Mitsubishi DPLUS 74SB -BKA Service Manual page 146

OPTIMIZING TRANSIENT RESPONSE
Referring to Figure 4, there are three components (R1, R2 and
L1) that can be adjusted to optimize the transient response of
the application circuit. Increasing the values of R1 and R2 will
slow the circuit down while decreasing overshoot. Increasing
the value of L1 will speed up the circuit as well as increase
overshoot. It is very important to use inductors with very high
self-resonant frequencies, preferably above 300MHz. Ferrite
core inductors from J.W. Miller Magnetics (part #78FR39K) were
used for optimizing the performance of the device in the NSC
application board. The values shown in Figure 4 can be used
as a good starting point for the evaluation of the LM2469. Us-
ing variable resistors for R1 and the parallel resistor will sim-
plify 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 13 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 increased
approximately 1.8% per pF.
Effect of Offset
Figure 11 shows the variation in rise and fall times when the
output offset of the device is varied from 40VDC to 50VDC.
The rise time shows a maximum variation relative to the center
data point (45VDC) of less than 1.3%. The fall time shows a
variation of about 3.9% relative to the center data points.
THERMAL CONSIDERATIONS
Figure 10 shows the performance of the LM2469 video ampli-
fiers 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 temperature in-
creases from 30°C to 100°C. This corresponds to a speed deg-
radation of 1.3% for very 10°C rise in case temperature.
Figure 9 shows the maximum power dissipation of the LM2469
vs. Frequency when all three channels of the device are driv-
ing an 8pF load with a 40V
one pixel off. The graph assumes a 72% active time (device
operating at the specified frequency) which is typical in a moni-
tor application. The other 28% of the time the device is as-
sumed 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 9, 50MHz
bandwith), then a maximum heat sink thermal resistance can be
calculated:
° − °
100 C 70
= =
R 7 8 .
TH
. 3 85 W
This example assumes a capacitive load of 8pF and no resistive
load.
signal alternating one pixel on,
p-p
°
C / W
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 frequencies up top 75MHz.
Figure 6 are the schematic for the NSC demonstration board
that can be used to evaluate the LM126X/246X/2480 combina-
tion in a monitor.
PC Board Layout Considerations
For optimum performance, an adequate ground plane, isola-
tion between channels, good supply bypassing and minimizing
unwanted feedback are necessary. Also, the length of the sig-
nal 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 recommended:
Ott, Henry W., "Noise Reduction Techniques in Electronic
Systems", John Wiley & Sons, New York, 1976.
"Video Amplifier Design for Computer Monitors", National Semi-
conductor Application Note 1013.
Pease, Robert A., "Troubleshooting Analog Circuits",
Butterworth-Heinemann, 1991.
Because of its high small signal bandwith, the part may oscil-
late 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 6. This board provides a good ex-
ample of a layout that can be used as a guide for future layouts.
Note the location of the following components:
• C16, C19 – V bypass capacitor, located very close to pin 4
CC
ground pins.
• C17, C20 – V bypass capacitors, located close to pin 8 and
BB
ground.
• C46, C47, C48 – V
clamp diodes. Very important for arc protection.
The routing of the LM2469 video outputs to the CRT is very
critical to achieving optimum performance. It 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 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 LM2469 ground pins. The
cathode of D9 is connected to V
pacitor C48 which is connected 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 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.
7-45
bypass capacitors near LM2469 V
CC
very close to decoupling ca-
CC
CC