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LM2450 Datasheet(PDF) 8 Page - Texas Instruments |
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LM2450 Datasheet(HTML) 8 Page - Texas Instruments |
8 / 14 page OBSOLETE 15A§LM2450 SNOSAN6D – SEPTEMBER 2005 – REVISED APRIL 2013 www.ti.com OPTIMIZING TRANSIENT RESPONSE Referring to Figure 13, 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 # 78FR--K) were used for optimizing the performance of the device in the application board. The values shown in Figure 13 can be used as a good starting point for the evaluation of the LM2450. Using a variable resistor for R1 will simplify finding the value needed for optimum performance in a given application. Once the optimum value is determined the variable resistor can be replaced with a fixed value. Due to arc over considerations it is recommended that the values shown in Figure 13 not be changed by a large amount. Figure 12 shows the typical cathode pulse response with an output swing of 130VPP inside a modified production TV set using the LM1237 pre-amp. PC BOARD LAYOUT CONSIDERATIONS For optimum performance, an adequate ground plane, isolation between channels, good supply bypassing and minimizing unwanted feedback are necessary. Also, the length of the signal traces from the signal inputs to the LM2450 and from the LM2450 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”, 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 TV 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. TYPICAL APPLICATION A typical application of the LM2450 is shown in Figure 14. Used in conjunction with a pre-amp with a 1.2V black level output no buffer transistors are required to obtain the correct black level at the cathodes. If the pre-amp has a black level closer to 2V, then an NPN transistor should be used to drop the video black level voltage closer to 1.2V. When using only one NPN transistor as an emitter follower, a jumper needs to be added in each channel. In the red channel a jumper needs to be added between C7 and R25. With just one transistor neither of these components would be installed. In addition to the video inputs are the DAC inputs. These inputs are used to vary the LM2450 output black level by a DAC. in the past when a driver was used with a CMOS AVP there was not enough range on the video output to vary the black level. A clamp circuit had to be used in conjunction with the AVP and the driver. The DAC inputs of the LM2450 are driven in the same way the clamp circuit had been driven, eliminating the need for a clamp circuit. Figure 4 shows the variation in the black level as the DAC input voltage is changed. This is shown for both VIN = 1.2V and VIN = 2.1V. The neck board in Figure 14 has two transistors in each channel enabling this board to work with pre-amps with a black level output as high as 2.5V. Each transistor stage has a gain of −1. This setup still gives the two diode drop at the driver input; however, now additional peaking can be done on the video signal before reaching the driver inputs. Some popular AVPs do have a black level of 2.5V. For lower black levels either one or both transistors would not be used. It is important that the TV designer use component values for the driver output stage close to the values shown in Figure 14. These values have been selected to protect the LM2450 from arc over. Diodes D1,D8, D9, and D13–D15 must also be used for proper arc over protection. The demonstration board can be used to evaluate the LM2450 in a TV. 8 Submit Documentation Feedback Copyright © 2005–2013, Texas Instruments Incorporated Product Folder Links: 15A§LM2450 |
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