lm2421 National Semiconductor Corporation, lm2421 Datasheet - Page 8

• Manufacturer Part Number: lm2421
• Description: Monolithic Triple Channel Hdtv Driver
• Manufacturer: National Semiconductor Corporation

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Application Hints
NSC DEMONSTRATION BOARD
Figure 13 shows the routing and component placement on
the NSC LM126X/2421 demonstration board. The schematic
of the board is shown in Figure 11 and Figure 12 . 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
components:
The routing of the LM2421 outputs to the CRT is very critical
to achieving optimum performance. Figure 14 shows the
routing and component placement from pin 1 of the LM2421
to the blue cathode. Note that the components are placed so
that they almost line up from the output pin of the LM2421 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 D8, D9, R24 and D6 are placed
to minimize the size of the video nodes that they are at-
tached to. This minimizes parasitic capacitance in the video
path and also enhances the effectiveness of the protection
diodes. The anode of protection diode D8 is connected
directly to a section of the the ground plane that has a short
and direct path to the LM2421 ground pins. The cathode of
D9 is connected to V
C48 (see Figure 14 ) which is connected to the same section
of the ground plane as D8. The diode placement and routing
is very important for minimizing the voltage stress on the
LM2421 during an arcover event. Lastly, notice that S3 is
placed very close to the blue cathode and is tied directly to
CRT ground.
• C19 — V
• C20 — V
• C46, C47, C48 — V
and ground pins
and ground
and V
CC
CC
BB
clamp diodes. Very important for arc protection.
bypass capacitor, located very close to pin 6
bypass capacitors, located close to pin 10
CC
CC
very close to decoupling capacitor
bypass capacitors, near LM2421
(Continued)
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