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LM2445 Datenblatt(PDF) 7 Page - National Semiconductor (TI) |
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LM2445 Datenblatt(HTML) 7 Page - National Semiconductor (TI) |
7 / 14 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 and Figure 12 can be used as a good starting point for the evaluation of the LM2445. Using variable resistors for R1 and the parallel resistor will simplify finding the values needed for optimum performance in a given application. Once the optimum values are determined the variable resistors 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 to 50 V DC. The rise time shows a maximum variation relative to the center data point (45 V DC) less than 5%. The fall time shows a variation of less than 8% relative to the center data point. THERMAL CONSIDERATIONS Figure 4 shows the performance of the LM2445 in the test circuit shown in Figure 2 as a function of case temperature. The figure shows that the rise time of the LM2445 increases by approximately 10% as the case temperature increases from 50˚C to 100˚C. This corresponds to a speed degrada- tion of 2% for every 10˚C rise in case temperature. The fall time increases by approximately 7% as the case tempera- ture increases from 50˚C to 100˚C. Figure 6 shows the maximum power dissipation of the LM2445 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 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 LM2445 case temperature must be maintained below 100˚C. If the maximum expected ambient temperature inside the monitor is 70˚C and the power dissipation is 4.4W (from Figure 6, 50 MHz max. video frequency), 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 LM2445 is shown in Figure 11 and Figure 12. Used in conjunction with an LM1267, 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 95 MHz. Figure 11 and Figure 12 are the schematic for the NSC demonstration board that can be used to evaluate the LM1267/2466 com- bination in a monitor, and Figure 10 shows the typical re- sponse at the red cathode for this application. The input video rise time is 3.2ns, and the peaking component values are those recommended in Figure 12. Table 1 shows the typical cathode response of all three channels. 20063710 FIGURE 9. One Channel of the LM2445 with the Recommended Application Circuit www.national.com 7 |
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Ähnliche Beschreibung - LM2445 |
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