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LM2413 데이터시트(PDF) 4 Page - National Semiconductor (TI) |
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LM2413 데이터시트(HTML) 4 Page - National Semiconductor (TI) |
4 / 11 page Theory of Operation The LM2413 is a high voltage monolithic three channel CRT driver suitable for very high resolution display applications, up to 1600 x 1200 at 70 Hz refresh rate. The LM2413 oper- ates using 80V and 12V power supplies. The part is housed in the industry standard 11-lead TO-220 molded plastic power package. The simplified circuit diagram of one channel of the LM2413 is shown in Figure 1. A PNP emitter follower, Q5, provides in- put buffering. This minimizes the current loading of the video pre-amp. R9 is used to turn on Q5 when there is no input. With Q5 turn on, Q1 will be almost completely off, minimizing the current flow through Q1 and Q2. This will drive the output stage near the V CC rail, minimizing the power dissipation with no inputs. R6 is a pull-up resistor for Q5 and also limits the current flow through Q5. R3 and R2 are used to set the current flow through Q1 and Q2. The ratio of R1 to R2 is used to set the gain of the LM2413. R1, R2, and R3 are all related when calculating the output voltage of the CRT driver. R b limits the current through the base of Q2. Q1 and Q2 are in a cascode configuration. Q1 is a low voltage and very fast transistor. Q2 is a higher voltage transistor. The cascode configuration gives the equivalent of a very fast and high voltage transistor. The two output transistors, Q3 and Q4, form a class B amplifier output stage. R4 and R5 are used to limit the current through the output stage and set the output impedance of the LM2413. Q6, along with R7 and R8 set the bias current through Q3 and Q4 when there is no change in the signal level. This bias current minimizes the crossover distortion of the output stage. With this bias cur- rent the output stage now becomes a class AB amplifier with a crossover distortion much lower than a class B amplifier. Figure 2 shows a typical test circuit for evaluation of the LM2413. Due to the very wide bandwidth of the LM2413, a specially designed output circuit is used with the required se- ries resistor and C LOAD to emulate the actual application when evaluating the performance of the LM2413 in a 50 Ω environment without the use of an expensive FET probe. The combined resistors of 4950 Ω at the output form a 200:1 voltage divider when connected to a 50 Ω load. The input sig- nal from the generator is ac coupled to the input of the CRT driver. V ADJ input sets the DC operating range of the LM2413. Application Hints INTRODUCTION National Semiconductor (NSC) is committed to providing 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. POWER SUPPY BYPASS Since the LM2413 is a very high bandwidth amplifier, proper power supply bypassing is critical for optimum performance. Improper power supply bypassing can result in large over- shoot, ringing and oscillation. A 0.1 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 LM2413’s supply and ground pins. A 0.1 µF capacitor should be connected from the bias pin, V BB, 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 volt- age, but to a value that is much higher than allowable on the LM2413. This fast, high voltage, high-energy pulse can dam- age the LM2413 output stage. The application circuit shown in Figure 9 is designed to help clamp the voltage at the out- put of the LM2413 to a safe level. The clamp diodes should have a fast transient response, high peak current rating, low series impedance and low shunt capacitance. 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 capacitor. The ground connection of the diode and the decoupling capacitor should be very close to the LM2413 ground. This will significantly re- duce the high frequency voltage transients that the LM2413 would be subjected to during an arc-over condition. Resistor R2 limits the arc-over current that is seen by the diodes while R1 limits the current into the LM2413 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. Inductor L1 is critical to reduce the inital high frequency voltage levels that the LM2413 would be sub- jected to during an arc-over. Having large value resistors for R1 and R2 would be desirable, but this has the effect of in- creasing rise and fall times. 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 components shown in Figure 9. The values of L1 and R1 may need to be adjusted for a particular application. The recommended mini- mum value for R1 is 110 Ω, with L1 = .12 µH. www.national.com 4 |
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