Figure 12 shows the typical cathode pulse response with an

output swing of 130V

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 LM2429 and

from the LM2429 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 TV 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.

TYPICAL APPLICATION

A typical application of the LM2429 is shown in the sche-

matic for the NSC demonstration board in Figure 14 and

Figure 15. Used in conjunction with an LM126X preamplifier,

a complete video channel from input to CRT cathode can be

achieved. Performance is ideal for DTV applications. The

NSC demonstration board can be used to evaluate the

LM126X/LM2429 combination in a TV.

NSC DEMONSTRATION BOARD

Figure 16 shows the routing and component placement on

the NSC LM126X/LM2429 demonstration board. 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, C48 — V

V

The routing of the LM2429 outputs to the CRT is very critical

to achieving optimum performance. Figure 17 shows the

routing and component placement from pin 10 (V1

the LM2429 to the blue cathode. Note that the components

are placed so that they almost line up from the output pin of

the LM2429 to the blue cathode pin of the CRT connector.

This is done to minimize the length of the video path be-

tween these two components. Note also that D8, D9, R24

and D6 are placed to minimize the size of the video nodes

that they are attached to. This minimizes parasitic capaci-

tance 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 LM2429 ground pins. The

cathode of D9 is connected to V

capacitor C48 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

LM2429 during an arcover event. Lastly, notice that S3 is

placed very close to the blue cathode and is tied directly to

CRT ground.

This demonstration board uses large PCB holes to accom-

modate socket pins, which function to allow for multiple

insertions of the LM2429 in a convenient manner. To benefit

from the enhanced LM2429 package with thin leads, the

device should be secured in small PCB holes to optimize the

metal-to-metal spacing between the leads.

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