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LM2460TA データシート(PDF) 6 Page - National Semiconductor (TI) |
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LM2460TA データシート(HTML) 6 Page - National Semiconductor (TI) |
6 / 11 page Application Hints (Continued) 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. Effect of Load Capacitance Figure 8 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. The rise time increased about 0.8 ns for an increase of 1 pF in the load capacitance. The fall time does remain almost the same as the load capacitance is increased. Effect of Offset Figure 7 shows the variation in rise and fall times when the output offset of the device is varied from 70 to 80 V DC. The rise time has very little increase over its fastest point near 75V. The fall time becomes a little faster as the offset voltage increases. THERMAL CONSIDERATIONS Figure 4 shows the performance of the LM2460 in the test circuit shown in Figure 2 as a function of case temperature. The figure shows that the rise time of the LM2460 increases by approximately 13% as the case temperature increases from 30˚C to 100˚C. This corresponds to a speed degrada- tion of 2.0% for every 10˚C rise in case of temperature. The fall time has almost no change as the case temperature increases. Figure 6 shows the maximum power dissipation of the LM2460 vs Frequency when all three channels of the device are driving an 8 pF load with a 60 V p-p 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 (105V in this case). This graph gives the designer the information needed to determine the heat sink requirement for his appli- cation. The designer should note that if the load capacitance is increased the AC component of the total power dissipation would also increase. The LM2460 case temperature must be maintained below 100˚C. If the maximum expected ambient temperature is 70˚C and the maximum power dissipation is 11W (from Figure 6, 70 MHz bandwidth) then a maximum heat sink thermal resistance can be calculated: This example assumes a capacitive load of 8 pF and no resistive load. TYPICAL APPLICATION A typical application of the LM2460 is shown in Figure 10 and Figure 11. Used in conjunction with a LM1267 pre-amp and a LM2479 bias clamp, a complete 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 80 MHz. Figure 10 and Figure 11 are the schematic for the NSC demonstration board that can be used to evaluate the LM1267/2460/2479 combination in a monitor. 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 LM2460 and from the LM2460 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 12 shows the routing and component placement on the NSC LM126X/246X/LM2479/80 demonstration board. The schematic of the board is shown in Figure 10 and Figure 11. 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 ground pins • C17, C20 —V BB bypass capacitors, located close to pin 8 and ground • C46, C47, C48 —V CC bypass capacitors, near LM2460 and V CC clamp diodes. Very important for arc protection. The routing of the LM2460 outputs to the CRT is very critical to achieving optimum performance. Figure 13 shows the routing and component placement from pin 3 of the LM2460 to the blue cathode. The blue video path from the LM2460 output is shown by the darker traces. Note that the compo- nents are placed so that they almost line up from the output pin of the LM2460 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 effec- tiveness of the protection diodes. The anode of protection diode D8 is connected directly to a section of the ground plane that has a short and direct path to the LM2460 ground pins. The cathode of D9 is connected to V CC very close to decoupling capacitor C48 (see 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 LM2460 during an arc over event. Lastly, notice that S3 is placed very close to the blue cathode and is tied directly to CRT ground. www.national.com 6 |
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