OPTIMIZING TRANSIENT RESPONSE

Referring to

Figure 9, there are three components (R1, R2

and L1) that can be adjusted to optimize the transient re-

sponse of the application circuit. Increasing the values of R1

and R2 will slow the circuit down while decreasing over-

shoot. Increasing the value of L1 will speed up the circuit as

well as increase overshoot. It is very important to use induc-

tors with very high self-resonant frequencies, preferably

above 300 MHz. Ferrite core inductors from J.W. Miller

Magnetics (part # 78FR--k) were used for optimizing the

performance of the device in the NSC application board. The

values shown in

Figure 10 and Figure 11 can be used as a

good starting point for the evaluation of the LM2463. Using a

variable resistor for R1 will simplify finding the value needed

for optimum performance in a given application. Once the

optimum value is determined, the variable resistor can be

replaced 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.

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 time shows a maximum variation relative to the center

data point (45 V

less than 6% relative to the center data point.

THERMAL CONSIDERATIONS

Figure 4 shows the performance of the LM2463 in the test

circuit shown in

Figure 2 as a function of case temperature.

The figure shows that the rise and fall times of the LM2463

increase by approximately 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 maximum power dissipation of the

LM2463 vs. Frequency when all three channels of the device

are driving an 8 pF load with a 40 V

on, one pixel off signal. 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 appli-

cation. The designer should note that if the load capacitance

is increased the AC component of the total power dissipation

will also increase.

The LM2463 case temperature must be maintained below

100˚C. If the maximum expected ambient temperature is

70˚C and the maximum power dissipation is 11.9W (from

Figure 6, 180 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 LM2463 is shown in

Figure 10

and

Figure 11. Used in conjunction with an LM1263, a com-

plete video channel from monitor input to CRT cathode can

be achieved. Performance is ideal for 1600 x 1200 resolution

displays with pixel clock frequencies up to 180 MHz.

Figure

10 and Figure 11 are the schematic for the NSC demonstra-

tion board that can be used to evaluate the LM126X/2463

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 LM2463 and

from the LM2463 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/2463 demonstration board. The schematic

of the board is shown in

Figure 10 and Figure 11. This board

DS200128-10

FIGURE 9. One Channel of the LM2463 with the Recommended Arc Protection Circuit

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