lm2468ta National Semiconductor Corporation, lm2468ta Datasheet - Page 5

• Manufacturer Part Number: lm2468ta
• Description: Monolothic Triple 14ns High Gain Crt Driver
• Manufacturer: National Semiconductor Corporation

APPLICATION HINTS
resistors for R1 will simplify finding the values needed for
optimum performance in a given application. Once the opti-
mum value is determined, the variable resistors can be
replaced with fixed values.
Effect of Load Capacitance
Figure 9 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 40VDC to 50VDC.
The rise and fall times shows a maximum variation relative to
the center data point (45VDC) of about 7%.
THERMAL CONSIDERATIONS
Figure 6 shows the performance of the LM2468 video am-
plifiers in the test circuit shown in Figure 3 as a function of
case temperature. The figure shows that the rise time of the
LM2468 increases by approximately 9% as the case tem-
perature increases from 30˚C to 100˚C. This corresponds to
a speed degradation of 1.3% for every 10˚C rise in case
temperature. The fall time degrades around 0.6% for every
10˚C in case temperature.
Figure 5 shows the maximum power dissipation of the
LM2468 vs. Frequency when all three channels of the device
are driving an 8pF load with a 40 V
pixel 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
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 LM2468 case temperature must be maintained below
115˚C. If the maximum expected ambient temperature is
70˚C and the maximum power dissipation is 2.95W (from
Figure 5 , 50 MHz bandwith), then a maximum heat sink
thermal resistance can be calculated:
This example assumes a capacitive load of 8pF and no
resistive load.
TYPICAL APPLICATION
A typical application of the LM2468 is shown in Figures 11,
12 . Used in conjunction with an LM1269 coupled with an
LM2479/2480 bias clamp, a complete video channel from
monitor input to CRT cathode can be achieved. Performance
is ideal for 1024x768 resolution displays with pixel clock
frequencies up to 50 MHz. Figures 11, 12 are the schematic
for the NSC demonstration board that can be used to evalu-
ate the LM1269/2468 combination in a monitor.
PC Board Layout Considerations
For optimum performance, an adequate ground plane, iso-
lation between channels, good supply bypassing and the
minimization of unwanted feedback are necessary. Also, the
length of the signal traces from the preamplifier to the
p-p
(Continued)
signal alternating one
5
LM2468 and from the LM2468 to the CRT cathode should be
as short as possible. The following references are recom-
mended:
Ott, Henry W., Noise Reduction Techniques in Electronic
Systems , John Wiley & Sons, New York, 1976.
Semiconductor Application Note 1013.
Pease, Robert A.,
Butterworth-Heinemann, 1991.
Because of its high small signal bandwith, 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
wiring should be spaced as far as possible from output circuit
wiring.
NSC Demonstration Board
Figure 13 shows the routing and component placement on
the NSC LM126X/246X demonstration board. The sche-
matic of the board is shown in Figures 11, 12 . This board
provides a good example of a layout that can be used as a
guide for future layouts. Note the location of the following
components:
The routing of the LM2468 video outputs to the CRT is very
critical to achieving optimum performance. Figure 13 shows
the routing and component placement from pin 3 of the
LM2468 to the blue cathode. Note that the components are
placed so that they almost line up from the output pin of the
LM2468 to the blue cathode pin of the CRT connector. This
is done to minimize the length of the video path between
these two components. Note also that D8, D9, R24, and D6
are placed to minimize the size of the video nodes that they
are attached to. This minimizes parasitic capacitance in the
video path and also enhances the effectiveness of the pro-
tection diodes. The anode of protection diode D8 is con-
nected directly to a section of the ground plane that has a
short and direct path to the LM2468 ground pins. The cath-
ode of D9 is connected to V
capacitor C48 (near the center of Figure 13 ), which is con-
nected to the same section of the ground plane as D8. The
diode placement and routing is very important for minimizing
the voltage stress on the LM2468 video outputs during an
arc over event. Lastly, notice that S3 is placed very close to
the blue cathode and is tied directly to the ground under the
CRT connector.
• C16, C19 — V
• C20 — V
• C46, C47, C48 — V
Video Amplifier Design for Computer Monitors , National
pin 4 and the ground plane near the device.
ground.
V
CC
clamp diodes. Very important for arc protection.
BB
bypass capacitors, located close to pin 8 and
CC
bypass capacitor, located very close to
CC
Troubleshooting Analog Circuits ,
bypass capacitors, near LM2468
CC
very close to decoupling
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