lm2421 National Semiconductor Corporation, lm2421 Datasheet - Page 7

• Manufacturer Part Number: lm2421
• Description: Monolithic Triple Channel Hdtv Driver
• Manufacturer: National Semiconductor Corporation

Application Hints
OPTIMIZING TRANSIENT RESPONSE
Referring to Figure 10 , 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 # 78FR56k) 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 LM2421. Using a
variable resistor for R1 will simplify finding the value needed
for optimum performance in a given application. Once the
optimum value is determined, the variable resistor can be
replaced with a fixed value.
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 75 to 85V
rise time shows a maximum variation relative to the center
data point (80V
less than 5% relative to the center data point.
THERMAL CONSIDERATIONS
Figure 4 shows the performance of the LM2421 in the test
circuit shown in Figure 2 as a function of case temperature.
The figure shows that the rise and fall times of the LM2421
increase by approximately 5% as the case temperature in-
creases from 50˚C to 70˚C. This corresponds to a speed
degradation of 2.5% for every 10˚C rise in case temperature.
Figure 6 shows the maximum power dissipation of the
LM2421 vs. Frequency when all three channels of the device
are driving an 8pF load with a 100V
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 (130V in
this case). This graph gives the designer the information
needed to determine the heat sink requirement for his appli-
DC
FIGURE 10. One Channel of the LM2421 with the Recommended Arc Protection Circuit
) of 4% . The fall time shows a variation of
(Continued)
p-p
alternating one pixel
DC
. The
7
cation. The designer should note that if the load capacitance
is increased the AC component of the total power dissipation
will also increase.
Figures 6 and 9 are used to design the heatsink for the
LM2421. For example, if the maximum bandwith needed will
be 40MHz (from Figure 6, 40MHz or Frequency = 20MHz),
the power dissipated will be 14.5W. Figure 9 shows that the
maximum allowed case temperature is 110˚C when 14.5W is
dissipated. If the maximum expected ambient temperature is
70˚C, then a maximum heatsink 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 LM2421 is shown in Figure 11 and
Figure 12 . Used in conjunction with an LM126X, a complete
video channel from input to CRT cathode can be achieved.
Performance is ideal for HDTV applications. Figure 11 and
Figure 12 are the schematic for the NSC demonstration
board that can be used to evaluate the LM126X/2421 com-
bination in a monitor.
PC BOARD LAYOUT CONSIDERATIONS
For optimum performance, an adequate ground plane, iso-
lation between channels, good supply bypassing and mini-
mizing unwanted feedback are necessary. Also, the length of
the signal traces from the preamplifier to the LM2421 and
from the LM2421 to the CRT cathode should be as short as
possible. The following references are recommended:
Ott, Henry W., “Noise Reduction Techniques in Electronic
Systems”, John Wiley & Sons, New York, 1976.
“Video Amplifier Design for Computer Monitors”, National
Semiconductor Application Note 1013.
Pease, Robert A., “Troubleshooting Analog Circuits”,
Butterworth-Heinemann, 1991.
Because of its high small signal bandwidth, the part may
oscillate in a monitor if feedback occurs around the video
channel through the chassis wiring. To prevent this, leads to
the video amplifier input circuit should be shielded, and input
circuit wiring should be spaced as far as possible from output
circuit wiring.
DS200233-10
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