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LM2432TE Datasheet(PDF) 10 Page - National Semiconductor (TI) |
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LM2432TE Datasheet(HTML) 10 Page - National Semiconductor (TI) |
10 / 13 page Application Hints (Continued) ever, it was decided to preserve the TV’s original RGB mainboard circuitry (the PNP buffer) and use two NPN tran- sistors on the neck board. The input stage from the RGB processor to the LM2432 will be determined by the system designer for his specific appli- cation. The input stage required depends mainly on the following system parameters: • Nominal CRT cut-off voltage • Nominal black level output voltage of the RGB processor • V CC &VBB supply voltages of the LM2432 (determines DC transfer characteristic) Once the nominal black level input to the LM2432 estab- lishes a cathode black level near the CRT cut-off voltage, it can be fine-tuned using the processor’s cut-off adjustment or calibrated automatically using the IK feedback system, if applicable. Lastly, some RGB processor video outputs can- not adequately drive the capacitive load introduced by the cabling between the RGB mainboard and neck boards. To prevent loading the processor’s output, a NPN or PNP buffer stage can be applied close to the output on the mainboard to sufficiently drive the video signal through cabling to the neck board. It is important to bias the buffer stage(s) properly to obtain optimal video performance and maintain the full video adjustment range of the RGB processor. Video Output and Arc Protection The routing of the LM2432 output to the CRT is very critical to achieve optimal video performance and robustness against arcover. Figure 17 shows the routing and component placement from V OUT (pin 2) of the LM2432 to the cathode pin of the CRT socket. The components are placed so that there is a short, direct path from the LM2432 output to the cathode. This is done to reduce the PCB parasitic capaci- tance on the LM2432 output and minimize EMI. Note also that L3, D3, D4, and R6 are placed to minimize the size of the video nodes that they are attached to. This enhances the effectiveness of the arc protection diodes. The anode of protection diode D3 is connected directly to a section of the ground plane that has a short, direct path to ground. The cathode of D4 is connected to V CC very close to decoupling capacitor C7, which is connected to the same section of the ground plane as D3. The diode placement and routing is very important to shunt arcover current away from the output and minimize the voltage stress on the LM2432. The internal CRT socket spark gap is essential to significantly reduce the initial arcover voltage seen at the cathode. The DAG con- nector should be connected to CRT ground for arc return current. IK Feedback Circuit The NSC demonstration neck board was made so that no modifications to the existing TV circuitry were necessary (except for replacing the original neck boards). Therefore, the video interface and IK feedback circuits are designed to be compatible with those original TV circuits. Referring to Figure 16, the LM2432 IK output (pin 5) is connected to protection circuitry before the IK current signal is routed to pin 4 of connector J8. Diode D1 protects the LM2432 IK pin from excessive voltage during normal video by clamping to the V BB supply. Diode D2 isolates the IK output from the other channels during the active IK measurement interval. Resistors R17 and R12 limit the current through D1 and D2, and C12 is used for filtering. From connector J8, the IK current signal is passed through cabling and combined with the other two IK signals on the RGB mainboard. Note: The following paragraph describes circuitry that is not part of the NSC demonstration neck board. The RGB pro- cessor, which operates with a voltage feedback IK topology, uses a single “IK resistor” on the RGB mainboard to convert all three IK currents into a voltage signal. The IK voltage signal is then buffered through a PNP transistor before it is filtered and AC coupled to the RGB processor’s IK input. This is one implementation of the IK feedback circuit based on this TV’s specific RGB processor. The system designer should refer to the RGB processor data sheet to determine the appropriate feedback circuit implementation for his ap- plication. Supply Decoupling Note the location of the following components: • C5—V CC bypass capacitor with short traces to the VCC and GND pins of LM2432. • C7—V CC bypass capacitor with short traces to the VCC arc protection diode and ground. This capacitor is very important for arc protection. • C9—V BB bypass capacitor with short traces to the VBB and GND pins. • C10 and C11 — V CC and V BB electrolytic capacitors placed near supply pins of LM2432. Other Items Connector J1 and switches JP2 & JP4 can be used to bypass the TV’s internal 200V and 12V supplies and evalu- ate the LM2432 with external V CC and VBB supplies. Also, this demonstration board uses medium-sized PCB holes to accommodate socket pins, which function to allow for mul- tiple insertions of the LM2432 in a convenient manner. To benefit from the enhanced LM2432 package with thin leads, the device should be secured with solder in small PCB holes to optimize the metal-to-metal spacing between the leads. www.national.com 10 |
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