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LM2422TE Arkusz danych(PDF) 9 Page - National Semiconductor (TI) |
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LM2422TE Arkusz danych(HTML) 9 Page - National Semiconductor (TI) |
9 / 13 page Application Hints (Continued) same area of the ground trace as D2. The diode placement and routing is very important for minimizing the voltage stress on the LM2422 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 LM2422 in a convenient manner. To benefit from the enhanced LM2422 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 LM2422 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 LM2422 to a preamplifier with an ac coupled IK input. This feedback system consists of the preamp, LM2422, 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. The output at pin 9 of the LM2422 is filtered of high fre- quency noise by C14. D7 is used to limit the peak voltage at pin 9. Without this clamp diode the voltage would easily exceed 12V during active video, in which the cathode cur- rents are much greater than the small currents being de- tected during vertical blanking. Exceeding 12V could dam- age Q1 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 lower to meet higher current requirement. A higher value of R35 will do the opposite, raising the cathode voltage because less IK cur- rent is needed to maintain the same voltage at R35. The emitter follower, Q1, isolates R35 from the input imped- ance of the preamp. R20 and R21 bias the emitter of Q1 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. C12 is used for further filtering of the IK signal. 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. www.national.com 9 |
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