TABLE 1. LM2436 Cathode Response

t

t

Red

7.7 ns / 7%

7.7 ns / 5%

Green

7.7 ns / 6%

7.5 ns / 6%

Blue

7.4 ns / 7%

7.3 ns / 7%

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 LM2436 and

from the LM2436 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 12 shows the routing and component placement on

the NSC LM1279/2436 demonstration board. The schematic

of the board is shown in Figure 11. 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 compo-

nents:

•

C55—V

and ground pins

•

C43, C44 — V

and ground

•

C53–C56 — V

V

The routing of the LM2436 outputs to the CRT is very critical

to achieving optimum performance. Figure 13 shows the

routing and component placement from pin 1 of the LM2436

to the blue cathode. Note that the components are placed so

that they almost line up from the output pin of the LM2436 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 D14, D15, R29 and D13 are

placed to minimize the size of the video nodes that they are

attached to. This minimizes parasitic capacitance in the

video path and also enhances the effectiveness of the pro-

tection diodes. The anode of protection diode D14 is con-

nected directly to a section of the the ground plane that has

a short and direct path to the LM2436 ground pins. The

cathode of D15 is connected to V

capacitor C55 (see Figure 13) which is connected to the

same section of the ground plane as D14. The diode place-

ment and routing is very important for minimizing the voltage

stress on the LM2436 during an arcover event. Lastly, notice

that S3 is placed very close to the blue cathode and is tied

directly to CRT ground.

20043521

Red Cathode Response

FIGURE 10.

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