lm2466 National Semiconductor Corporation, lm2466 Datasheet - Page 7

• Manufacturer Part Number: lm2466
• Description: Monolithic Triple 7.5 Ns Crt Driver
• Manufacturer: National Semiconductor Corporation

Application Hints
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 LM2466. 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 40 to 50 V
rise time shows a maximum variation relative to the center
data point (45 V
variation of less than 8% relative to the center data point.
THERMAL CONSIDERATIONS
Figure 4 shows the performance of the LM2466 in the test
circuit shown in Figure 2 as a function of case temperature.
The figure shows that the rise time of the LM2466 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
LM2466 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
DC
FIGURE 9. One Channel of the LM2466 with the Recommended Application Circuit
) less than 5%. The fall time shows a
(Continued)
p-p
alternating one pixel
DC
. The
7
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 LM2466 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 4.4W (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 LM2466 is shown in Figure 11 and
Figure 12. Used in conjunction with an LM1267, 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 LM1267/2466 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 3.2ns, and the peaking component values
are those recommended in Figure 12. Table 1 shows the
typical cathode response of all three channels.
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