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LM2413T Fiches technique(PDF) 5 Page - National Semiconductor (TI) |
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LM2413T Fiches technique(HTML) 5 Page - National Semiconductor (TI) |
5 / 11 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. The values shown in Figure 9 can be used as a good starting point for the evaluation of the LM2413. Effect of Load Capacitance The output rise and fall times as well as overshoot will vary as the load capacitance varies. The values of the output cir- cuit (R1, R2 and L1 in Figure 9) should be chosen based on the nominal load capacitance. Once this is done the perfor- mance of the design can be checked by varying the load based on what the expected variation will be during produc- tion. Effect of Offset Figure 5 shows the variation in rise and fall times when the output offset of the device is varied from 35 to 55 VDC. The rise and fall times show about the same overall variation. The slightly faster rise and fall times are fastest near the cen- ter point of 45V, making this the optimum operating point. At the low and high output offset range, the characteristic of rise/fall time is slower due to the saturation of Q3 and Q4. The recovery time of the output transistors takes longer com- ing out of saturation thus slows down the rise and fall times. THERMAL CONSIDERATIONS Figure 4 shows the performance of the LM2413 in the test circuit shown in Figure 2 as a function of case temperature. Figure 4 shows that both the rise and fall times of the LM2413 become slightly longer as the case temperature in- creases from 40˚C to 100˚C. Please note that the LM2413 is never to be operated over a case temperature of 100˚C. In addition to exceeding the safe operating temperature, the rise and fall times will typically exceed 3.7/4.4 ns. Figure 6 shows that total power dissipation of the LM2413 vs. Frequency when all three channels of the device are driv- ing an 8 pF load. Typically the active time is about 72% of the total time for one frame. Worst-case power dissipation is when a one on, one off pixel is displayed over the active time of the video input. This is the condition used to measure the total power dissipation of the LM2413 at different input fre- quencies. Figure 6 gives all the information a monitor de- signed normally needs for worst case power dissipation. However, if the designer wants to calculate the power dissi- pation for an active time different from 72%, this can be done using the information in Figure 14. The recommended input black level voltage is 1.9V. From Figure 14, if a 1.9V input is used for the black level, then power dissipation during the in- active video time is 1.95W. This includes both the 80V and 12V supplies. If the monitor designer chooses to calculate the power dissi- pation for the LM2413 using an active video time different from 72%, then he needs to use the following steps when us- ing a 1.9V input black level: 1. Multiply the black level power dissipation, 1.95W, by 0.28, the result is 0.6W. 2. Choose the maximum frequency to be used. A typical application would use 90 MHz, or a 180 MHz pixel clock. The power dissipation is 12.4W. 3. Subtract the 0.6W from the power dissipation from Fig- ure 6. For 100 MHz this would be 12.4 − 0.6 = 11.8W. 4. Divide the result from step 3 by 0.72. For 90 MHz, the re- sult is 16.4W 5. Multiply the result in 4 by the new active time percent- age. 6. Multiply 1.95W by the new inactive time. 7. Add together the results of steps 5 and 6. This is the ex- pected power dissipation for the LM2413 in the design- er’s application. The LM2413 case temperature must be maintained below 100˚C. If the maximum expected ambient temperature is 70˚C and the maximum power dissipation is 12.2W ( Figure 6) then a maximum heat sink thermal resistance can be cal- culated: This example assumes a capacitive load of 8 pF and no re- sistive load. TYPICAL APPLICATION A typical application of the LM2413 is shown in Figure 10. Used in conjunction with an LM1283, a complete video chan- nel from monitor input to CRT cathode can be achieved. Per- formance is excellent for resolutions up to 1600 x 1200 and pixel clock frequencies at 180 MHz. Figure 10 is the sche- matic for the NSC demonstration board that can be used to evaluate the LM1283/2413 combination in a monitor. DS101275-10 FIGURE 9. One Channel of the LM2413 with the Recommended Arc Protection Circuit DS101275-11 www.national.com 5 |
Numéro de pièce similaire - LM2413T |
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Description similaire - LM2413T |
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