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LM2407 Arkusz danych(PDF) 4 Page - National Semiconductor (TI) |
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LM2407 Arkusz danych(HTML) 4 Page - National Semiconductor (TI) |
4 / 9 page Theory of Operation The LM2407 is a high voltage monolithic three channel CRT driver suitable for high resolution display applications. The LM2407 operates using 80V and 12V power supplies. The part is housed in the industry standard 11-lead TO-220 molded plastic power package. The circuit diagram of the LM2407 is shown in Figure 1.A 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 -14. Emitter followers Q3 and Q4 iso- late the high output impedance of the cascode stage from the capacitance of the CRT cathode which decreases the sensitivity of the device to load capacitance. Q6 provides bi- asing to the output emitter follower stage to reduce cross- over distortion at low signal levels. Figure 2 shows a typical test circuit for evaluation of the LM2407. This circuit is designed to allow testing of the LM2407 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 totaling 4.95 k Ω form a 100:1 wideband, low capacitance probe when connected to a 50 Ω coaxial cable anda50 Ω load (such as a 50Ω oscilloscope input). The in- put signal from the generator is ac coupled to the base of Q1. 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 follow- ing information is provided in order to support this commit- ment. 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 may 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 pur- poses. When working with high bandwidth circuits, good lay- out practices are also critical to achieving maximum perfor- mance. IMPORTANT INFORMATION The LM2407 performance is targeted for the XGA resolution market (1024 x 768, 85 Hz refresh). It is not designed to be a direct replacement for the LM2405 or LM2406. The appli- cation circuits required to optimize performance and to pro- tect against damage from CRT arcover are different for each part. The application section in this document provides infor- mation for the LM2407. Please refer to the LM2405 and LM2406 data sheets for specific application information on each of those devices. POWER SUPPLY BYPASS Since the LM2407 is a high bandwidth amplifier, proper power supply bypassing is critical for optimum performance. Improper power supply bypassing can result in large over- shoot, ringing and oscillation. A 0.01 µF 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 10 µF to 100 µF electrolytic capacitor should be connected from the supply pin to ground. The electrolytic capacitor should also be placed reasonably close to the LM2407’s supply and ground pins. A 0.1 µF capacitor should be connected from the bias pin, V BB, to 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 volt- age, but to a value that is much higher than allowable on the LM2407. This fast, high voltage, high energy pulse can dam- age the LM2407 output stage. The application circuit shown in Figure 9 is designed to help clamp the voltage at the out- put of the LM2407 to a safe level. The clamp diodes should have a fast transient response, high peak current rating, low series impedance and low shunt capacitance. FDH400 or equivalent diodes are recommended. 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 9). The ground connection of the diode and the decoupling ca- pacitor should be very close to the LM2407 ground. This will significantly reduce the high frequency voltage transients that the LM2407 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 LM2407 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. Inductor L1 is critical to reduce the initial high frequency voltage levels that the LM2407 would be subjected to. Having large value resistors for R1 and R2 would be desirable, but this has the effect of increasing rise and fall times. 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 9. DS100093-10 FIGURE 9. One Channel of the LM2407 with the Recommended Arc Protection Circuit www.national.com 4 |
Podobny numer części - LM2407 |
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Podobny opis - LM2407 |
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