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LM2423TE Scheda tecnica(PDF) 7 Page - National Semiconductor (TI) |
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LM2423TE Scheda tecnica(HTML) 7 Page - National Semiconductor (TI) |
7 / 11 page Application Hints (Continued) Figure 10 shows the maximum power dissipation of the LM2423 vs. Frequency when all three channels of the device are driving into a 10 pF load with a 130V P-P alternating one pixel on, one pixel off. 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 (190V in this case). This graph gives the designer the information needed to determine the heat sink requirement for his application. The designer should note that if the load capacitance is in- creased the AC component of the total power dissipation would also increase. The LM2423 case temperature must be maintained below 110˚C. If the maximum expected ambient temperature is 60˚C and the maximum power dissipation is 17W (from Figure 10, 15 MHz) then a maximum heat sink thermal resistance can be calculated: This example assumes a capacitive load of 10 pF 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. OPTIMIZING TRANSIENT RESPONSE Referring to Figure 13, there are three components (R1, R2 and L1) that can be adjusted to optimize the transient re- sponse of the application circuit. Increasing the values of R1 and R2 will slow the circuit down while decreasing over- shoot. Increasing the value of L1 will speed up the circuit as well as increase overshoot. It is very important to use induc- tors 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 13 can be used as a good starting point for the evaluation of the LM2423. Using a variable resistor for R1 will simplify finding the value needed for optimum performance in a given application. Once the opti- mum value is determined the variable resistor can be re- placed with a fixed value. Due to arc over considerations it is recommended that the values shown in Figure 13 not be changed by a large amount. Figure 12 shows the typical cathode pulse response with an output swing of 130V PP inside a modified Sony TV using a Sony pre-amp. PC BOARD LAYOUT CONSIDERATIONS For optimum performance, an adequate ground plane, iso- lation between channels, good supply bypassing and mini- mizing unwanted feedback are necessary. Also, the length of the signal traces from the signal inputs to the LM2423 and from the LM2423 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 LM2423 is shown in the sche- matic for the NSC demonstration board in Figure 14. Used in conjunction with an LM1246 preamplifier, a complete video channel from input to CRT cathode can be achieved. Perfor- mance is ideal for DTV applications. The NSC demonstration board can be used to evaluate the LM2423 combination with the LM2485 and the LM1246 in a TV. It is important that the TV designer use component values for the driver output stage close to the values shown in Figure 14. These values have been selected to protect the LM2423 from arc over. Diodes D1, D2, D4, and D7–D9 must also be used for proper arc over protection. The NSC demonstration board can be used to evaluate the LM2423 in a TV. If the NSC demonstration board is used for evaluating the LM2423, then U3, the +5V voltage regulator may be used, eliminating the need to route +5V to the neck board for the LM1246. NSC DEMONSTRATION BOARD Figure 15 shows the routing and component placement on the NSC LM2423/LM1246/LM2486 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—V CC bypass capacitor, located very close to pin 2 and ground pins • C20—V BB bypass capacitor, located close to pin 11 and ground • C46, C48 — V CC bypass capacitors, near LM2423 and V CC clamp diodes. Very important for arc protection. The routing of the LM2423 outputs to the CRT is very critical to achieving optimum performance. Figure 16 shows the routing and component placement from pin 10 (V OUT1)ofthe LM2423 to the blue cathode. Note that the components are placed so that they almost line up from the output pin of the LM2423 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 pro- tection diodes. The anode of protection diode D8 is con- nected directly to a section of the ground plane that has a short and direct path to the LM2423 ground pins. The cath- ode of D9 is connected to V CC very close to decoupling capacitor C7 which is connected to the same area of the ground trace as D8. The diode placement and routing is very important for minimizing the voltage stress on the LM2423 during an arc over event. This demonstration board uses large PCB holes to accom- modate socket pins, which function to allow for multiple insertions of the LM2423 in a convenient manner. To benefit from the enhanced LM2423 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 7 |
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