mance of the device in the NSC application board. The val-

ues shown in

Figure 9 can be used as a good starting point

for the evaluation of the LM2402.

Effect of Load Capacitance

The output rise and fall times as well as overshoot will vary

as the load capacitance varies. The values of the output cir-

cuit (R1, R2 and L1 in

Figure 9) should be chosen based on

the nominal load capacitance. Once this is done the perfor-

mance of the design can be checked by varying the load

based on what the expected variation will be during produc-

tion.

Effect of Offset

Figure 7 shows the variation in rise and fall times when the

output offset of the device is varied from 40 to 50 V

rise and fall times show about the same overall variation.

The slightly faster fall time is fastest near the center point of

45V, making this the optimum operating point since there is

little increase in the rise time.

THERMAL CONSIDERATIONS

Figure 4 shows the performance of the LM2402 in the test

circuit shown in

Figure 2 as a function of case temperature.

Figure 4 shows that both the rise and fall times of the

LM2402 become slightly slower as the case temperature in-

creases from 40˚C to 125˚C. In addition to exceeding the

safe operating temperature, the rise and fall times will typi-

cally exceed 3 nsec. Please note that the LM2402 is never

to be operated over a case temperature of 100˚C.

Figure 6 shows the total power dissipation of the LM2402 vs.

Frequency when all three channels of the device are driving

both an 8 pF load and a 20 pF load. This graph gives the de-

signer the information needed to determine the heat sink re-

quirement for his application. The designer should note that

if the load capacitance is increased the AC component of the

total power dissipation will also increase as shown in

Figure

6. The designer should also remember that the actual video

signal has a period of around 70% to 75%. The remainder of

the time the video signal is inactive, or at the black level (be-

low the black level if blanked). During this time the LM2402

will be at the black level, or below, dissipating under 4W. Re-

ferring to

Figure 14 and using an input black level voltage of

1.9V, the power dissipation during the inactive video time is

3.8W, including both the 80V and 12V supplies.

The LM2402 case temperature must be maintained below

100˚C. Assume the worst case operating condition is a

100 MHz square wave during active video (a pixel clock of

200 MHz with one pixel on, one pixel off). From

Figure 6 one

can see that the power dissipation of the LM2402 is 28W if

the 100 MHz square wave is applied all the time. One must

also compensate for the inactive period of video. From

Fig-

ure 14 it has been calculated that the power dissipation dur-

ing the inactive video is 4W. Therefore there is an additional

24W of power dissipation due to the AC signal. Assume that

the AC signal is active 72% of the time. Now the AC power

dissipation is:

24W x 0.72 = 17W

The total power dissipation for 72% active video time is:

17W+4W = 21W

If the maximum expected ambient temperature is 50˚C and

using the maximum power dissipation of 21W (video being

active only 72% of the frame), 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 LM2402 is shown in

Figure 10.

Used in conjunction with three LM2202s, a complete video

channel from monitor input to CRT cathode can be achieved.

Performance is excellent for resolutions up to 1600 x 1200

and pixel clock frequencies at 200 MHz.

Figure 10 is the

schematic for the NSC demonstration board that can be

used to evaluate the LM2202/2402 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 LM2402 and from

the LM2402 to the CRT cathode should be as short as pos-

sible. The red video trace from the buffer transistor to the

LM2402 input is about the absolute maximum length one

should consider on a PCB layout. If possible the traces

should actually be shorter than the red video trace. The fol-

lowing references are recommended for video board design-

ers:

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 11, 12 show routing and component placement on

the NSC LM2202/2402 demonstration board. The schematic

of the board is shown in

Figure 10. 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:

•

C47 -V

and ground pins. (

Figure 12)

•

C49 -V

ground. (

Figure 12)

•

C46 and C77 -V

V

ure 11)

The routing of the LM2402 outputs to the CRT is very critical

to achieving optimum performance.

Figure 13 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 LM2402 to the blue

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