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LM2403 数据表(PDF) 7 Page - Texas Instruments |
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LM2403 数据表(HTML) 7 Page - Texas Instruments |
7 / 15 page OBSOLETE LM2403 www.ti.com SNOS019B – AUGUST 1999 – REVISED APRIL 2013 OPTIMIZING TRANSIENT RESPONSE Referring to Figure 11, there are three components (R1, R2 and L1) that can be adjusted to optimize the transient response of the application circuit. Increasing the values of R1 and R2 will slow the circuit down while decreasing overshoot. Increasing the value of L1 will speed up the circuit as well as increase overshoot. It is very important to use inductors with very high self-resonant frequencies, preferably above 300 MHz. Ferrite core inductors from J.W. Miller Magnetics (part # 78FR12M) were used for optimizing the performance of the device in the TI application board. The values shown in Figure 11 can be used as a good starting point for the evaluation of the LM2403. The TI demo board also has a position open to add a resistor in parallel with L1. This resistor can be used to help control overshoot. Using variable resistors for R1 and the parallel resistor is a great way to help dial in the values needed for optimum performance in a given application. Pull-up Resistors Optimizing the performance of the LM2403 does require the use of pull-up resistors at the outputs of the CRT driver. These resistors are shown as R100, R101, and R102 in the schematic. If you have a demo board form TI please note that these resistors have been added on the back of the board since there is no PCB location for the pull-up resistors. Because of the improved performance with these resistors, all demo boards have been shipped with the added pull-up resistors. The LM2403 does have some crossover distortion, normal for any AB amplifier such as the LM2403. Adding the pull-up resistors does add more bias to Q3 (Figure 2) thus minimizing the crossover distortion. The LM2403 is normally used in high end monitors, so it is highly recommended that the 12k pull-up resistors be used in any design using the LM2403. Selecting a 12k resistor provides the needed pull- up current and limits the worst case power dissipation to 1/4W (white level at 25V). In some applications pull-down resistors may be preferred. Using 12k resistors gives acceptable performance, but this will require the use of 1/2W resistors. Normally the power save mode establishes whether pull-up or pull- down resistors are preferred. If the setup of the power save mode in the monitor gives a low output at the LM2403, then the pull-down resistors would be preferred, if the 80V supply is still turned on. 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 circuit (R1, R2 and L1 in Figure 11) should be chosen based on the nominal load capacitance. Once this is done the performance of the design can be checked by varying the load based on what the expected variation will be. For example, suppose you needed to drive a 10 pF (±20%) load with a 40Vp-p waveform. First, you would pick the values of R1, R2 and L1 that give the desired response with a 10 pF load. Then you would test the design when driving an 8 pF load and a 12 pF load. The table below summarizes the results from doing this exercise in a test board in the TI lab. The output signal swing was 40Vp-p from 65V to 25V. Parameter 8 pF 10 pF 12 pF Rise Time 4.1 4.2 4.3 Overshoot 1% 5% 10% Fall Time 4.4 4.6 4.7 Overshoot 1% 2% 5% The example above clearly demonstrates the importance of having a good estimate of the range of the load capacitance. Effect of Offset Figure 8 shows the variation in rise and fall times when the output offset of the device is varied from 30 VDC to 50 VDC. The rise time shows about twice as much variation as the fall time, however the maximum variation relative to the center data point (40 VDC) is less than 10%. Copyright © 1999–2013, Texas Instruments Incorporated Submit Documentation Feedback 7 Product Folder Links: LM2403 |
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类似说明 - LM2403 |
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