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LM2476 Fiches technique(PDF) 8 Page - National Semiconductor (TI) |
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LM2476 Fiches technique(HTML) 8 Page - National Semiconductor (TI) |
8 / 12 page Application Hints (Continued) will be increased. This increase in package stress will result in reduced contact area with the heat sink increasing die operating temperature and possible catastrophic IC failure. TYPICAL APPLICATION A typical application of the LM2476 is shown in Figure 14. Used in conjunction with an LM123X/4X preamplfier, a com- plete video channel from monitor input to CRT cathode can be achieved. Performance is ideal for 1024 x 768 resolution displays with pixel clock frequencies up to 95 MHz. Please see the next two sections below for hints on how to properly evaluate the LM123X/4X and LM2476 combination in a monitor. Figure 13 shows the typical cathode response for this application. The peaking component values used are shown in Figure 12 and Figure 14. PC BOARD LAYOUT CONSIDERATIONS For optimum performance, use single-point grounds sys- tems with adequate ground planes, isolate between chan- nels, apply good supply bypassing, and minimize unwanted feedback and parasitic capacitance. Also, the length of the video signal traces from the preamplifier to the LM2476 and from the LM2476 to the CRT cathodes 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 14 is the schematic for the NSC LM1276/3X/4X_2476 Demonstration PCB that can be used to evaluate the LM1276, LM123X, or LM124X preamp with the LM2476 in a monitor. Figure 15 shows the routing and component place- ment on the NSC Demonstration PCB. 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: • C26—V CC bypass capacitor, located very close to Pin 14 and GND Pins • C27—V BB bypass capacitors, located close to Pin 2 and GND Pins • C28, C30, C33 — V CC bypass capacitors, near LM2476 and V CC clamp diodes. These are very important for arc protection. The routing of the LM2476 driver outputs to the CRT is very critical to achieving optimum performance. Figure 16 shows the routing and component placement from Pin 17 of the LM2476 to the Red cathode. Note that the components are placed so that they almost line up from the output pin of the LM2476 to the Red cathode pin of the CRT connector. This is done to minimize the length of the video path between these two components for optimal performance and to mini- mize EMI. Note also that L4, D9, D13 and R23, R19, D10 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 D13 is con- nected directly to a section of the ground trace that has a short and direct path to CRT ground to shunt arcover current away from the LM2476. The cathode of D9 is connected to V CC very close to decoupling capacitor C30 (see Figure 16) which is connected to the same section of the ground trace as D13. The diode placement and routing is very important for minimizing the voltage stress on the LM2476 during an arcover event. R37 and C29 are placed very close to the clamp output on Pin 11. These components will help limit the current and voltage stress to the clamp output. Lastly, notice that S2 is placed very close to the Red cathode and is connected directly to CRT ground. 20121916 40V P-P (35V – 75V) FIGURE 13. Typical Cathode Response www.national.com 8 |
Numéro de pièce similaire - LM2476 |
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Description similaire - LM2476 |
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