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LM2468 Datenblatt(PDF) 5 Page - National Semiconductor (TI) |
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LM2468 Datenblatt(HTML) 5 Page - National Semiconductor (TI) |
5 / 9 page APPLICATION HINTS (Continued) resistors for R1 will simplify finding the values needed for optimum performance in a given application. Once the opti- mum value is determined, the variable resistors can be replaced with fixed values. Effect of Load Capacitance Figure 9 shows the effect of increased load capacitance on the speed of the device. This demonstrates the importance of knowing the load capacitance in the application. Effect of Offset Figure 7 shows the variation in rise and fall times when the output offset of the device is varied from 40VDC to 50VDC. The rise and fall times shows a maximum variation relative to the center data point (45VDC) of about 7%. THERMAL CONSIDERATIONS Figure 6 shows the performance of the LM2468 video am- plifiers in the test circuit shown in Figure 3 as a function of case temperature. The figure shows that the rise time of the LM2468 increases by approximately 9% as the case tem- perature increases from 30˚C to 100˚C. This corresponds to a speed degradation of 1.3% for every 10˚C rise in case temperature. The fall time degrades around 0.6% for every 10˚C in case temperature. Figure 5 shows the maximum power dissipation of the LM2468 vs. Frequency when all three channels of the device are driving an 8pF load with a 40 V p-p signal 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 monitor application. The other 28% of the time the device is assumed to be sitting at the black level (65V 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 increased, the AC component of the total power dissipation will also increase. The LM2468 case temperature must be maintained below 115˚C. If the maximum expected ambient temperature is 70˚C and the maximum power dissipation is 2.95W (from Figure 5, 50 MHz bandwith), then a maximum heat sink thermal resistance can be calculated: This example assumes a capacitive load of 8pF and no resistive load. TYPICAL APPLICATION A typical application of the LM2468 is shown in Figures 11, 12. Used in conjunction with an LM1269 coupled with an LM2479/2480 bias clamp, a complete video channel from monitor input to CRT cathode can be achieved. Performance is ideal for 1024x768 resolution displays with pixel clock frequencies up to 50 MHz. Figures 11, 12 are the schematic for the NSC demonstration board that can be used to evalu- ate the LM1269/2468 combination in a monitor. PC Board Layout Considerations For optimum performance, an adequate ground plane, iso- lation between channels, good supply bypassing and the minimization of unwanted feedback are necessary. Also, the length of the signal traces from the preamplifier to the LM2468 and from the LM2468 to the CRT cathode should be as short as possible. The following references are recom- mended: 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 bandwith, 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 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 LM126X/246X demonstration board. The sche- matic of the board is shown in Figures 11, 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 following components: • C16, C19 —V CC bypass capacitor, located very close to pin 4 and the ground plane near the device. • C20 —V BB bypass capacitors, located close to pin 8 and ground. • C46, C47, C48 —V CC bypass capacitors, near LM2468 V CC clamp diodes. Very important for arc protection. The routing of the LM2468 video outputs to the CRT is very critical to achieving optimum performance. Figure 13 shows the routing and component placement from pin 3 of the LM2468 to the blue cathode. Note that the components are placed so that they almost line up from the output pin of the LM2468 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 LM2468 ground pins. The cath- ode of D9 is connected to V CC very close to decoupling capacitor C48 (near the center of Figure 13), which is con- nected 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 LM2468 video outputs during an arc over event. Lastly, notice that S3 is placed very close to the blue cathode and is tied directly to the ground under the CRT connector. www.national.com 5 |
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