tors for R1 will simplify finding the values needed for opti-

mum performance in a given application. Once the optimum

value is determined, the variable resistors can be replaced

with fixed values.

Effect of Load Capacitance

Figure 14 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. Note that

the fall time stayed fairly constant while the rise time in-

creased approximately 1.8% per pF.

Effect of Offset

Figure 12 shows the variation in rise and fall times when the

output offset of the device is varied from 40VDC to 50 VDC.

The rise time shows a maximum variation relative to the

center data point (45 VDC) of less than 1.3%. The fall time

shows a variation of about 3.9% relative to the center data

point.

THERMAL CONSIDERATIONS

Figure 11 shows the performance of the LM2469 video

amplifiers in the test circuit shown in Figure 3 as a function of

case temperature. The figure shows that the rise time of the

LM2469 increases by approximately 9% as the case tem-

perature increases from 30˚C to 100˚C. This corresponds to

a speed degradation of 1.3% for every 10˚C rise in case

temperature. The fall time degrades around 0.6% for every

10˚C in case temperature.

Figure 10 shows the maximum power dissipation of the

LM2469 vs. Frequency when all three channels of the device

are driving an 8pF load with a 40 V

pixel 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 (65V in this

case). This graph gives the designer the information needed

to determine the heat sink requirement 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.

The LM2469 case temperature must be maintained below

100˚C. If the maximum expected ambient temperature is

70˚C and the maximum power dissipation is 3.85W (from

Figure 10, 50MHz bandwith), then a maximum heat sink

thermal resistance can be calculated:

This example assumes a capacitive load of 8pF and no

resistive load.

TYPICAL APPLICATION

The typical application of the LM2469 is shown in Figure 5 &

6. Used in conjunction with an LM126X and an LM2479/

2480 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 frequen-

cies up top 75MHz. Figure5&6are the schematic for the

NSC demonstration board that can be used to evaluate the

LM126X/246X/2480 combination in a monitor.

PC Board Layout Considerations

For optimum performance, an adequate ground plane, iso-

lation between channels, good supply bypassing and the

minimization of unwanted feedback are necessary. Also, the

length of the signal traces from the preamplifier to the

LM2469 and from the LM2469 to the CRT cathode should be

as short as possible. The following references are recom-

mended:

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 bandwith, 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

wiring should be spaced as far as possible from output circuit

wiring.

NSC Demonstration Board

Figure 7 shows the routing and component placement on the

NSC LM126X/246X demonstration board. The schematic of

the board is shown in Figure5&6. 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 compo-

nents:

•

C16, C19 —V

pin 4 and the ground plane near the device.

•

C20 —V

ground.

•

C46, C47, C48 —V

V

The routing of the LM2469 video outputs to the CRT is very

critical to achieving optimum performance. Figure 8 shows

the routing and component placement from pin 3 of the

LM2469 to the blue cathode. Note that the components are

placed so that they almost line up from the output pin of the

LM2469 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 effectiveness of the pro-

tection diodes. The anode of protection diode D8 is con-

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

short and direct path to the LM2469 ground pins. The cath-

ode of D9 is connected to V

capacitor C48 (see Figure 8), which is connected to the

same section of the ground plane as D8. The diode place-

ment and routing is very important for minimizing the voltage

stress on the LM2469 video outputs during an arc over

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

cathode and is tied directly to the ground under the CRT

connector.

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