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LM2421 Folha de dados(PDF) 7 Page - National Semiconductor (TI) |
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LM2421 Folha de dados(HTML) 7 Page - National Semiconductor (TI) |
7 / 13 page Application Hints (Continued) OPTIMIZING TRANSIENT RESPONSE Referring to Figure 10, there are three components (R1, R2 and L1) that can be adjusted to optimize the transient re- sponse of the application circuit. Increasing the values of R1 and R2 will slow the circuit down while decreasing over- shoot. Increasing the value of L1 will speed up the circuit as well as increase overshoot. It is very important to use induc- tors with very high self-resonant frequencies, preferably above 300 MHz. Ferrite core inductors from J.W. Miller Magnetics (part # 78FR56k) were used for optimizing the performance of the device in the NSC application board. The values shown in Figure 11 and Figure 12 can be used as a good starting point for the evaluation of the LM2421. Using a variable resistor for R1 will simplify finding the value needed for optimum performance in a given application. Once the optimum value is determined, the variable resistor can be replaced 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. EFFECT OF OFFSET Figure 7 shows the variation in rise and fall times when the output offset of the device is varied from 75 to 85V DC. The rise time shows a maximum variation relative to the center data point (80V DC) of 4% . The fall time shows a variation of less than 5% relative to the center data point. THERMAL CONSIDERATIONS Figure 4 shows the performance of the LM2421 in the test circuit shown in Figure 2 as a function of case temperature. The figure shows that the rise and fall times of the LM2421 increase by approximately 5% as the case temperature in- creases from 50˚C to 70˚C. This corresponds to a speed degradation of 2.5% for every 10˚C rise in case temperature. Figure 6 shows the maximum power dissipation of the LM2421 vs. Frequency when all three channels of the device are driving an 8pF load with a 100V p-p alternating one pixel on, one pixel off signal. 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 (130V 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 will also increase. Figures 6 and 9 are used to design the heatsink for the LM2421. For example, if the maximum bandwith needed will be 40MHz (from Figure 6, 40MHz or Frequency = 20MHz), the power dissipated will be 14.5W. Figure 9 shows that the maximum allowed case temperature is 110˚C when 14.5W is dissipated. If the maximum expected ambient temperature is 70˚C, then a maximum heatsink 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 LM2421 is shown in Figure 11 and Figure 12. Used in conjunction with an LM126X, a complete video channel from input to CRT cathode can be achieved. Performance is ideal for HDTV applications. Figure 11 and Figure 12 are the schematic for the NSC demonstration board that can be used to evaluate the LM126X/2421 com- bination 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 LM2421 and from the LM2421 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. DS200233-10 FIGURE 10. One Channel of the LM2421 with the Recommended Arc Protection Circuit www.national.com 7 |
Nº de peça semelhante - LM2421 |
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Descrição semelhante - LM2421 |
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