GND Top View Order Number LM2426TE 20066401 FIGURE 1. Simplified Connection and Pinout Diagram FIGURE 2. Simplified Schematic Diagram (One Channel) © 2003 National Semiconductor Corporation DS200664 www.national.com LM2426TE Absolute Maximum Ratings 3) (Notes 1, Operating Ranges (Note 2) VCC VBB VIN VOUT Case Temperature +130V to +180V +7V to +13V +0V to +4V +15V to +175V Refer to Figure 11 If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Supply Voltage (VCC) Bias Voltage (VBB) Input Voltage (VIN) Storage Temperature Range (TSTG) Lead Temperature (Soldering, < 10 sec.) ESD Tolerance, Human Body Model Machine Model Junction Temperature θJC (typ) 2kV 200V 150˚C 2.9˚C/W 300˚C +200V +15V -0.5V to VBB +0.5V −65˚C to +150˚C Do not operate the part without a heat sink. Electrical Characteristics (See Figure 3 for Test Circuit) Unless otherwise noted: VCC = +180V, VBB = +8V, CL = 8pF, TC = 50˚C. DC Tests: VIN = 2.5VDC. AC Tests: Output = 110VPP (55V - 165V) at 1MHz. Symbol ICC IBB VOUT, 1 VOUT, 2 AV ∆AV LE tR tF OS Parameter Supply Current Bias Current DC Output Voltage DC Output Voltage DC Voltage Gain Gain Matching Linearity Error Rise Time Fall Time Overshoot Conditions All Three Channels, No Input Signal, No Output Load All Three Channels No AC Input Signal, VIN = 2.5VDC No AC Input Signal, VIN = 1.2VDC No AC Input Signal (Note 4), No AC Input Signal (Notes 4, 5), No AC Input Signal (Note 6), 10% to 90% (Note 6), 90% to 10% (Note 6) 93 160 -50 LM2426TE Min Typical 28 15 98 165 -53 1.0 8 13 13 7 Max 40 22 103 170 -56 dB % ns ns % Units mA mA VDC VDC Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Note 2: Operating ratings indicate conditions for which the device is functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics. Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis. The guaranteed specifications apply only for the test conditions listed. Some performance characteristics may change when the device is not operated under the listed test conditions. Note 3: All voltages are measured with respect to GND, unless otherwise specified. Note 4: Calculated value from Voltage Gain test on each channel. Note 5: Linearity Error is the variation in DC gain from VIN = 1.1V to VIN = 3.8V. Note 6: Input from signal generator: tr, tf < 1 ns. www.national.com 2 LM2426TE AC Test Circuit 20066403 Note: 8pF load includes parasitic capacitance. FIGURE 3. Test Circuit (One Channel) Figure 3 shows a typical test circuit for evaluation of the LM2426TE. This circuit is designed to allow testing of the LM2426TE in a 50Ω environment without the use of an expensive FET probe. The two 4990Ω resistors form a 400:1 divider with the 50Ω resistor and the oscilloscope. A test point is included for easy use of an oscilloscope probe.The compensation capacitor is used to compensate the network to achieve flat frequency response. 3 www.national.com LM2426TE Typical Performance Characteristics (VCC = +180VDC, VBB = +8VDC, CL = 8pF, VOUT = 110VPP (55V − 165V), Test Circuit - Figure 3 unless otherwise specified) 20066404 20066405 FIGURE 4. VOUT vs VIN FIGURE 7. Speed vs Load Capacitance 20066406 20066408 FIGURE 5. LM2426TE Pulse Response FIGURE 8. Speed vs Offset 20066418 20066409 FIGURE 6. Bandwidth FIGURE 9. Speed vs Case Temperature www.national.com 4 LM2426TE Typical Performance Characteristics (VCC = +180VDC, VBB = +8VDC, CL = 8pF, VOUT = 110VPP (55V − 165V), Test Circuit - Figure 3 unless otherwise specified) (Continued) 20066407 FIGURE 10. Power Dissipation vs Frequency 20066416 FIGURE 11. Power Derating Curve 20066419 FIGURE 12. Cathode Pulse Response 5 www.national.com LM2426TE Typical Performance Characteristics (VCC = +180VDC, VBB = +8VDC, CL = 8pF, VOUT = 110VPP (55V − 165V), Test Circuit - Figure 3 unless otherwise specified) (Continued) TABLE 1. Power Dissipation for Various Video Patterns Power Dissipation (W) Pattern 480i Raster Full White Field White Box, 75% Screen Size Gray Bars Color Bars 75% Amplitude Color Bars 100% Amplitude SMPTE Color Bars SMPTE 133 Cross Hatch 16x12 Resolution Chart Multiburst White Text on Black Background Windows Pattern Windows Pattern Windows Pattern Vertical Lines 5 On 5 Off Vertical Lines 4 On 4 Off Vertical Lines 3 On 3 Off Vertical Lines 2 On 2 Off Vertical Lines 1 On 1 Off Note: Input from signal generator: tr, tf < 2 ns. Format 480p 2.4 6.1 4.6 4.8 4.0 4.3 3.8 5.2 