NSC DEMONSTRATION BOARD

Figure 12 shows the routing and component placement on

the NSC LM1279/2438 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 fu-

ture layouts. Note the location of the following components:

•

C54, C56 — V

pin 4 and ground pins

•

C43, C44 — V

8 and ground

•

C53, C55 — Additional V

CC

bypass capacitors, near

LM2438 and V

protection.

The routing of the LM2438 outputs to the CRT is very critical

to achieving optimum performance.

Figure 13 shows the

routing and component placement from pin 1 of the LM2438

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

that they almost line up from the output pin of the LM2438 to

the red cathode pin of the CRT connector. This is done to

minimize the length of the video path between these two

components. Note also that D16, D17, R21 and D9 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 D17 is con-

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

a short and direct path to the LM2438 ground pins. The cath-

ode of D16 is connected to V

pacitor C53 (see

Figure 13) which is connected to the same

section of the ground plane as D17. The diode placement

and routing is very important for minimizing the voltage

stress on the LM2438 during an arcover event. Lastly, notice

that S1 is placed very close to the red cathode and is tied di-

rectly to CRT ground.

DS101043-15

FIGURE 10. Waveform at Cathode with LM1279/243X Demo Board

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