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LM2469 датащи(PDF) 4 Page - National Semiconductor (TI) |
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LM2469 датащи(HTML) 4 Page - National Semiconductor (TI) |
4 / 10 page THEORY OF OPERATION The LM2469 is a high voltage monolithic three channel CRT driver suitable for color monitor applications. The LM2469 operates with 80V and 12V power supplies. The part is housed in the industry standard 9-lead TO-220 molded plas- tic power package. The circuit diagram of the LM2469 is shown in Figure 1. The PNP emitter follower, Q5, provides input buffering. Q1 and Q2 form a fixed gain cascode amplifier with resistors R1 and R2 setting the gain at -20. Emitter followers Q3 and Q4 isolate the high output impedance of the amplifier from the capacitive load on the output of the amplifier, decreasing the sensitivity of the device to changes in load capacitance. Q6 provides biasing to the output emitter follower stage to re- duce crossover distortion at low signal levels. Figure 3 shows a typical test circuit for evaluation of the LM2469. This circuit is designed to allow testing of the LM2469 in a 50 Ω environment without the use of an expen- sive FET probe. In this test circuit, two low inductance resis- tors in series totalling 4.95K Ω form a 200:1 wideband, low capacitance probe when connected to a 50 Ω load (such as 50 Ω oscilloscope input). The input signal from the generator is AC coupled to the base of Q5. APPLICATION HINTS INTRODUCTION National Semiconductor (NSC) is committed to provide ap- plication information that assists our customers in obtaining the best performance possible from our products. The fol- lowing information is provided in order to support this com- mitment. The reader should be aware that the optimization of performance was done using a specific printed circuit board designed at NSC. Variations in performance can be realized due to physical changes in the printed circuit board and the application. Therefore, the designer should know that com- ponent value changes might be required in order to optimize performance in a given application. The values shown in this document can be used as a starting point for evaluation purposes. When working with high bandwidth circuits, good layout practices are always critical to achieving maximum performance. IMPORTANT INFORMATION The LM2469 performance is targeted for the VGA (640 x 480) to XGA (1024 x 768, 70Hz refresh) resolution market. It is designed to be a replacement for discrete CRT drivers. The application circuits shown in this document to optimize performance and to protect against damage from CRT arc- over are designed specifically for the LM2469. If another member of the LM246X family is used, please refer to its datasheet. POWER SUPPLY BYPASS Since the LM2469 is a wide bandwith amplifier, proper power supply bypassing is critical for optimum performance. Im- proper power supply bypassing can result in large over- shoot, ringing or oscillation. A 0.1uF capacitor should be connected from the supply pin, V CC, to ground, as close to the supply and ground pins as is practical. Additionally, a 10uF to 100uF electrolytic capacitor should be connected from the supply pin to ground. The electrolytic capacitor should also be placed reasonably close to the LM2469’s supply and ground pins. A 0.1uF capacitor should be con- nected from the bias pin (V bb) to the ground, as close as is practical to the part. ARC PROTECTION During normal CRT operation, internal arcing may occasion- ally 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 LM2469. This fast, high voltage, high energy pulse can damage the LM2469 output stage. The application circuit shown in Figure 4 is designed to help clamp the voltage at the output of the LM2469 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 capaci- tance. 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 V cc and ground respectively. The cathode of D1 should be located very close to a separately decoupled bypass capacitor (C3 in Figure 4). The ground connection of D2 and the decoupling capacitor should be very close to the LM2469 ground. This will significantly reduce the high frequency voltage transients that the LM2469 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 LM2469 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 has the effect of increasing rise and fall times. Inductor L1 is critical to reduce the initial high frequency voltage levels that the LM2469 would be subjected to. The inductor will not only help protect the device, but 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 compo- nents shown in Figure 4. OPTIMIZING TRANSIENT RESPONSE Referring to Figure 4, 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 # 78FR39K) were used for optimizing the performance of the device in the NSC application board. The values shown in Figure 4 can be used as a good starting point for the evaluation of the LM2469. Using variable resis- DS200006-10 FIGURE 4. One Video Channel of the LM2469 with the Recommended Arc Protection Circuit www.national.com 4 |
Аналогичный номер детали - LM2469 |
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Аналогичное описание - LM2469 |
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