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LM2435T データシート(PDF) 5 Page - National Semiconductor (TI) |
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LM2435T データシート(HTML) 5 Page - National Semiconductor (TI) |
5 / 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 Mag- netics (part # 78FR22K) were used for optimizing the perfor- mance of the device in the NSC application board. The val- ues shown in Figure 9 can be used as a good starting point for the evaluation of the LM2435. Using a variable resistor for R1 will simplify finding the value needed for optimum per- formance in a given application. Once the optimum values are determined the variable resistor can be replaced with fixed values. 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 40 V DC to 50 VDC. The rise time shows a maximum variation relative to the cen- ter data point (45 V DC) of about 13%. The fall time shows a maximum variation of about 3% relative to the center data point. THERMAL CONSIDERATIONS Figure 4 shows the performance of the LM2435 in the test circuit shown in Figure 2 as a function of case temperature. The figure shows that the rise time of the LM2435 increases by approximately 12% as the case temperature increases from 50˚C to 100˚C. This corresponds to a speed degrada- tion of 2.4% for every 10˚C rise in case temperature. There is a negligible change in fall time vs. temperature in the test circuit. Figure 6 shows the maximum power dissipation of the LM2435 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 (65V in this case). This graph gives the designer the information needed to determine the heat sink requirement for the 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 LM2435 case temperature must be maintained below 100˚C. If the maximum expected ambient temperature is 70˚C and the maximum power dissipation is 8.7W (from Fig- ure 6, 72.5 MHz bandwidth) then a maximum heat sink ther- mal resistance can be calculated: This example assumes a capacitive load of 8 pF and no re- sistive load. TYPICAL APPLICATION A typical application of the LM2435 is shown in Figure 10. Used in conjunction with an LM1279, a complete video chan- nel from monitor input to CRT cathode can be achieved. Per- formance is ideal for 1280 x 1024 resolution displays with pixel clock frequencies up to 135 MHz. Figure 10 is the sche- matic for the NSC demonstration board that can be used to evaluate the LM1279/2435 combination in a monitor. PC BOARD LAYOUT CONSIDERATIONS For optimum performance, an adequate ground plane, isola- tion between channels, good supply bypassing and minimiz- ing unwanted feedback are necessary. Also, the length of the signal traces from the preamplifier to the LM2435 and from the LM2435 to the CRT cathode should be as short as pos- sible. 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 os- cillate in a monitor if feedback occurs around the video chan- nel through the chassis wiring. To prevent this, leads to the video amplifier input circuit should be shielded, and input cir- cuit wiring should be spaced as far as possible from output circuit wiring. It is very important that the tab of the heatsink is connected to PCB ground. The single ground pin does not provide an adequate return path at high frequencies. The ground con- nection can be made using the heatsink. The NSC LM1279 & LM243X (Nov. 1998, Rev. B) demo board, shown in Figure 11 and Figure 12, provides a good example of how this can be done. A Thermalloy 6698B heatsink is used in the demo DS101044-10 FIGURE 9. One Channel of the LM2435 with the Recommended Application Circuit www.national.com 5 |
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