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LM2439T Datenblatt(PDF) 4 Page - National Semiconductor (TI) |
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LM2439T Datenblatt(HTML) 4 Page - National Semiconductor (TI) |
4 / 9 page Theory of Operation The LM2439 is a high voltage monolithic three channel CRT driver suitable for high resolution display applications. The LM2439 operates with 80V and 12V power supplies. The part is housed in the industry standard 9-lead TO-220 molded plastic power package. The circuit diagram of the LM2439 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 −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 LM2439. This circuit is designed to allow testing of the LM2439 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 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 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 LM2439 performance is targeted for the VGA (640 x 480) to XGA (1024 x 768, 70 Hz 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 LM2439. If another member of the LM243X family is used, please refer to its datasheet. POWER SUPPLY BYPASS Since the LM2439 is a wide bandwidth amplifier, proper power supply bypassing is critical for optimum performance. Improper power supply bypassing can result in large over- shoot, ringing or 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 LM2439’s supply and ground pins. A 0.1 µF capacitor should be con- nected from the bias pin, V BB, to ground, as close as is prac- tical 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 LM2439. This fast, high voltage, high energy pulse can dam- age the LM2439 output stage. The application circuit shown in Figure 9 is designed to help clamp the voltage at the out- put of the LM2439 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 or equivalent 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 lo- cated very close to a separately decoupled bypass capacitor (C3 in Figure 9). The ground connection of D2 and the de- coupling capacitor should be very close to the LM2439 ground. This will significantly reduce the high frequency volt- age transients that the LM2439 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 LM2439 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 lev- els that the LM2439 would be subjected to. The inductor will not only help protect the device but it will also help maximize 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. DS100988-10 FIGURE 9. One Channel of the LM2439 with the Recommended Application Circuit www.national.com 4 |
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