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LM2460 Arkusz danych(PDF) 6 Page - Texas Instruments |
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LM2460 Arkusz danych(HTML) 6 Page - Texas Instruments |
6 / 13 page OBSOLETE LM2460 SNOSA92B – JUNE 2002 – REVISED APRIL 2013 www.ti.com ARC PROTECTION During normal CRT operation, internal arcing may occasionally occur. Spark gaps, in the range of 200V, connected from the CRT cathodes to CRT ground will limit the maximum voltage, but to a value that is much higher than allowable on the LM2460. This fast, high voltage, high energy pulse can damage the LM2460 output stage. The application circuit shown in Figure 10 is designed to help clamp the voltage at the output of the LM2460 to a safe level. The clamp diodes, D1 and D2, should have a fast transient response, high peak current rating, low series impedance and low shunt capacitance. FDH400 or equivalent diodes are recommended. Do not use 1N4148 diodes for the clamp diodes. D1 and D2 should have short, low impedance connections to VCC and ground respectively. The cathode of D1 should be located very close to a separately decoupled bypass capacitor (C3 in Figure 10). The ground connection of D2 and the decoupling capacitor should be very close to the LM2460 ground. This will significantly reduce the high frequency voltage transients that the LM2460 would be subjected to during an arcover condition. Resistor R2 limits the arcover current that is seen by the diodes while R1 limits the current into the LM2460 as well as the voltage stress at the outputs of the device. R2 should be a 1/2W solid carbon type resistor. R1 can be a 1/4W metal or carbon film type resistor. Having large value resistors for R1 and R2 would be desirable, but this has the effect of increasing rise and fall times. Inductor L1 is critical to reduce the initial high frequency voltage levels that the LM2460 would be subjected to. The inductor will not only help protect the device but it will also help optimize rise and fall times as well as minimize EMI. For proper arc protection, it is important to not omit any of the arc protection components shown in Figure 10. Figure 10. One Channel of the LM2460 with the Recommended Application Circuit OPTIMIZING TRANSIENT RESPONSE Referring to Figure 10, 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, perferably 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 TI application board. The values shown in Figure 10 can be used as a good starting point for the evaluation of the LM2460. 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. Effect of Load Capacitance Figure 9 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. The rise time increased about 0.8 ns for an increase of 1 pF in the load capacitance. The fall time does remain almost the same as the load capacitance is increased. Effect of Offset Figure 8 shows the variation in rise and fall times when the output offset of the device is varied from 70 to 80 VDC. The rise time has very little increase over its fastest point near 75V. The fall time becomes a little faster as the offset voltage increases. 6 Submit Documentation Feedback Copyright © 2002–2013, Texas Instruments Incorporated Product Folder Links: LM2460 |
Podobny numer części - LM2460 |
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Podobny opis - LM2460 |
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