Output

Swing

Rise Time

Fall Time

10V–50V

4.0 ns

5.0 ns

15V–55V

4.2 ns

4.8 ns

20V–60V

4.4 ns

5.0 ns

THERMAL CONSIDERATIONS

Figure 4 shows the performance of the LM2403 in the test

circuit shown in

Figure 2 as a function of case temperature.

The figure shows that the speed of the LM2403 decreases

by less than 10% as the case temperature increases from

50˚C to 100˚C. This corresponds to a speed degradation of

2% for every 10˚C rise in case temperature.

Figure 6 shows the total power dissipation of the LM2403 vs.

Frequency when all three channels of the device are driving

an 8 pF load with a 40V

active time (device operating at the specified frequency)

which is typical in a monitor application. The other 28% of

the time the device is assumed to be sitting at the black level

(65V in this case). This graph gives the designer the informa-

tion needed to determine the heat sink requirement for his

application. The designer should note that if the load capaci-

tance is increased the AC component of the total power dis-

sipation will also increase.

The LM2403 case temperature must be maintained below

100˚C. If the maximum expected ambient temperature is

50˚C and the maximum power dissipation is 12W, then a

maximum heat sink thermal resistance can be calculated:

This example assumes a capacitive load of 8 pF and no re-

sistive load.

TYPICAL APPLICATION

A typical application of the LM2403 is shown in

Figure 11.

Used in conjunction with an LM1283, a complete video chan-

nel from monitor input to CRT cathode can be achieved. Per-

formance is satisfactory for resolutions up to 1600 x 1200

and pixel clock frequencies up to 160 MHz.

Figure 11 is the

schematic for the NSC demonstration board that can be

used to evaluate the LM1283/2403 combination in a monitor.

PC Board Layout Considerations

For optimum performance, an adequate ground plane, isola-

tion between channels, good supply bypassing and minimiz-

ing unwanted feedback are necessary. Also, the length of the

signal traces from the preamplifier to the LM2403 and from

the LM2403 to the CRT cathode should be as short as pos-

sible. The following references are recommended:

Ott, Henry W., “Noise Reduction Techniques in Electronic

Systems”, John Wiley & Sons, New York, 1976.

“Guide to CRT Video Design”, National Semiconductor Appli-

cation Note 861.

“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 os-

cillate in a monitor if feedback occurs around the video chan-

nel through the chassis wiring. To prevent this, leads to the

video amplifier input circuit should be shielded, and input cir-

cuit wiring should be spaced as far as possible from output

circuit wiring.

NSC Demonstration Board

Figures 12, 13 show routing and component placement on

the NSC LM1283/2403 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:

•

C79 — V

and ground pins

•

C55 — V

ground

•

C75–C77 — V

V

The routing of the LM2403 outputs to the CRT is very critical

to achieving optimum performance.

Figure 14 shows the

routing and component placement from pin 1 to the blue

cathode. Note that the components are placed so that they

almost line up from the output pin of the LM2403 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 D7, D8, R32 and D3 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 protection diodes. The an-

ode of protection diode D8 is connected directly to a section

of the the ground plane that has a short and direct path to the

LM2403 ground pins. The cathode of D7 is connected to V

CC

very close to decoupling capacitor C77 (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 LM2403 during an arc

over event. Lastly, notice that S1 is placed very close to the

blue cathode and is tied directly to CRT ground.

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