LM2460 National Semiconductor Corporation, LM2460 Datasheet - Page 6

• Manufacturer Part Number: LM2460
• Description: Monolithic Triple Channel High Swing CRT Driver
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Hints
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 0.8 ns for an increase of 1 pF in the
load capacitance. The fall time does remain almost the same
as the load capacitance is increased.
Effect of Offset
Figure 7 shows the variation in rise and fall times when the
output offset of the device is varied from 70 to 80 V
rise time has very little increase over its fastest point near
75V. The fall time becomes a little faster as the offset voltage
increases.
THERMAL CONSIDERATIONS
Figure 4 shows the performance of the LM2460 in the test
circuit shown in Figure 2 as a function of case temperature.
The figure shows that the rise time of the LM2460 increases
by approximately 13% as the case temperature increases
from 30˚C to 100˚C. This corresponds to a speed degrada-
tion of 2.0% for every 10˚C rise in case of temperature. The
fall time has almost no change as the case temperature
increases.
Figure 6 shows the maximum power dissipation of the
LM2460 vs Frequency when all three channels of the device
are driving an 8 pF load with a 60 V
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 (105V in
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
would also increase.
The LM2460 case temperature must be maintained below
100˚C. If the maximum expected ambient temperature is
70˚C and the maximum power dissipation is 11W (from
Figure 6, 70 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 LM2460 is shown in Figure 10
and Figure 11. Used in conjunction with a LM1267 pre-amp
and a LM2479 bias clamp, 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 80 MHz. Figure 10 and Figure 11 are the
(Continued)
p-p
alternating one pixel
DC
. The
6
schematic for the NSC demonstration board that can be
used to evaluate the LM1267/2460/2479 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 LM2460 and
from the LM2460 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 LM126X/246X/LM2479/80 demonstration board.
The schematic of the board is shown in Figure 10 and Figure
11. 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:
• C16, C19 — V
• C17, C20 — V
• C46, C47, C48 — V
The routing of the LM2460 outputs to the CRT is very critical
to achieving optimum performance. Figure 13 shows the
routing and component placement from pin 3 of the LM2460
to the blue cathode. The blue video path from the LM2460
output is shown by the darker traces. Note that the compo-
nents are placed so that they almost line up from the output
pin of the LM2460 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 effec-
tiveness of the protection diodes. The anode of protection
diode D8 is connected directly to a section of the ground
plane that has a short and direct path to the LM2460 ground
pins. The cathode of D9 is connected to V
decoupling capacitor C48 (see 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 LM2460 during an arc over event.
Lastly, notice that S3 is placed very close to the blue cathode
and is tied directly to CRT ground.
pin 4 and ground pins
8 and ground
and V
CC
clamp diodes. Very important for arc protection.
CC
BB
bypass capacitors, located close to pin
bypass capacitor, located very close to
CC
bypass capacitors, near LM2460
CC
very close to