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LM2471 Arkusz danych(PDF) 6 Page - National Semiconductor (TI) |
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LM2471 Arkusz danych(HTML) 6 Page - National Semiconductor (TI) |
6 / 10 page Application Hints (Continued) OPTIMIZING TRANSIENT RESPONSE Referring to Figure 9, 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 # 78FR--k) were used for optimizing the performance of the device in the NSC application board. The values shown in Figure 11 can be used as a good starting point for the evaluation of the LM2471. Using a variable 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. EFFECT OF OFFSET Figure 7 shows the variation in rise and fall times when the output offset of the device is varied from 50 to 60 V DC. The rise time shows a maximum variation relative to the center data point (55 V DC) of 15%. The fall time shows a variation of less than 4% relative to the center data point. THERMAL CONSIDERATIONS Figure 4 shows the performance of the LM2471 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 LM2471 increase by approximately 10% as the case temperature increases from 50˚C to 100˚C. This corresponds to a speed degradation of 2% for every 10˚C rise in case temperature. Figure 6 shows the maximum power dissipation of the LM2471 vs. Frequency when all three channels of the device are driving an 8 pF load with a 40 V 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 (75V 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. The LM2471 case temperature must be maintained below 100˚C. If the maximum expected ambient temperature is 70˚C and the maximum power dissipation is 8.45W (from Figure 6, 160 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 LM2471 is shown in Figure 11. Used in conjunction with an LM1262, a complete video channel from monitor input to CRT cathode can be achieved. Performance is ideal for 1600 x 1200 resolution displays with pixel clock frequencies up to 180 MHz. Please see the next two sections below for hints on how to properly evaluate the LM126X and LM2471 combination in a monitor. Figure 10 shows the typical cathode response for this application. The peaking component values used are shown in Figure 11. 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 LM2471 and from the LM2471 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. 20103710 FIGURE 9. One Channel of the LM2471 with the Recommended Application Circuit 20103716 40V P-P (35V–75V) FIGURE 10. Typical Cathode Response www.national.com 6 |
Podobny numer części - LM2471 |
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Podobny opis - LM2471 |
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