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LM2421 데이터시트(PDF) 6 Page - National Semiconductor (TI) |
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LM2421 데이터시트(HTML) 6 Page - National Semiconductor (TI) |
6 / 13 page Theory of Operation The LM2421 is a high voltage monolithic three channel CRT driver suitable for HDTV applications. The LM2421 operates with 145V and 8V power supplies. The part is housed in the industry standard 11-lead TO-220 molded plastic power package. The circuit diagram of the LM2421 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 −42. Emitter followers Q3 and Q4 isolate 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 biasing 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 LM2421. This circuit is designed to allow testing of the LM2421 in a 50 Ω environment without the use of an expen- sive FET probe. In this test circuit, the two 4.99k Ω resistors form a 400:1 wideband, low capacitance probe when con- nected to a 50 Ω coaxial cable and a 50Ω load (such as a 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 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 purposes. When working with high bandwidth circuits, good layout practices are also critical to achieving maximum per- formance. IMPORTANT INFORMATION The LM2421 performance is targeted for the HDTV market. The application circuits shown in this document to optimize performance and to protect against damage from CRT ar- cover are designed specifically for the LM2421. If another member of the LM242X family is used, please refer to its datasheet. POWER SUPPLY BYPASS Since the LM2421 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. 0.1 µF capacitors should be connected from the supply pins, V CC and VBB, to ground, as close to the LM2421 as is practical. Additionally, a 47 µF or larger electrolytic capacitor should be connected from both supply pins to ground reasonably close to the LM2421. ARC PROTECTION During normal CRT operation, internal arcing may occasion- ally occur. Spark gaps, in the range of 300V, 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 LM2421. This fast, high voltage, high energy pulse can damage the LM2421 output stage. The application circuit shown in Figure 10 is designed to help clamp the voltage at the output of the LM2421 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. 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 capaci- tor (C3 in Figure 10). The ground connection of D2 and the decoupling capacitor should be very close to the LM2421 ground. This will significantly reduce the high frequency voltage transients that the LM2421 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 LM2421 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 LM2421 would be subjected to. The inductor will not only help protect the device but it will also help minimize 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. www.national.com 6 |
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