lm2462ta National Semiconductor Corporation, lm2462ta Datasheet - Page 6

• Manufacturer Part Number: lm2462ta
• Description: Monolithic Triple 3 Ns Crt Driver
• Manufacturer: National Semiconductor Corporation

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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 # 78-FRK) 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 LM2462. 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 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. The rise
time increased about 0.08 nsec for an increase of 1 pF in the
load capacitance. The fall time increased about 0.14 nsec for
a 1 pF increase in the load capacitance.
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 varies less than 0.10 nsec. The fall time varies a
little under 0.50 nsec, but only 0.15 nsec from the fastest fall
time at 40V offset.
THERMAL CONSIDERATIONS
Figure 4 shows the performance of the LM2462 in the test
circuit shown in Figure 2 as a function of case temperature.
The figure shows that both the rise and fall times of the
LM2462 increase by approximately 46% as the case tem-
perature increases from 30˚C to 95˚C. This corresponds to a
speed degradation of 7.1% for every 10˚C rise in case
temperature.
Figure 6 shows the maximum power dissipation of the
LM2462 vs. Frequency when all three channels of the device
are driving an 8pF load with a 40V
on, one pixel off. 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
FIGURE 9. One Channel of the LM2462 with the Recommended Arc Protection Circuit
(Continued)
p-p
alternating one pixel
DC
. The
6
case). This graph gives the designer the information needed
to determine the heat sink requirement for his application.
The designer should note that if the load capacitance is
increased the AC component of the total power dissipation
will also increase.
The LM2462 case temperature must be maintained below
100˚C. If the maximum expected ambient temperature is
65˚C and the maximum power dissipation is 16.5W (from
Figure 6 , 100MHz bandwidth) 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 LM2462 is shown in Figure 10
and Figure 11 . Used in conjunction with an LM1262 video
pre-amp and an LM2479/2480 bias clamp, a complete video
channel from monitor input to CRT cathode can be achieved.
Performance is ideal for 1600 X 1200 resolution displays
with pixel clock frequencies up to 230MHz. Figure 10 and
Figure 11 are the schematic for the NSC demonstration
board that can be used to evaluate the LM1262/2462/2480
combination 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 LM2462 and
from the LM2462 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.
DS200376-10