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LM2485NA bảng dữ liệu(PDF) 5 Page - National Semiconductor (TI) |
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LM2485NA bảng dữ liệu(HTML) 5 Page - National Semiconductor (TI) |
5 / 8 page NSC Demonstration Board Figure 6 shows the recommended application schematic for the NSC Demonstration Board that can be used to evaluate the LM2423/LM2485 AC coupled solution in a Direct-View CRT Digital TV application. Figure 7 shows the routing and component placement on the NSC LM2423/LM2485 Dem- onstration Board. This board provides a good example of a PCB layout that was designed for robust arc protection and optimal video performance. PCB LAYOUT CONSIDERATIONS Note that the NSC Demonstration Board complies with the layout guidelines outlined below. For convenience, the com- ponent names are shown in parenthesis to cross-reference with the schematic and layout images provided. Power Supplies and Grounds • 0.1uF bypass capacitors (C3 and C6) should be placed very close to V BB and VCC of the LM2485. These capaci- tors should have a short, direct return to the LM2485 ground. • 1uF electrolytic capacitors (C1 and C2) should also be placed reasonably close to both supply pins. • GND Pins 8 and 14 should connect to a solid ground plane under the LM2485. The LM2485 ground plane should connect directly to the CRT Driver ground at one point (above C21). Do not connect the LM2485 ground plane directly to CRT ground to protect the LM2485 from arcing. Inputs • The LM2485 input traces should be routed away from all output traces of the CRT Driver. This will help protect the video preamplifier and LM2485 from high frequency, large amplitude video coupling and potential damage from arcing at the cathodes. • If the LM2485 inputs are coming from the main board, 0.1uF capacitors (C4, C5, C7) should be placed at the V IN pins of the input connector. If a LM12XX preamplifier is placed on the neck board, these 0.1uF capacitors should be placed very close to its DAC output pins to protect it during arcing. In both cases, these capacitors should be connected to the LM2485 ground and should not be connected directly to CRT ground. • 1k Ω series resistors (R1, R3, R4) should be placed close to the V IN pins and a small resistor may also be placed close to the VB IN pin. These resistors will protect the preamplifier in the event that the LM2485 fails. Outputs • 1.5k Ω resistors (R6, R7, R8) and 0.1uF capacitors (C21, C24, C25) should be placed near the V OUT pins to limit the current and voltage surges on the outputs during an arc event. These bypass capacitors should have a short, direct return to the LM2485 ground. • Because the clamp outputs carry DC, the clamp output traces routed to each AC coupled video output can be long. Avoid cutting a ground trace(s) simply to route clamp output traces and use wire jumpers, if necessary. • Clamp diodes (D3, D5, D6) and pull-up resistors (R20, R23, R24) should be placed closest to the video output traces to minimize the size of the video nodes and reduce parasitic capacitance on the video outputs. The clamp diodes must be 1SS83 or equivalent diodes; do not use 1N4148 type diodes. These diodes must have a fast transient response, low shunt capacitance, low series impedance, and a high peak current rating to clamp the video output voltage to the V CC supply during an arc event. • 0.1uF bypass capacitors (C46, C48, C9) should be placed very close to the 1SS83 type arc protection diodes (D1/D2, D4/D7, D8/D9) between V CC and ground. The ground connection should have a short, direct path to CRT ground. Therefore, arc currents shunted by the pro- tection diodes can return to the CRT DAG (J6). • 300V spark gaps (S1, S2, S3, S4) are strongly recom- mended and should be placed very close to the cathode and G1 pins and grounded directly to the CRT ground plane, so arc current can return to the nearby CRT DAG. Spark gaps help to minimize voltage stress on the CRT Driver and LM2485. • If the G1 blanking feature is used, all G1 circuit compo- nents should be placed as close as possible to the G1 pin of the CRT connector, with the exception of a small resistor (R2) placed close to the VB OUT pin to limit any current toward the LM2485. The sequence of circuit com- ponents from the VB OUT pin to the G1 pin is recom- mended to protect the LM2485 against arcing. All these components must be grounded directly to the CRT ground plane. Therefore, arc currents can easily return to the CRT DAG. Do not omit any of the components in the G1 circuit as they are required to protect the LM2485 against arcing and for correct G1 blank operation. • If the G1 blanking feature is not used, simply place a 470pF capacitor from the VB OUT pin to LM2485 ground and do not connect the output to the G1 circuitry. Pin Options and No Connects • Depending on the pin options required, Pin 12 and Pin 13 can be connected to ground via a ground plane, trace, or jumper wire. See the Section Theory of Operation to determine the appropriate pin option for the application. • The “No Connect” or N/C pins (pins 16, 18, 20, 22, 24) are not connected to the die internally in order to achieve larger spacing between the high voltage V CC and VOUT pins. These N/C pins should not be used to route other traces through. www.national.com 5 |
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