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LM2432TE bảng dữ liệu(PDF) 8 Page - National Semiconductor (TI) |
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LM2432TE bảng dữ liệu(HTML) 8 Page - National Semiconductor (TI) |
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8 / 13 page  Application Hints (Continued) Tips for Reducing Power Dissipation The following methods can be used to reduce the power dissipated by the LM2432 in order to optimize heatsink size and cost: • Use a lower V CC supply voltage while maintaining suffi- cient operating range for cutoff, brightness, and drive adjustments. • Reduce the input bandwidth to the LM2432 while main- taining acceptable picture performance. • Lower the maximum V PP swing while maintaining accept- able picture contrast and brightness. • Minimize capacitive load on the LM2432 output by using good PCB layout practices. OPTIMIZING TRANSIENT RESPONSE Referring to Figure 13, 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, preferably 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 16 can be used as a good starting point for the evaluation of the LM2432. Using a variable resistor for R1 will simplify finding the value needed for optimum performance in a given application. Once the opti- mum value is determined, the variable resistor can be re- placed with a fixed value. Figure 12 shows a typical cathode pulse response with an output swing of 110V PP using a RGB video processor that provides input speeds with 12 ns rise and fall times. Note: The RGB processor’s sharpness feature adds emphasis (preshoots and overshoots) to the rising and falling edges of the input pulse, which consequently adds emphasis to the cathode pulse response. CATHODE CURRENT OUTPUT FOR IK FEEDBACK SYSTEMS IK Feedback Systems IK feedback was developed to accurately bias the CRT and continuously calibrate it to the correct cut-off and/or drive levels over the useful life of the CRT. RGB video processors that use IK feedback to automatically adjust only cut-off, or black level, are realized by a 1-point calibration system. A few trade names for this system are Auto Kine Bias (AKB) and Black Current Stabilization (BCS). RGB processors that can automatically adjust both cut-off and drive, or white level, are realized by a 2-point calibration system. This is commonly known as Continuous Cathode Calibration (CCC). For convenience, some 2-point RGB processors may be programmed to 1-point operation if drive calibration is not required. The LM2432 is compatible with both 1- and 2-point systems. To be compatible with various RGB processors, an interface circuit may be needed in the feedback path between the LM2432 IK output and the processor’s IK input. This feed- back circuit depends on the RGB processor and feedback topology (voltage or current) used. Because each processor has its own IK input signal and topology requirements, it is outside the scope of this data sheet to describe each feed- back circuit in detail. For more information, please refer to the RGB processor data sheet or contact your local National Semiconductor Sales Office with your specific application requirements. Feedback Topologies RGB processors that use voltage feedback require the LM2432 IK current to be converted to voltage via a resistor (R IK) to ground. This IK voltage, VIK, will be fed back to the IK input of the RGB processor through an interface circuit, which will be AC or DC coupled depending on the proces- sor’s IK input requirement. For proper feedback operation, some processors may require an emitter follower to isolate the IK input from the high impedance of the resistor. During the closed-loop IK measurement interval, the IK input volt- age will be sampled and compared with the processor’s internal reference voltage to automatically calibrate the video levels for the next field. The value of R IK is crucial, since it establishes the IK voltage and consequently, the operating point of the CRT. Once a stable operating point is estab- lished with a properly chosen resistor, this point can be fine-tuned using the adjustment range of the processor’s RGB cut-off and/or gain controls via the I 2C-bus. After the IK measurement interval (usually at the end of blanking), nor- mal video will resume and high currents will flow out of the IK output. These high video currents will produce large IK volt- ages across the resistor that can exceed the maximum voltage rating for V IK. Therefore, it is recommended to use a high-speed diode (D PROT) to clamp the LM2432 IK output to a safe level (preferably V BB or a lower supply). If a zener diode is used instead, it may be necessary for the RGB processor to have IK leakage compensation for the leakage current attributed to the zener. Lastly, it is possible to use a single R IK resistor to set the IK voltage for all three LM2432s. See a simplified voltage feedback interface circuit in Figure 14. RGB processors that use current feedback do not require voltage conversion. The LM2432 IK current can be fed back directly to the IK input of the RGB processor, although some protection circuitry will be needed to protect the RGB pro- cessor and LM2432. During the closed-loop IK measure- ment interval, the voltage of the RGB processor’s IK pin will be internally clamped, and the IK current will be sampled and compared with the processor’s internal reference current to calibrate the video levels. The operating point of the CRT can be fine-tuned using the adjustment range of the proces- 20137724 FIGURE 14. Simplified IK Interface for Voltage Feedback Systems www.national.com 8 |
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