lm2467 National Semiconductor Corporation, lm2467 Datasheet - Page 6

• Manufacturer Part Number: lm2467
• Description: Monolithic Triple 7.5 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 # 78FR--k) were used for optimizing the
performance of the device in the NSC application board. The
values shown in Figure 10 and Figure 11 can be used as a
good starting point for the evaluation of the LM2467. 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 5% relative to the center data point.
THERMAL CONSIDERATIONS
Figure 4 shows the performance of the LM2467 in the test
circuit shown in Figure 2 as a function of case temperature.
The figure shows that the rise time of the LM2467 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.There is a
negligible change in fall time vs. temperature in the test
circuit.
Figure 6 shows the maximum power dissipation of the
LM2467 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
this case). This graph gives the designer the information
needed to determine the heat sink requirement for his appli-
DC
FIGURE 9. One Channel of the LM2467 with the Recommended Arc Protection Circuit
) less than 5%. The fall time shows a
(Continued)
p-p
alternating one pixel
DC
. The
6
cation. The designer should note that if the load capacitance
is increased the AC component of the total power dissipation
will also increase.
The LM2467 case temperature must be maintained below
100˚C. If the maximum expected ambient temperature is
70˚C and the maximum power dissipation is 4.3W (from
Figure 6 , 50 MHz 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 LM2467 is shown in Figure 10
and Figure 11 . Used in conjunction with an LM1267, a com-
plete 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 10
and Figure 11 are the schematic for the NSC demonstration
board that can be used to evaluate the LM1267/2467 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 LM2467 and
from the LM2467 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.
NSC DEMONSTRATION BOARD
Figure 12 shows the routing and component placement on
the NSC LM1267/2467 demonstration board. The schematic
of the board is shown in Figure 10 and Figure 11 . This board
DS200078-10