3.0 5.5 6.6 7.1 4.5 5.2 6.7 5.9 6.3 7.0 8.5 12.7 720p 2.4 6.0 4.0 4.7 4.0 4.3 3.8 5.4 3.0 5.6 10.1 11.1 6.4 6.7 8.6 8.7 9.8 11.7 14.4 21.8 1080i 2.4 6.5 4.2 5.0 4.1 4.5 4.0 5.7 2.9 5.8 10.9 12.3 6.6 7.0 9.4 9.5 10.8 12.9 16.2 24.6 2.4 6.1 4.6 4.8 3.9 4.3 3.8 5.0 2.8 5.3 5.4 5.2 4.0 4.7 6.0 5.1 5.3 5.6 6.3 8.5 Theory of Operation The LM2426TE is a high voltage monolithic three channel CRT driver suitable for HDTV applications. The LM2426TE operates with 180V and 8V power supplies. The part is housed in the industry standard 11-lead TO-220 molded plastic power package with thin leads for improved metal-tometal spacing. The circuit diagram of the LM2426TE is shown in Figure 2. The PNP emitter follower, Q5, provides input buffering. Q1 and Q2 form a fixed gain cascode amplifier with resistors R1 and R2 setting the gain at −53. Emitter followers Q3 and Q4 isolate the high output impedance of the cascode stage from the capacitance of the CRT cathode which decreases the sensitivity of the device to load capacitance. Q6 provides biasing to the output emitter follower stage to reduce crossover distortion at low signal levels. Figure 3 shows a typical test circuit for evaluation of the LM2426TE. This circuit is designed to allow testing of the LM2426TE in a 50Ω environment without the use of an expensive FET probe. In this test circuit, the two 4.99kΩ resistors form a 400:1 wideband, low capacitance probe when connected to a 50Ω coaxial cable and a 50Ω load (such as a 50Ω oscilloscope input). The input signal from the generator is ac coupled to the base of Q5. Application Hints INTRODUCTION National Semiconductor (NSC) is committed to provide application information that assists our customers in obtaining the best performance possible from our products. The following information is provided in order to support this commitment. The reader should be aware that the optimization of performance was done using a specific printed circuit board designed at NSC. Variations in performance can be realized due to physical changes in the printed circuit board and the application. Therefore, the designer should know that component value changes may be required in order to optimize performance in a given application. The values shown in this document can be used as a starting point for evaluation purposes. When working with high bandwidth circuits, good layout practices are also critical to achieving maximum performance. IMPORTANT INFORMATION The LM2426TE performance is targeted for the HDTV market. The application circuits shown in this document to optimize performance and to protect against damage from CRT arcover are designed specifically for the LM2426TE. If another member of the LM242X family is used, please refer to its datasheet. www.national.com 6 LM2426TE Application Hints POWER SUPPLY BYPASS (Continued) EFFECT OF OFFSET Figure 8 shows the variation in rise and fall times when the output offset of the device is varied from 105 to 115VDC. The rise time shows a variation of less than 7% relative to the center data point (110VDC). The fall time shows a variation of less than 2% relative to the center data point. THERMAL CONSIDERATIONS Figure 9 shows the performance of the LM2426TE in the test circuit shown in Figure 3 as a function of case temperature. The figure shows that the rise and fall times of the LM2426TE increase by approximately 10% and 4%, respectively, as the case temperature increases from 50˚C to 90˚C. This corresponds to a speed degradation of 2.5% and 1% for every 10˚C rise in case temperature. Figure 10 shows the power dissipation of the LM2426TE vs. Frequency when all three channels of the device are driving an 8pF load with a 110VPP alternating one pixel on, one pixel off signal. The graph assumes a 72% active time (device operating at the specified frequency) which is typical in a TV application. The other 28% of the time the device is assumed to be sitting at the black level (165V in this case). Table 1 also shows the typical power dissipation of the LM2426TE for various video patterns in the 480i, 480p, 720p, and 1080i video formats. Figure 10, Figure 11, and Table 1 give the designer the information needed to determine the heatsink requirement for the LM2426TE. For example, if an HDTV application uses the 720p format and "Vertical Lines 2 On 2 Off" is assumed to be the worst-case pattern to be displayed, then the power dissipated will be 14.4W (from Table 1). Figure 11 shows that the maximum allowed case temperature is 108˚C when 14.4W is dissipated. If the maximum expected ambient temperature is 70˚C, then a maximum heatsink thermal resistance can be calculated: Since the LM2426TE is a wide bandwidth amplifier, proper power supply bypassing is critical for optimum performance. Improper power supply bypassing can result in large overshoot, ringing or oscillation. 