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 11 and Figure 12 can be used as a

good starting point for the evaluation of the LM2470. Using

variable resistors for R1 and the parallel resistor will simplify

finding the values needed for optimum performance in a

given application. Once the optimum values are determined

the variable resistors can be replaced with fixed values.

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 50 to 60 V

rise time shows a maximum variation relative to the center

data point (55 V

variation of less than 1% relative to the center data point.

THERMAL CONSIDERATIONS

Figure 4 shows the performance of the LM2470 in the test

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

The figure shows that the rise time of the LM2470 increases

by approximately 10% as the case temperature increases

from 50˚C to 100˚C. This corresponds to a speed degrada-

tion of 2% for every 10˚C rise in case temperature. The fall

time increases by approximately 7% as the case tempera-

ture increases from 50˚C to 100˚C.

Figure 6 shows the maximum power dissipation of the

LM2470 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 (75V 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 LM2470 case temperature must be maintained below

100˚C. If the maximum expected ambient temperature inside

the monitor is 70˚C and the power dissipation is 5.3W (from

Figure 6, 50 MHz max. video frequency), 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 LM2470 is shown in Figure 11 and

Figure 12. Used in conjunction with an LM1246, 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 95 MHz. Figure 11

and Figure 12 are the schematic for the NSC demonstration

board that can be used to evaluate the LM1246/2471 com-

bination in a monitor, and Figure 10 shows the typical re-

sponse at the red cathode for this application. The input

video rise time is 4.5ns, and the peaking component values

are those recommended in Figure 12. Table 1 shows the

typical 40Vpp cathode response of all three channels. Table

2 shows the typical 60Vpp cathode response of all three

channels.

20087110

FIGURE 9. One Channel of the LM2470 with the Recommended Application Circuit

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