PDF LM2460 Data sheet ( Hoja de datos )

Número de pieza	LM2460
Descripción	Monolithic Triple Channel High Swing CRT Driver
Fabricantes	National Semiconductor
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No Preview Available ! September 2003 LM2460 Monolithic Triple Channel High Swing CRT Driver General Description The LM2460 is an integrated high voltage CRT driver circuit designed for use in high brightness monitor applications. The IC contains three high input impedance, wide band amplifi- ers which directly drive the RGB cathodes of a CRT. Each channel has its gain internally set to −30 and can drive CRT capacitive loads as well as resistive loads present in other application, limited only by the package’s power dissipation. The IC is packaged in an industry standard 9 lead TO-220 molded plastic package. n Stable with 0–20 pF capacitive loads and inductive peaking networks n Convenient TO-220 staggered lead package style n Matched to LM126X/3X/4X pre-amplifier families Applications n High brightness CRT monitors Features n 0V to 5V input range n Capable of up to a 70 Vp-p output swing Schematic and Connection Diagrams 20082601 FIGURE 1. Simplified Schematic Diagram (One Channel) Top View 20082602 Order Number LM2460TA See NS Package Number TA09A © 2003 National Semiconductor Corporation DS200826 www.national.com 1 page Theory of Operation The LM2460 is a high voltage monolithic three channel CRT driver with a higher output swing suitable for driving the new high brightness CRTs. The LM2460 operates with 120V and 8V power supplies. The part is housed in the industry stan- dard 9-lead TO-220 molded plastic power package. The circuit diagram of the LM2460 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 −32. 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 2 shows a typical test circuit for evaluation of the LM2460. This circuit is designed to allow testing of the LM2460 in a 50Ω environment without the use of an expen- sive FET probe. In this test circuit, two low inductance resis- tors in series totaling 4.95 kΩ form a 200:1 wideband, low capacitance probe when connected to a 50Ω coaxial cable and a 50Ω load (such as a 50Ω oscilloscope input). The input signal from the generator is AC coupled to the base of Q5. VBIAS is used to adjust the DC level of the output. 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 LM2460 performance is targeted for the 15" and 17" market with resolutions up to 1024 x 7684 and 75 Hz refresh rate. 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 LM2460. If another member of the LM246X family is used, please refer to its datasheet. POWER SUPPLY BYPASS Since the LM2460 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 0.1 µF capacitor should be connected from the supply pin, VCC, to ground, as close to the supply and ground pins as is practical. Additionally, a 22 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 LM2460’s supply and ground pins. A 0.1 µF capacitor should be con- nected from the bias pin (VBB) 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 voltage, but to a value that is much higher than allowable on the LM2460. This fast, high voltage, high energy pulse can damage the LM2460 output stage. The application circuit shown in Figure 9 is designed to help clamp the voltage at the output of the LM2460 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 VCC 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 LM2460 ground. This will significantly reduce the high frequency voltage transients that the LM2460 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 LM2460 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 LM2460 would be subjected to. 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 compo- nents shown in Figure 9. 20082609 FIGURE 9. One Channel of the LM2460 with the Recommended Application Circuit OPTIMIZING TRANSIENT RESPONSE Referring to Figure 9, 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, perferably above 300 MHz. Ferrite core inductors from J.W. Miller Magnetics (part # 78FR--K) were used for optimizing the performance of the device in the NSC application board. The values shown in Figure 9 can be used as a good starting point for the evaluation of the LM2460. Using a variable 5 www.national.com 5 Page Physical Dimensions inches (millimeters) unless otherwise noted NS Package Number TA09A Order Number LM2460TA LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. 2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. National Semiconductor Americas Customer Support Center Email: [email protected] Tel: 1-800-272-9959 www.national.com National Semiconductor Europe Customer Support Center Fax: +49 (0) 180-530 85 86 Email: [email protected] Deutsch Tel: +49 (0) 69 9508 6208 English Tel: +44 (0) 870 24 0 2171 Français Tel: +33 (0) 1 41 91 8790 National Semiconductor Asia Pacific Customer Support Center Email: [email protected] National Semiconductor Japan Customer Support Center Fax: 81-3-5639-7507 Email: [email protected] Tel: 81-3-5639-7560 National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications. 11 Page

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