PDF LM2438T Data sheet ( Hoja de datos )

Número de pieza	LM2438T
Descripción	Monolithic Triple 13.5 ns CRT Driver
Fabricantes	National Semiconductor
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No Preview Available ! August 1999 LM2438 Monolithic Triple 13.5 ns CRT Driver General Description The LM2438 is an integrated high voltage CRT driver circuit designed for use in color monitor applications. The IC con- tains three high input impedance, wide band amplifiers which directly drive the RGB cathodes of a CRT. Each chan- nel has its gain internally set to −14 and can drive CRT ca- pacitive loads as well as resistive loads present in other ap- plications, limited only by the package’s power dissipation. The IC is packaged in an industry standard 9-lead TO-220 molded plastic power package. See Thermal Considerations section. Features n Well matched with LM1279 video preamp n 0V to 5V input range n Stable with 0–20 pF capacitive loads and inductive peaking networks n Convenient TO-220 staggered lead package style n Standard LM243X Family Pinout which is designed for easy PCB layout Applications n 1024 x 768 displays up to 60 Hz refresh n Pixel clock frequencies up to 60 MHz n Monitors using video blanking Schematic and Connection Diagrams DS101043-1 FIGURE 1. Simplified Schematic Diagram (One Channel) DS101043-2 Note: Tab is at GND Top View Order Number LM2438T © 1999 National Semiconductor Corporation DS101043 www.national.com 1 page Theory of Operation The LM2438 is a high voltage monolithic three channel CRT driver suitable for high resolution display applications. The LM2438 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 LM2438 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 LM2438. This circuit is designed to allow testing of the LM2438 in a 50Ω environment without the use of an expen- sive FET probe. In this test circuit, the two 2.49 kΩ resistors form a 200:1 wideband, low capacitance probe when con- nected 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. 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 LM2438 performance is targeted for the XGA (1024 x 768, 60 Hz refresh) resolution market. The application cir- cuits shown in this document to optimize performance and to protect against damage from CRT arcover are designed spe- cifically for the LM2438. If another member of the LM243X family is used, please refer to its datasheet. POWER SUPPLY BYPASS Since the LM2438 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 con- nected from the supply pins, VCC and VBB, to ground, as close to the LM2438 as is practical. Additionally, a 47 µF or larger electrolytic capacitor should be connected from both supply pins to ground reasonably close to the LM2438. 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 LM2438. This fast, high voltage, high energy pulse can dam- age the LM2438 output stage. The application circuit shown in Figure 9 is designed to help clamp the voltage at the out- put of the LM2438 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 diodes for the clamp diodes. D1 and D2 should have short, low impedance connections to VCC and ground re- spectively. 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 LM2438 ground. This will signifi- cantly reduce the high frequency voltage transients that the LM2438 would be subjected to during an arcover condition. Resistor R2 limits the arcover current that is seen by the di- odes while R1 limits the current into the LM2438 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 increas- ing rise and fall times. Inductor L1 is critical to reduce the ini- tial high frequency voltage levels that the LM2438 would be subjected to. The inductor will not only help protect the de- vice 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 components shown in Figure 9. 5 www.national.com 5 Page Physical Dimensions inches (millimeters) unless otherwise noted CONTROLLING DIMENSION IS INCH VALUES IN [ ] ARE MILLIMETERS NS Package Number TA09A Order Number LM2438T 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 Corporation Americas Tel: 1-800-272-9959 Fax: 1-800-737-7018 Email: [email protected] www.national.com National Semiconductor Europe Fax: +49 (0) 1 80-530 85 86 Email: [email protected] Deutsch Tel: +49 (0) 1 80-530 85 85 English Tel: +49 (0) 1 80-532 78 32 Français Tel: +49 (0) 1 80-532 93 58 Italiano Tel: +49 (0) 1 80-534 16 80 National Semiconductor Asia Pacific Customer Response Group Tel: 65-2544466 Fax: 65-2504466 Email: [email protected] National Semiconductor Japan Ltd. Tel: 81-3-5639-7560 Fax: 81-3-5639-7507 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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