LM2422TE
Application Hints
(Continued)
Figure 10
shows the maximum power dissipation of the
LM2422 vs. Frequency when all three channels of the device
are driving into a 10 pF load with a 110V
P-P
alternating one
pixel on, one pixel off. Note that the frequency given in
Figure 10
is half of the pixel frequency. The graph assumes
a 72% active time (device operating at the specified fre-
quency), which is typical in a TV application. The other 28%
of the time the device is assumed to be sitting at the black
level (190V in this case). A TV picture will not have frequency
content over the whole picture exceeding 20 MHz. It is
important to establish the worst case condition under normal
viewing to give a realistic worst-case power dissipation for
the LM2422. One test is a 1 to 30 MHz sine wave sweep
over the active line. This would give a slightly lower power
than taking the average of the power between 1 and 30 MHz.
This average is 23.5 W. A sine wave will dissipate slightly
less power, probably about 21 W or 22 W of power dissipa-
tion. All of this information is critical for the designer to
establish the heat sink requirement for his application. The
designer should note that if the load capacitance is in-
creased the AC component of the total power dissipation will
also increase.
The LM2422 case temperature must be maintained below
110藲C given the maximum power dissipation estimate of
22W. If the maximum expected ambient temperature is 60藲C
and the maximum power dissipation is 22W then a maximum
heat sink thermal resistance can be calculated:
the signal traces from the signal inputs to the LM2422 and
from the LM2422 to the CRT cathode should be as short as
possible. The following references are recommended:
Ott, Henry W., 鈥淣oise Reduction Techniques in Electronic
Systems鈥? John Wiley & Sons, New York, 1976.
鈥淰ideo Amplifier Design for Computer Monitors鈥? National
Semiconductor Application Note 1013.
Pease, Robert A., 鈥淭roubleshooting Analog Circuits鈥?
Butterworth-Heinemann, 1991.
Because of its high small signal bandwidth, the part may
oscillate in a TV 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.
TYPICAL APPLICATION
A typical application of the LM2422 is shown in
Figure 14.
Used in conjunction with a pre-amp with a 1.2V black level
output no buffer transistors are required to obtain the correct
black level at the cathodes. If the pre-amp has a black level
closer to 2V, then an NPN transistor should be used to drop
the video black level voltage closer to 1.2V.
The neck board in
Figure 14
has two transistors in each
channel enabling this board to work with pre-amps with a
black level output as high as 2.5V. Some popular AVPs do
have a black level of 2.5V. For lower black levels either one
or both transistors would not be used.
It is important that the TV designer use component values for
the driver output stage close to the values shown in
Figure
14.
These values have been selected to protect the LM2422
from arc over. Diodes D1鈥揇6 must also be used for proper
arc over protection. The NSC demonstration board can be
used to evaluate the LM2422 in a TV.
NSC DEMONSTRATION BOARD
Figure 15
shows the routing and component placement on
the NSC LM2422 demonstration board. 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:
鈥?/div>
C4 鈥?V
CC
bypass capacitor, located very close to pin 2
and ground pins
鈥?/div>
C6 鈥?V
BB
bypass capacitor, located close to pin 11 and
ground
鈥?/div>
C5, C8 鈥?V
CC
bypass capacitors, near LM2422 and V
CC
clamp diodes. Very important for arc protection.
The routing of the LM2422 outputs to the CRT is very critical
to achieving optimum performance.
Figure 16
shows the
routing and component placement from pin 10 (V
OUT1
) of the
LM2422 to the blue cathode. Note that the components are
placed so that they almost line up from the output pin of the
LM2422 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 D1, D2 and R3 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 D2 is con-
nected directly to a section of the ground plane that has a
short and direct path to the heater ground and the LM2422
ground pins. The cathode of D1 is connected to V
CC
very
close to decoupling capacitor C5 which is connected to the
This example assumes a capacitive load of 10 pF and no
resistive load. The designer should note that if the load
capacitance is increased the AC component of the total
power dissipation will also increase.
OPTIMIZING TRANSIENT RESPONSE
Referring to
Figure 13,
there are three components (R1, R2
and L1) that can be adjusted to optimize the transient re-
sponse of the application circuit. Increasing the values of R1
and R2 will slow the circuit down while decreasing over-
shoot. Increasing the value of L1 will speed up the circuit as
well as increase overshoot. It is very important to use induc-
tors with very high self-resonant frequencies, preferably
above 300 MHz. Ferrite core inductors from J.W. Miller
Magnetics (part # 78FR--K) were used for optimizing the
performance of the device in the NSC application board. The
values shown in
Figure 13
can be used as a good starting
point for the evaluation of the LM2422. Using a variable
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. Due to arc over considerations it is
recommended that the values shown in
Figure 13
not be
changed by a large amount.
Figure 12
shows the typical cathode pulse response with an
output swing of 110V
PP
inside a modified production TV set
using the LM1237 pre-amp.
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
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