0.1µF capacitors should be connected from the supply pins, VCC and VBB, to ground, as close to the LM2426TE as is practical. Additionally, a 22µF or larger electrolytic capacitor should be connected from both supply pins to ground reasonably close to the LM2426TE. ARC PROTECTION During normal CRT operation, internal arcing may occasionally occur. Spark gaps, in the range of 300V, connected from the CRT cathodes to CRT ground will limit the maximum voltage, but to a value that is much higher than allowable on the LM2426TE. This fast, high voltage, high energy pulse can damage the LM2426TE output stage. The application circuit shown in Figure 13 is designed to help clamp the voltage at the output of the LM2426TE to a safe level. The clamp diodes, D1 and D2, should have a fast transient response, high peak current rating, low series impedance and low shunt capacitance. 1SS83 or equivalent diodes are recommended. D1 and D2 should have short, low impedance connections to VCC and ground respectively. The cathode of D1 should be located very close to a separately decoupled bypass capacitor (C3 in Figure 13). The ground connection of D2 and the decoupling capacitor should be very close to the LM2426TE ground. This will significantly reduce the high frequency voltage transients that the LM2426TE would be subjected to during an arcover condition. Resistor R2 limits the arcover current that is seen by the diodes while R1 limits the current into the LM2426TE as well as the voltage stress at the outputs of the device. R2 should be a 1⁄2W solid carbon type resistor. R1 can be a 1⁄4W metal or carbon film type resistor. Having large value resistors for R1 and R2 would be desirable, but this has the effect of increasing rise and fall times. Inductor L1 is critical to reduce the initial high frequency voltage levels that the LM2426TE would be subjected to. The inductor will not only help protect the device but it will also help minimize rise and fall times as well as minimize EMI. For proper arc protection, it is important to not omit any of the arc protection components shown in Figure 13. This example assumes a capacitive load of 8pF and no resistive load. The designer should note that if the load capacitance is increased the AC component of the total power dissipation will also increase. Note: An LM126X preamplifier, with rise and fall times of about 2 ns, was used to drive the LM2426TE for these power measurements. Using a preamplifier with rise and fall times slower than the LM126X will cause the LM2426TE to dissipate less power than shown in Table 1. OPTIMIZING TRANSIENT RESPONSE In Figure 13, there are three components (R1, R2 and L1) that can be adjusted to optimize the transient response of the application circuit. Increasing the values of R1 and R2 will slow the circuit down while decreasing overshoot. Increasing the value of L1 will speed up the circuit as well as increase overshoot. It is very important to use inductors with very high self-resonant frequencies, preferably above 300 MHz. Ferrite core inductors from J.W. Miller Magnetics (part # 78FR_ _k) were used for optimizing the performance of the device in the NSC application board. The values shown in Figure 14 and Figure 15 can be used as a good starting point for the evaluation of the LM2426TE. Using a variable resistor for R1 will simplify finding the value needed for optimum performance in a given application. Once the optimum value is determined, the variable resistor can be replaced with a fixed value. 