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LM2432 Arkusz danych(PDF) 9 Page - National Semiconductor (TI) |
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LM2432 Arkusz danych(HTML) 9 Page - National Semiconductor (TI) |
9 / 13 page Application Hints (Continued) sor’s RGB cut-off and/or gain controls via the I 2C-bus. When normal video resumes, a protection element should shunt high video current away from the IK input of the RGB pro- cessor. Since the processor’s IK pin is not clamped during normal video, V IK of the LM2432 must not exceed +16V (V IKMAX). A properly chosen high speed, low-leakage zener diode (D PROT) can be used to protect both the RGB proces- sor input and LM2432 IK output in this case. Again, it may be necessary for the RGB processor to have IK leakage com- pensation for the leakage current attributed to the zener. See a simplified current feedback interface circuit in Figure 15. LM2432 IK Output and Protection Requirements The LM2432 IK output sources a copy of the actual cathode current to the interface circuit during the closed-loop IK measurement interval and during normal video when the IK feedback loop is opened. Because the cathode current dur- ing normal video is much higher than the low current being measured during the measurement interval, V IK may exceed it’s maximum rating. To protect and prevent improper opera- tion of the LM2432, V IK must be maintained within the range specified in the Section Operating Ranges . For voltage feedback topologies, it is recommended to use a high-speed diode to clamp the IK voltage to V BB or a lower supply during normal video. A small series resistor (R D in Figure 14) can be placed at the IK pin to limit the current through the diode when clamping. See the NSC Demonstra- tion Board for an example. For current feedback topologies, it is recommended to use a high-speed, low-leakage zener diode to clamp V IK to a properly chosen zener voltage and shunt the high video current away from the RGB processor’s IK input. The zener voltage should be higher than the clamp- ing voltage of the processor’s IK pin and lower than the maximum voltage rating of either the processor’s IK pin or LM2432 V IK, whichever is less. In a Direct-View TV application with a single neck PCB, it is possible for the three LM2432 IK outputs to share the one feedback circuit and protection diode by connecting the IK pins together on the neck PCB. This will reduce component count. In a Rear-Projection TV application with three neck PCBs, the IK pins can be connected on the central neck PCB or the RGB processor mainboard through cabling. This way, they can share the interface circuit to feed back the IK voltage or current signal to the RGB processor. However, each LM2432 IK output should have its own protection diode on its PCB. PC BOARD LAYOUT CONSIDERATIONS For optimum performance, an adequate ground plane, iso- lation between channels, good supply bypassing and mini- mizing unwanted feedback are necessary. Also, the length of the signal traces from the preamplifier to the LM2432 and from the LM2432 to the CRT cathode should be as short as possible. The following references are recommended: Ott, Henry W., “Noise Reduction Techniques in Electronic Systems”, John Wiley & Sons, New York, 1976. “Video Amplifier Design for Computer Monitors”, National Semiconductor Application Note 1013. Pease, Robert A., “Troubleshooting Analog Circuits”, Butterworth-Heinemann, 1991. Because of its high small signal bandwidth, the part may oscillate in a TV if feedback occurs around the video channel through the chassis wiring. To prevent this, leads to the video amplifier input circuit should be shielded, and input circuit wiring should be spaced as far as possible from output circuit wiring. TYPICAL APPLICATION The high bandwidth, large swing capability, and simple ap- plication make the LM2432 ideal for Rear-Projection and Direct-View HDTV CRT applications. The IK output can be made compatible with any RGB video processor with IK feedback. If the IK output is not used in the application, it should be connected to the same ground as pin 3 (GND). See the section “Cathode Current Output for IK Feedback Systems” for more information. NSC DEMONSTRATION BOARD Figure 16 and Figure 17 show the schematic and PCB layout for the NSC demonstration neck board for a typical Rear- Projection HDTV application with IK feedback. This single channel neck board could be used for all three channels, since each neck board receives video-related signals directly from the RGB mainboard. The power supplies are daisy- chained between each channel using inboard and outboard connectors J6 and J7. This board provides a good example of a layout that can be used as a guide for future layouts. Samples of the NSC demonstration neck board are available upon request to your local National Semiconductor Sales Office. Input Video Interface On the RGB mainboard, the video output of the RGB pro- cessor is buffered with a PNP transistor to drive the video through flat cabling to the NSC neck board. The cabling from the mainboard plugs into the neck board at connector J8 to supply it with video, IK, GND, and other signals. Between the video input (pin 3) of J8 and V IN (pin 7) of the LM2432 is another buffer stage consisting of two NPN transistors. Both NPN transistors drop the video levels from the preceding PNP buffer by a total of two V BE. This shifts the nominal input black level such that the LM2432 output (or cathode) black level voltage is near the nominal cut-off voltage of the CRT. The overall voltage shift from the processor output to the LM2432 input is one V BE drop. Note: The same video level shifting could have been accomplished using one NPN buffer on the RGB mainboard to drive the processor’s video output through cabling directly to the LM2432 input. How- 20137725 FIGURE 15. Simplified IK Interface for Current Feedback Systems www.national.com 9 |
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