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PDF LM2469 Data sheet ( Hoja de datos )
| Número de pieza | LM2469 | |
| Descripción | Monolothic Triple 9nS High Gain CRT Driver | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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October 2000
LM2469
Monolothic Triple 9nS High Gain CRT Driver
General Description
The LM2469 is an integrated high voltage CRT driver circuit
designed for use in color monitor applications. The IC con-
tains three high input impedance wide band amplifiers,
which directly drive the RGB cathodes of a CRT. Each
channel has its gain internally set to -20 and can drive CRT
capacitive loads as well as resistive loads present in other
applications, limited only by the package’s power dissipation.
The IC is packaged in an industry standard 9 lead TO-220
molded plastic package.
Features
n Higher gain to match LM126X CMOS preamplifiers
n 0V to 3.75V input range
n Stable with 0-20pF capacitive loads and inductive
peaking networks
n Maintains standard LM243X Family Pinout which is
designed for easy PCB layout
n Convenient TO-220 staggered 9 lead package style
Applications
n Up to 1024 X 768 at 70Hz
n Pixel clock frequencies up to 75MHz
n Monitors using video blanking
Schematic Diagram
Connection Diagram
DS200006-1
FIGURE 1. Simplified Schematic Diagram (One
Channel)
Note: Tab is at GND.
DS200006-2
FIGURE 2. Top View
Order Number: LM2469TA
NS package Number: TA09A
© 2000 National Semiconductor Corporation DS200006
www.national.com
1 page  
APPLICATION HINTS (Continued)
tors for R1 will simplify finding the values needed for opti-
mum performance in a given application. Once the optimum
value is determined, the variable resistors can be replaced
with fixed values.
Effect of Load Capacitance
Figure 14 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. Note that
the fall time stayed fairly constant while the rise time in-
creased approximately 1.8% per pF.
Effect of Offset
Figure 12 shows the variation in rise and fall times when the
output offset of the device is varied from 40VDC to 50 VDC.
The rise time shows a maximum variation relative to the
center data point (45 VDC) of less than 1.3%. The fall time
shows a variation of about 3.9% relative to the center data
point.
THERMAL CONSIDERATIONS
Figure 11 shows the performance of the LM2469 video
amplifiers in the test circuit shown in Figure 3 as a function of
case temperature. The figure shows that the rise time of the
LM2469 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 10 shows the maximum power dissipation of the
LM2469 vs. Frequency when all three channels of the device
are driving an 8pF load with a 40 Vp-p signal alternating one
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 LM2469 case temperature must be maintained below
100˚C. If the maximum expected ambient temperature is
70˚C and the maximum power dissipation is 3.85W (from
Figure 10, 50MHz 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
The typical application of the LM2469 is shown in Figure 5 &
6. Used in conjunction with an LM126X and an LM2479/
2480 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 frequen-
cies up top 75MHz. Figure 5 & 6 are the schematic for the
NSC demonstration board that can be used to evaluate the
LM126X/246X/2480 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
LM2469 and from the LM2469 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.
″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 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 7 shows the routing and component placement on the
NSC LM126X/246X demonstration board. The schematic of
the board is shown in Figure 5 & 6. 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 compo-
nents:
• C16, C19 — VCC bypass capacitor, located very close to
pin 4 and the ground plane near the device.
• C20 — VBB bypass capacitors, located close to pin 8 and
ground.
• C46, C47, C48 — VCC bypass capacitors, near LM2469
VCC clamp diodes. Very important for arc protection.
The routing of the LM2469 video outputs to the CRT is very
critical to achieving optimum performance. Figure 8 shows
the routing and component placement from pin 3 of the
LM2469 to the blue cathode. Note that the components are
placed so that they almost line up from the output pin of the
LM2469 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 LM2469 ground pins. The cath-
ode of D9 is connected to VCC very close to decoupling
capacitor C48 (see Figure 8), which is connected to the
same section of the ground plane as D8. The diode place-
ment and routing is very important for minimizing the voltage
stress on the LM2469 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.
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| PDF Descargar | [ Datasheet LM2469.PDF ] | |
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