NEC MultiSync FE700 Serviceanleitung Seite 150

OPTIMIZING TRANSIENT RESPONSE
Referring to Figure 9, there are three components (R1, R2 and L1) that can be adjusted to optimize the
transient response of the application circuit. Increasing the values of R1 and R2 will slow the circuit down
while decreasing overshoot. Increasing the value of L1 will speed up the circuit as well as increase
overshoot. It is very important to use inductors with very high self-resonant frequencies, preferably above
300 MHz. Ferrite core inductors from J.W. Miller Magnetics (part # 78FR82K) were used for optimizing the
performance of the device in the NSC application board. The values shown in Figure 9can be used as a
good starting point for the evaluation of the LM2439. 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 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 7shows the variation in rise and fall times when the output offset of the device is varied from 40 V
DC
to 50 V . The rise time shows a maximum variation relative to the center data point (45 V ) of about 21% .
DC DC
The fall time shows a variation of about 3% relative to the center data point.
THERMAL CONSIDERATIONS
Figure 4 shows the performance of the LM2439 in the test circuit shown in Figure 2as a function of case
temperature. The figure shows that the rise time of the LM2439 increases by approximately 3% as the case
temperature increases from 50°C to 100°C. This corresponds to a speed degradation of 0.6% for every 10°C
rise in case temperature. The fall time increases by approximately 3% which corresponds to a speed
degradation of 0.6% for every 10°C rise in case temperature. Figure 6 shows the maximum power
dissipation of the LM2439 vs Frequency when all three channels of the device are driving an 8pF load with a
40 Vp-p alternating one pixel 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 the application. The designer should note that
if the load capacitance is increased the AC component of the total power dissipation will also increase. The
LM2439 case temperature must be maintained below 115°C. If the maximum expected ambient temperature
is 70°C and the maximum power dissipation is 3.4W (from Figure 6,40MHz bandwidth) then a maximum
heat sink thermal resistance can be calculated:
This example assumes a capacitive load of 8pF and no resistive load.
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