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LM2470TA Scheda tecnica(PDF) 8 Page - National Semiconductor (TI) |
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LM2470TA Scheda tecnica(HTML) 8 Page - National Semiconductor (TI) |
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8 / 14 page  Application Hints (Continued) TABLE 1. LM2470 40V PP Cathode Response Channel t r/OS t f/OS Red 7.6ns / 8% 7.6ns / 7% Green 7.1ns / 7% 7.3ns / 6% Blue 7.2ns / 8% 7.1ns / 8% TABLE 2. LM2470 60V PP Cathode Response Channel t r/OS t f/OS Red 8.4ns / 7% 7.9ns / 6% Green 8.0ns / 6% 7.9ns / 4% Blue 8.2ns / 6% 7.4ns / 6% 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 LM2470 and from the LM2470 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 monitor 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. NSC DEMONSTRATION BOARD Figure 13 shows the routing and component placement on the NSC LM123X/246X demonstration board. The sche- matic of the board is shown in Figure 11 and Figure 12. This board provides a good example of a layout that can be used as a guide for future layouts. Note the location of the follow- ing components: • C19—V CC bypass capacitor, located very close to pin 4 and ground pins • C20—V BB bypass capacitors, located close to pin 8 and ground • C46, C47, C48 — V CC bypass capacitors, near LM2470 and V CC clamp diodes. Very important for arc protection. The routing of the LM2470 outputs to the CRT is very critical to achieving optimum performance. Figure 14 shows the routing and component placement from pin 1 of the LM2470 to the blue cathode. Note that the components are placed so that they almost line up from the output pin of the LM2470 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 at- tached 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 LM2470 ground pins. The cathode of D9 is connected to V CC very close to decoupling capacitor C19 (see Figure 14) 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 LM2470 during an arcover event. Lastly, notice that S3 is placed very close to the blue cathode and is tied directly to CRT ground. 20087116 FIGURE 10. Red Cathode Response www.national.com 8 |
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