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LM2451 Datasheet(PDF) 8 Page - National Semiconductor (TI) |
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LM2451 Datasheet(HTML) 8 Page - National Semiconductor (TI) |
8 / 12 page Application Hints (Continued) NSC DEMONSTRATION BOARD Figure 15 shows the routing and component placement on the NSC LM2451 demonstration board. This board provides a good example of a layout that can be used as a guide for future layouts. Note the location of the following compo- nents: • C26—V CC bypass capacitor, located very close to pin 12 and ground pins • C27—V BB bypass capacitor, located close to pin 7 and ground • C28, C30, C33 — V CC bypass capacitors, near LM2451 and V CC clamp diodes. Very important for arc protection. The routing of the LM2451 outputs to the CRT is very critical to achieving optimum performance. Figure 16 shows the routing and component placement from pin 13 (V OUT3)ofthe LM2451 to the blue cathode. Note that the components are placed so that they almost line up from the output pin of the LM2451 to the blue cathode pin of the CRT connector. This is done to minimize the length of the video path between these two components. Note also that D1, D8 and R36 are placed to minimize the size of the video nodes that they are attached to. This minimizes parasitic capacitance in the video path and also enhances the effectiveness of the pro- tection diodes. The anode of protection diode D1 is con- nected directly to a section of the ground plane that has a short and direct path to the CRT ground and the LM2451 ground pins. The cathode of D8 is connected to V CC very close to decoupling capacitor C28 which is connected to the same area of the ground trace as D1. The diode placement and routing is very important for minimizing the voltage stress on the LM2451 during an arc over event. This demonstration board uses large PCB holes to accom- modate socket pins, which function to allow for multiple insertions of the LM2451 in a convenient manner. To benefit from the enhanced LM2451 package with thin leads, the device should be secured in small PCB holes to optimize the metal-to-metal spacing between the leads. CURRENT OUTPUT FOR IK FEEDBACK SYSTEMS The LM2451 can be used in DTV applications that use an IK feedback system. Figure 14 shows an example of an inter- face circuit used to feed back the IK output of LM2451 to a preamplifier with an ac coupled IK input. This feedback system consists of the preamp, LM2451, and interface circuit, forming a closed loop to automatically ad- just the black level of the drive signals to the cutoff point of the RGB cathodes. Following is a description of the interface circuit operation used for AVPs that have a voltage input for their IK sense input. The output at pin 8 of the LM2451 is filtered of high fre- quency noise by C14. D7 is used to limit the peak voltage at pin 8. Without this clamp diode the voltage would easily exceed 12V during active video when the cathode currents are much greater than the small currents being detected during vertical blanking. Exceeding 12V could damage Q4 and result in improper operation of the driver. R35 is essential to convert the IK current to voltage. Choos- ing the value of R35 sets the gain of the feedback voltage, and consequently, the operating point of the tube. Once a stable operating point is established, this point can be fine- tuned using the adjustment range of the feedback system or standard preamp controls. Changing the value of R35 will change the cutoff voltage at the cathode. A smaller value of R35 requires more IK current to maintain the feedback loop. The cutoff voltage set at the cathode will be lower to adjust to the higher IK current. This additional current must come from the cathode; therefore, the cathode voltage is set lower to meet higher current requirement. A higher value of R35 will do the opposite, raising the cathode voltage because less IK current is needed to maintain the same voltage at R35. The emitter follower, Q7, isolates R35 from the input imped- ance of the preamp. R21 and R39 bias the emitter of Q7 to limit the maximum voltage to the preamp. These resistor values should be chosen to limit the maximum voltage at the emitter and protect the preamp from any large voltages that would otherwise occur during active video. C9 is used to AC couple the IK signal to the preamp. The advantage of AC coupling is that any DC component (leakage current from the driver) of the IK signal is not detected by the IK sense input of the preamp. Some AVPs do have a direct current input for their IK sense input. For interfacing to these AVPs the only components to be used in the IK sense section are D7 and R41. To com- plete the signal path a jumper must be used to replace R34, C9 and the base-emitter junction of Q7. C14 can still be used for high frequency filtering. www.national.com 8 |
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