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Zakładka z wyszukiwarką danych komponentów |
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Zakładka z wyszukiwarką danych komponentów |
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LM2445 Arkusz danych(PDF) 6 Page - National Semiconductor (TI) |
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LM2445 Arkusz danych(HTML) 6 Page - National Semiconductor (TI) |
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6 / 14 page  Theory of Operation The LM2445 is a high voltage monolithic three channel CRT driver suitable for high resolution display applications. The LM2445 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 LM2445 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 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 LM2445. This circuit is designed to allow testing of the LM2445 in a 50 Ω environment without the use of an expen- sive FET probe. In this test circuit, the two 2.49k Ω resistors form a 200:1 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 LM2445 performance is targeted for the VGA (640 x 480) to XGA (1024 x 768, 85 Hz refresh) resolution 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 LM2445. If another member of the LM246X family is used, please refer to its datasheet. POWER SUPPLY BYPASS Since the LM2445 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 LM2445 as is practical. Additionally, a 47 µF or larger electrolytic capacitor should be connected from both supply pins to ground reasonably close to the LM2445. 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 LM2445. This fast, high voltage, high energy pulse can damage the LM2445 output stage. The application circuit shown in Figure 9 is designed to help clamp the voltage at the output of the LM2445 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 9). The ground connection of D2 and the decoupling capacitor should be very close to the LM2445 ground. This will significantly reduce the high frequency voltage transients that the LM2445 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 LM2445 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 resis- tors 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 LM2445 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 compo- nents shown in Figure 9. www.national.com 6 |
Podobny numer części - LM2445 |
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Podobny opis - LM2445 |
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