TABLE 1. LM2445 Cathode Response

Channel

t

t

Red

6.3ns / 7%

6.3ns / 3%

Green

6.1ns / 6%

5.9ns / 4%

Blue

6.5ns / 3%

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 Analog

Circuits”,

Butterworth-Heinemann, 1991.

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

and ground pins

•

C20—V

ground

•

C46, C47, C48 — V

and V

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

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

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