Application Hints
(Continued)
TABLE 1. LM2445 Cathode Response
Channel
Red
Green
Blue
t
r
/OS
6.3ns / 7%
6.1ns / 6%
6.5ns / 3%
t
f
/OS
6.3ns / 3%
5.9ns / 4%
6.4ns / 2%
PC BOARD LAYOUT CONSIDERATIONS
For optimum performance, an adequate ground plane, iso-
lation between channels, good supply bypassing and mini-
mizing unwanted feedback are necessary.Also, the length of
the signal traces from the preamplifier to the LM2445 and
from the LM2445 to the CRT cathode should be as short as
possible. The following references are recommended:
Ott, Henry W., “Noise Reduction Techniques in Electronic
Systems”, John Wiley & Sons, New York, 1976.
“Video Amplifier Design for Computer Monitors”, National
Semiconductor Application Note 1013.
Pease,
Robert
A.,
“Troubleshooting
Butterworth-Heinemann, 1991.
Analog
Circuits”,
Because of its high small signal bandwidth, the part may
oscillate in a monitor if feedback occurs around the video
channel through the chassis wiring. To prevent this, leads to
the video amplifier input circuit should be shielded, and input
circuit wiring should be spaced as far as possible from output
circuit wiring.
NSC DEMONSTRATION BOARD
Figure 13
shows the routing and component placement on
the NSC LM1267/2466 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:
C19—V
bypass capacitor, located very close to pin 4
and ground pins
C20—V
BB
bypass capacitors, located close to pin 8 and
ground
C46, C47, C48—V
bypass capacitors, near LM2445
and V
CC
clamp diodes. Very important for arc protection.
The routing of the LM2445 outputs to the CRT is very critical
to achieving optimum performance.
Figure 14
shows the
routing and component placement from pin 1 of the LM2445
to the blue cathode. Note that the components are placed so
that they almost line up from the output pin of the LM2445 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 LM2445 ground pins. The cathode of
D9 is connected to V
CC
very close to decoupling capacitor
C19 (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
LM2445 during an arcover event. Lastly, notice that S3 is
placed very close to the blue cathode and is tied directly to
CRT ground.
20063716
FIGURE 10. Red Cathode Response
L
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