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 # 78FR56M) were used for optimizing the

performance of the device in the NSC application board. The

values shown in Figure 9 can be used as a good starting

point for the evaluation of the LM2436. The NSC demo

board also has a position open to add a resistor in parallel

with L1. This resistor can be used to help control overshoot.

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 deter-

mined the variable resistors can be replaced with fixed val-

ues.

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

variation less than 4% relative to the center data point.

THERMAL CONSIDERATIONS

Figure 4 shows the performance of the LM2436 in the test

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

The figure shows that the rise time of the LM2436 increases

by approximately 8% as the case temperature increases

from 40˚C to 100˚C. This corresponds to a speed degrada-

tion of 1.3% for every 10˚C rise in case temperature.The fall

time increases by approximately 4% as the case tempera-

ture increases from 40˚C to 100˚C.

Figure 6 shows the maximum power dissipation of the

LM2436 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 LM2436 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 4.2W (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 LM2436 is shown in Figure 11.

Used in conjunction with an LM1279, 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 100 MHz. Figure 11 is the

schematic for the NSC demonstration board that can be

used to evaluate the LM1279/2436 combination in a monitor,

and Figure 10 shows the response at the red cathode for this

application. The input video rise time is 3.2ns, and the peak-

ing component values are those recommended in Figure 11.

Table 1 shows the typical cathode response of all three

channels.

20043510

FIGURE 9. One Channel of the LM2436 with the Recommended Application Circuit

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