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LM2402 Datasheet(PDF) 5 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Part No. LM2402
Description  Monolithic Triple 3 ns CRT Driver
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Maker  NSC [National Semiconductor (TI)]
Homepage  http://www.national.com
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LM2402 Datasheet(HTML) 5 Page - National Semiconductor (TI)

 
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Application Hints (Continued)
mance of the device in the NSC application board. The val-
ues shown in
Figure 9 can be used as a good starting point
for the evaluation of the LM2402.
Effect of Load Capacitance
The output rise and fall times as well as overshoot will vary
as the load capacitance varies. The values of the output cir-
cuit (R1, R2 and L1 in
Figure 9) should be chosen based on
the nominal load capacitance. Once this is done the perfor-
mance of the design can be checked by varying the load
based on what the expected variation will be during produc-
tion.
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
DC. The
rise and fall times show about the same overall variation.
The slightly faster fall time is fastest near the center point of
45V, making this the optimum operating point since there is
little increase in the rise time.
THERMAL CONSIDERATIONS
Figure 4 shows the performance of the LM2402 in the test
circuit shown in
Figure 2 as a function of case temperature.
Figure 4 shows that both the rise and fall times of the
LM2402 become slightly slower as the case temperature in-
creases from 40˚C to 125˚C. In addition to exceeding the
safe operating temperature, the rise and fall times will typi-
cally exceed 3 nsec. Please note that the LM2402 is never
to be operated over a case temperature of 100˚C.
Figure 6 shows the total power dissipation of the LM2402 vs.
Frequency when all three channels of the device are driving
both an 8 pF load and a 20 pF load. This graph gives the de-
signer the information needed to determine the heat sink re-
quirement 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 as shown in
Figure
6. The designer should also remember that the actual video
signal has a period of around 70% to 75%. The remainder of
the time the video signal is inactive, or at the black level (be-
low the black level if blanked). During this time the LM2402
will be at the black level, or below, dissipating under 4W. Re-
ferring to
Figure 14 and using an input black level voltage of
1.9V, the power dissipation during the inactive video time is
3.8W, including both the 80V and 12V supplies.
The LM2402 case temperature must be maintained below
100˚C. Assume the worst case operating condition is a
100 MHz square wave during active video (a pixel clock of
200 MHz with one pixel on, one pixel off). From
Figure 6 one
can see that the power dissipation of the LM2402 is 28W if
the 100 MHz square wave is applied all the time. One must
also compensate for the inactive period of video. From
Fig-
ure 14 it has been calculated that the power dissipation dur-
ing the inactive video is 4W. Therefore there is an additional
24W of power dissipation due to the AC signal. Assume that
the AC signal is active 72% of the time. Now the AC power
dissipation is:
24W x 0.72 = 17W
The total power dissipation for 72% active video time is:
17W+4W = 21W
If the maximum expected ambient temperature is 50˚C and
using the maximum power dissipation of 21W (video being
active only 72% of the frame), then a maximum heat sink
thermal resistance can be calculated:
This example assumes a capacitive load of 8 pF and no re-
sistive load.
TYPICAL APPLICATION
A typical application of the LM2402 is shown in
Figure 10.
Used in conjunction with three LM2202s, a complete video
channel from monitor input to CRT cathode can be achieved.
Performance is excellent for resolutions up to 1600 x 1200
and pixel clock frequencies at 200 MHz.
Figure 10 is the
schematic for the NSC demonstration board that can be
used to evaluate the LM2202/2402 combination in a monitor.
PC Board Layout Considerations
For optimum performance, an adequate ground plane, isola-
tion between channels, good supply bypassing and minimiz-
ing unwanted feedback are necessary. Also, the length of the
signal traces from the preamplifier to the LM2402 and from
the LM2402 to the CRT cathode should be as short as pos-
sible. The red video trace from the buffer transistor to the
LM2402 input is about the absolute maximum length one
should consider on a PCB layout. If possible the traces
should actually be shorter than the red video trace. The fol-
lowing references are recommended for video board design-
ers:
Ott, Henry W., “Noise Reduction Techniques in Electronic
Systems”, John Wiley & Sons, New York, 1976.
“Guide to CRT Video Design”, National Semiconductor Appli-
cation Note 861.
“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 os-
cillate in a monitor if feedback occurs around the video chan-
nel through the chassis wiring. To prevent this, leads to the
video amplifier input circuit should be shielded, and input cir-
cuit wiring should be spaced as far as possible from output
circuit wiring.
NSC Demonstration Board
Figures 11, 12 show routing and component placement on
the NSC LM2202/2402 demonstration board. The schematic
of the board is shown in
Figure 10. This board provides a
good example of a layout that can be used as a guide for fu-
ture layouts. Note the location of the following components:
C47 -V
CC bypass capacitor, located very close to pin 6
and ground pins. (
Figure 12)
C49 -V
BB bypass capacitor, located close to pin 10 and
ground. (
Figure 12)
C46 and C77 -V
CC bypass capacitors, near LM2402 and
V
CC clamp diodes. Very important for arc protection. (Fig-
ure 11)
The routing of the LM2402 outputs to the CRT is very critical
to achieving optimum performance.
Figure 13 shows the
routing and component placement from pin 1 to the blue
cathode. Note that the components are placed so that they
almost line up from the output pin of the LM2402 to the blue
www.national.com
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