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LM2459 Datenblatt(PDF) 8 Page - National Semiconductor (TI) |
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LM2459 Datenblatt(HTML) 8 Page - National Semiconductor (TI) |
8 / 11 page Application Hints (Continued) 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 14 can be used as a good starting point for the evaluation of the LM2459. 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. Figure 12 shows the typical cathode pulse response with an output swing of 110V PP using a LM1269 preamplifier. The transient response can also be improved by adding a capacitor from pin 6 to the ground plane used by the LM2459. A small capacitor, such as a 22pF ceramic, will notably improve the fall time and only increase the over- shoots and settling times slightly. Note that increasing the capacitance beyond 22pF will only improve the fall time marginally, but will increase the settling times significantly. This option allows for better matching between the rise and fall time. 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 preamplifier to the LM2459 and from the LM2459 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 LM2459 is shown in the sche- matic for the NSC demonstration board in Figure 14. Used in conjunction with an LM126X 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 LM2459 in a TV. NSC DEMONSTRATION BOARD Figure 15 shows the routing and component placement on the NSC LM2459 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 compo- nents: • C7203 — V CC bypass capacitor, located very close to pin 1 and ground pins • C7205 — V BB bypass capacitor, located close to pin 4 and ground • C7207 — V CC bypass capacitor, near LM2459 and VCC clamp diodes. Very important for arc protection. The routing of the LM2459 output to the CRT is very critical to achieving optimum performance. Figure 16 shows the routing and component placement from pin 2 (V OUT)ofthe LM2459 to the cathode. Note that the components are placed so that they almost line up from the output pin of the LM2459 to the cathode pin of the CRT connector. This is done to minimize the length of the video path between these two components. Note also that D7204, D7205, R7251 and D7214 are placed to minimize the size of the video nodes that they are attached to. This minimizes parasitic capaci- tance in the video path and also enhances the effectiveness of the protection diodes. The anode of protection diode D7204 is connected directly to a section of the the ground plane that has a short and direct path to the LM2459 ground pins. The cathode of D7205 is connected to V CC very close to decoupling capacitor C7207 (see Figure 16) which is connected to the same section of the ground plane as D7204. The diode placement and routing is very important for minimizing the voltage stress on the LM2459 during an arcover event. Lastly, notice that SG7202 is placed very close to the cathode and is tied directly to CRT ground. This demonstration board uses medium-sized PCB holes to accommodate socket pins, which function to allow for mul- tiple insertions of the LM2459 in a convenient manner. To benefit from the enhanced LM2459 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 |
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