7 www.national.com 20066410 FIGURE 13. One Channel of the LM2426TE with the Recommended Application Circuit EFFECT OF LOAD CAPACITANCE Figure 7 shows the effect of increased load capacitance on the speed of the device. This demonstrates the importance of knowing the load capacitance in the application. LM2426TE Application Hints (Continued) • • • Figure 12 shows the typical cathode pulse response with an output swing of 110VPP using a LM1269 preamplifier. PC BOARD LAYOUT CONSIDERATIONS For optimum performance, an adequate ground plane, isolation between channels, good supply bypassing and minimizing unwanted feedback are necessary. Also, the length of the signal traces from the preamplifier to the LM2426TE and from the LM2426TE to the CRT cathode should be as short as possible. The following references are recommended: Ott, Henry W., “Noise Reduction Techniques in Electronic Systems”, John Wiley & Sons, New York, 1976. “Video Amplifier Design for Computer Monitors”, National Semiconductor Application Note 1013. Pease, Robert A., “Troubleshooting Analog Circuits”, Butterworth-Heinemann, 1991. Because of its high small signal bandwidth, the part may oscillate in a TV if feedback occurs around the video channel through the chassis wiring. To prevent this, leads to the video amplifier input circuit should be shielded, and input circuit wiring should be spaced as far as possible from output circuit wiring. TYPICAL APPLICATION A typical application of the LM2426TE is shown in the schematic for the NSC demonstration board in Figure 14 and Figure 15. Used in conjunction with an LM126X preamplifier, a complete video channel from input to CRT cathode can be achieved. Performance is ideal for HDTV applications. The NSC demonstration board can be used to evaluate the LM126X/2426 combination in a TV. NSC DEMONSTRATION BOARD Figure 16 shows the routing and component placement on the NSC LM126X/2426 demonstration board. This board provides a good example of a layout that can be used as a guide for future layouts. Note the location of the following components: C19 — VCC bypass capacitor, located very close to pin 2 and ground pins C20 — VBB bypass capacitor, located close to pin 11 and ground C46, C48 — VCC bypass capacitors, near LM2426TE and VCC clamp diodes. Very important for arc protection. The routing of the LM2426TE outputs to the CRT is very critical to achieving optimum performance. Figure 17 shows the routing and component placement from pin 10 (V1OUT) of the LM2426TE to the blue cathode. Note that the components are placed so that they almost line up from the output pin of the LM2426TE to the blue cathode pin of the CRT connector. This is done to minimize the length of the video path between these two components. Note also that D8, D9, R24 and D6 are placed to minimize the size of the video nodes that they are attached to. This minimizes parasitic capacitance in the video path and also enhances the effectiveness of the protection diodes. The anode of protection diode D8 is connected directly to a section of the the ground plane that has a short and direct path to the LM2426TE ground pins. The cathode of D9 is connected to VCC very close to decoupling capacitor C48 (see Figure 17) which is connected to the same section of the ground plane as D8. The diode placement and routing is very important for minimizing the voltage stress on the LM2426TE during an arcover event. Lastly, notice that S3 is placed very close to the blue cathode and is tied directly to CRT ground. This demonstration board uses large PCB holes to accommodate socket pins, which function to allow for multiple insertions of the LM2426TE in a convenient manner. To benefit from the enhanced LM2426TE package with thin leads, the device should be secured in small PCB holes to optimize the metal-to-metal spacing between the leads. www.national.com 8 LM2426TE Application Hints (Continued) 20066411 FIGURE 14. LM126X/LM242X/LM248X Demonstration Board Schematic 9 www.national.com LM2426TE Application Hints (Continued) 20066412 FIGURE 15. LM126X/LM242X/LM248X Demonstration Board Schematic (continued) www.national.com 10 LM2426TE Application Hints (Continued) 20066413 FIGURE 16. LM126X/LM242X/LM248X Demonstration Board Layout 20066414 FIGURE 17. Trace Routing and Component Placement for Blue Channel Output 11 www.national.com LM2426TE Monolithic Triple Channel 30 MHz DTV Driver Physical Dimensions inches (millimeters) unless otherwise noted CONTROLLING DIMENSION IS INCH VALUES IN [ ] ARE MILLIMETERS NS Package Number TE11A Order Number LM2426TE 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. 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. 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