resistor for R1 will simplify finding the value needed for

optimum performance in a given application. Once the opti-

mum value is determined the variable resistor can be re-

placed with a fixed value.

Effect of Load Capacitance

Figure 8 shows the effect of increased load capacitance on

the speed of the device. This demonstrates the importance

of knowing the load capacitance in the application. The rise

time increased about 0.8 ns for an increase of 1 pF in the

load capacitance. The fall time does remain almost the same

as the load capacitance is increased.

Effect of Offset

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

output offset of the device is varied from 70 to 80 V

rise time has very little increase over its fastest point near

75V. The fall time becomes a little faster as the offset voltage

increases.

THERMAL CONSIDERATIONS

Figure 4 shows the performance of the LM2460 in the test

circuit shown in Figure 2 as a function of case temperature.

The figure shows that the rise time of the LM2460 increases

by approximately 13% as the case temperature increases

from 30˚C to 100˚C. This corresponds to a speed degrada-

tion of 2.0% for every 10˚C rise in case of temperature. The

fall time has almost no change as the case temperature

increases.

Figure 6 shows the maximum power dissipation of the

LM2460 vs Frequency when all three channels of the device

are driving an 8 pF load with a 60 V

on, one pixel off. The graph assumes a 72% 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 (105V in

this case). This graph gives the designer the information

needed to determine the heat sink requirement for his appli-

cation. The designer should note that if the load capacitance

is increased the AC component of the total power dissipation

would also increase.

The LM2460 case temperature must be maintained below

100˚C. If the maximum expected ambient temperature is

70˚C and the maximum power dissipation is 11W (from

Figure 6, 70 MHz bandwidth) then a maximum heat sink

thermal resistance can be calculated:

This example assumes a capacitive load of 8 pF and no

resistive load.

TYPICAL APPLICATION

A typical application of the LM2460 is shown in Figure 10

and Figure 11. Used in conjunction with a LM1267 pre-amp

and a LM2479 bias clamp, a complete video channel from

monitor input to CRT cathode can be achieved. Performance

is ideal for 1024 x 768 resolution displays with pixel clock

frequencies up to 80 MHz. Figure 10 and Figure 11 are the

schematic for the NSC demonstration board that can be

used to evaluate the LM1267/2460/2479 combination in a

monitor.

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 LM2460 and

from the LM2460 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 monitor 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.

NSC Demonstration Board

Figure 12 shows the routing and component placement on

the NSC LM126X/246X/LM2479/80 demonstration board.

The schematic of the board is shown in Figure 10 and Figure

11. 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:

•

C16, C19 —V

pin 4 and ground pins

•

C17, C20 —V

8 and ground

•

C46, C47, C48 —V

and V

The routing of the LM2460 outputs to the CRT is very critical

to achieving optimum performance. Figure 13 shows the

routing and component placement from pin 3 of the LM2460

to the blue cathode. The blue video path from the LM2460

output is shown by the darker traces. Note that the compo-

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

pin of the LM2460 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 ground

plane that has a short and direct path to the LM2460 ground

pins. The cathode of D9 is connected to V

decoupling capacitor C48 (see Figure 13) which is con-

nected 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 LM2460 during an arc over event.

Lastly, notice that S3 is placed very close to the blue cathode

and is tied directly to CRT ground.

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