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Moteur de recherche de fiches techniques de composants électroniques |
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Moteur de recherche de fiches techniques de composants électroniques |
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LM2451TB Fiches technique(PDF) 7 Page - National Semiconductor (TI) |
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LM2451TB Fiches technique(HTML) 7 Page - National Semiconductor (TI) |
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7 / 12 page  Application Hints (Continued) THERMAL CONSIDERATIONS Figure 9 shows the performance of the LM2451 in the test circuit shown in Figure 3 as a function of case temperature. The figure shows that the rise time of the LM2451 increases by about 7ns as the case temperature increases from 30˚C to 110˚C. Over the same case temperature range the fall time increased by about 9 ns. Figure 10 shows the maximum power dissipation of the LM2451 vs. Frequency when all three channels of the device are driving into a 10 pF load with a 130V P-P alternating one pixel on, one pixel off. Note that the frequency given in Figure 10 is half of the pixel frequency. The graph assumes an 80% active time (device operating at the specified fre- quency), which is typical in a TV application. The other 20% of the time the device is assumed to be sitting at the black level (190V in this case). A TV picture will not have frequency content over the whole picture exceeding 15 MHz. It is important to establish the worst case condition under normal viewing to give a realistic worst-case power dissipation for the LM2451. One test isa1to30MHz sine wave sweep over the active line. This would give a slightly lower power than taking the average of the power between 1 and 30 MHz. This average is 10.4 W. A sine wave will dissipate slightly less power, probably about an even 10W of power dissipa- tion. All of this information is critical for the designer to establish the heat sink requirement for his application. The designer should note that if the load capacitance is in- creased the AC component of the total power dissipation will also increase. The LM2451 case temperature must be maintained below 110˚C given the maximum power dissipation estimate of 10W. If the maximum expected ambient temperature is 60˚C and the maximum power dissipation is 10W then a maximum heat sink thermal resistance can be calculated: This example assumes a capacitive load of 10 pF and no resistive load. The designer should note that if the load capacitance is increased the AC component of the total power dissipation will also increase. If the worst case power dissipation is not 10W please refer to Figure 11 for the maximum case temperature for the expected maximum power dissipation. OPTIMIZING TRANSIENT RESPONSE Referring to Figure 13, 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 13 can be used as a good starting point for the evaluation of the LM2451. Using a variable resistor for R1 will simplify finding the value needed for optimum performance in a given application. Once the opti- mum value is determined the variable resistor can be re- placed 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 130V PP inside a modified production TV set using the LM1237 pre-amp. PC BOARD LAYOUT CONSIDERATIONS For optimum performance, an adequate ground plane, iso- lation between channels, good supply bypassing and mini- mizing unwanted feedback are necessary. Also, the length of the signal traces from the signal inputs to the LM2451 and from the LM2451 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”, National Semiconductor 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 LM2451 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 LM2451 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 LM2451 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 V IN = 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 LM2451 from arc over. Diodes D1,D8, D9, and D13–D15 must also be used for proper arc over protection. The NSC demonstra- tion board can be used to evaluate the LM2451 in a TV. www.national.com 7 |
Numéro de pièce similaire - LM2451TB |
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