OPTIMIZING TRANSIENT RESPONSE

Referring to Figure 12, 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 14 can be used as a good starting

point for the evaluation of the LM2476. Using a variable

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 OFFSET

Figure 8 shows the variation in rise and fall times when the

output offset of the device is varied from 50 to 60VDC. The

rise time shows a maximum variation of 13% relative to the

center data point (55 V

variation of less than 3% relative to the center data point.

EFFECT OF LOAD CAPACITANCE

Figure 9 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.

THERMAL CONSIDERATIONS

Figure 10 shows the performance of the LM2476 in the test

circuit shown in Figure 4 as a function of case temperature

(on the device tab). The figure shows that the rise and fall

times of the LM2476 increase by approximately 10% and

5%, respectively, as the case temperature increases from

50˚C to 100˚C. This corresponds to a speed degradation of

2% and 1%, respectively, for every 10˚C rise in case tem-

perature.

Figure 11 shows the maximum power dissipation of the

LM2476 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 LM2476 case temperature must be maintained below

100˚C. If the maximum expected ambient temperature is

70˚C and the maximum power dissipation is about 6W (from

Figure 11, 95 MHz bandwidth), then a maximum heat sink

thermal resistance can be calculated:

PACKAGE MOUNTING CONSIDERATIONS

Mounting of the TO-247 package to a heat sink must be

done such that there is sufficient pressure from the mounting

screws to insure good contact with the heat sink for efficient

heat flow. The surface of the heat sink should be free of

contaminants before mounting to insure good contact. Incor-

rect mounting may lead to both thermal and mechanical

problems. Over tightening the mounting screws will cause

the package to warp, reducing contact area with the heat

sink. Less contact with the heat sink will increase the thermal

resistance from the package case to the heat sink (

θ

resulting in higher operating die temperatures. Extreme over

tightening of the mounting screws will cause severe physical

stress resulting in a cracked or chipped package and pos-

sible catastrophic IC failure.

The recommended mounting screw size is M3 with a maxi-

mum torque of 50 N-cm. It is best to use fiber washers under

the screws to distribute the force over a wider area. Addi-

tionally, if the mounting screws are used to force the pack-

age into correct alignment with the heat sink, package stress

20121910

FIGURE 12. One Channel of the LM2476 with the Recommended Application Circuit

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