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LM2457 Arkusz danych(PDF) 9 Page - National Semiconductor (TI) |
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LM2457 Arkusz danych(HTML) 9 Page - National Semiconductor (TI) |
9 / 14 page Application Hints (Continued) clock frequencies up to 95MHz. A sample of this design along with all necessary support hardware and materials can be obtained from your local sales office. 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 LM2457 and from the LM2457 to the CRT cathode should be as short as possible. Due to the high gain and bandwidth of the LM2457, 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. IMPORTANT LAYOUT CONSIDERATIONS The NSC reference design can be used directly or it can be used as a guide for future layouts. Note the location of the following components: • C48 —V CC bypass capacitor, located very close to pin 7 and the device ground. • C8 —V BB bypass capacitor, located close to pin 8 and ground. • C34,C35, C46 —V CC bypass capacitors, near LM2457 V CC clamp diodes. Very important for arc protection. Note the direct path from the capacitor ground to the LM2457 ground. • C25-27 — clamp output bypass caps. Very important for a stable DC clamp voltage and for protecting the clamp outputs from damage due to a CRT arc. The routing of the LM2457 video outputs to the CRT is very critical to achieving optimum performance. Figure 20 shows the routing and component placement from pin 1 of the LM2457 to the blue cathode. Note that the components are placed so that they almost line up from the output pin of the LM2457 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 D16, D17, R17 and D7 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 D17 is connected directly to a section of the ground plane that has a short and direct path to the LM2457 ground pins. The cathode of D16 is connected to V CC very close to decoupling capacitor C34, which is connected to the same section of the ground plane as D17. The diode placement and routing is very important for minimizing the voltage stress on the LM2457 video outputs during an arc over event. Lastly, notice that S1 is placed very close to the blue cathode and is tied directly to the ground under the CRT connector. www.national.com 9 |
Podobny numer części - LM2457 |
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Podobny opis - LM2457 |
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