LM2426 datasheet(8/12 Pages) NSC

8 / 12 page

Application Hints (Continued)

Figure 12 shows the typical cathode pulse response with an

output swing of 110V

PP using a LM1269 preamplifier.

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 LM2426TE and

from the LM2426TE 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 LM2426TE 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 HDTV applications. The

NSC demonstration board can be used to evaluate the

LM126X/2426 combination in a TV.

NSC DEMONSTRATION BOARD

Figure 16 shows the routing and component placement on

the NSC LM126X/2426 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

CC bypass capacitor, located very close to pin 2

and ground pins

•

C20—V

BB bypass capacitor, located close to pin 11 and

ground

•

C46, C48 — V

CC bypass capacitors, near LM2426TE and

CC clamp diodes. Very important for arc protection.

The routing of the LM2426TE outputs to the CRT is very

critical to achieving optimum performance. Figure 17 shows

the routing and component placement from pin 10 (V1

OUT)

of the LM2426TE to the blue cathode. Note that the compo-

nents are placed so that they almost line up from the output

pin of the LM2426TE 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 attached to. This minimizes parasitic

capacitance in the video path and also enhances the effec-

tiveness 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 LM2426TE

ground pins. The cathode of D9 is connected to V

CC very

close to decoupling capacitor C48 (see Figure 17) which is

connected to the same section of the ground plane as D8.

The diode placement and routing is very important for mini-

mizing the voltage stress on the LM2426TE during an ar-

cover 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 LM2426TE in a convenient manner. To

benefit from the enhanced LM2426TE 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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