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LM2426 Datenblatt(PDF) 7 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Teilenummer LM2426
Bauteilbeschribung  Monolithic Triple Channel 30 MHz DTV Driver
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Hersteller  NSC [National Semiconductor (TI)]
Direct Link  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM2426 Datenblatt(HTML) 7 Page - National Semiconductor (TI)

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Application Hints (Continued)
POWER SUPPLY BYPASS
Since the LM2426TE is a wide bandwidth amplifier, proper
power supply bypassing is critical for optimum performance.
Improper power supply bypassing can result in large over-
shoot, ringing or oscillation. 0.1µF capacitors should be con-
nected from the supply pins, V
CC and VBB, to ground, as
close to the LM2426TE as is practical. Additionally, a 22µF or
larger electrolytic capacitor should be connected from both
supply pins to ground reasonably close to the LM2426TE.
ARC PROTECTION
During normal CRT operation, internal arcing may occasion-
ally occur. Spark gaps, in the range of 300V, connected from
the CRT cathodes to CRT ground will limit the maximum
voltage, but to a value that is much higher than allowable on
the LM2426TE. This fast, high voltage, high energy pulse
can damage the LM2426TE output stage. The application
circuit shown in Figure 13 is designed to help clamp the
voltage at the output of the LM2426TE to a safe level. The
clamp diodes, D1 and D2, should have a fast transient
response, high peak current rating, low series impedance
and low shunt capacitance. 1SS83 or equivalent diodes are
recommended. D1 and D2 should have short, low imped-
ance connections to V
CC and ground respectively. The cath-
ode of D1 should be located very close to a separately
decoupled bypass capacitor (C3 in Figure 13). The ground
connection of D2 and the decoupling capacitor should be
very close to the LM2426TE ground. This will significantly
reduce the high frequency voltage transients that the
LM2426TE would be subjected to during an arcover condi-
tion. Resistor R2 limits the arcover current that is seen by the
diodes while R1 limits the current into the LM2426TE as well
as the voltage stress at the outputs of the device. R2 should
be a 12W solid carbon type resistor. R1 can be a 14W metal
or carbon film type resistor. Having large value resistors for
R1 and R2 would be desirable, but this has the effect of
increasing rise and fall times. Inductor L1 is critical to reduce
the initial high frequency voltage levels that the LM2426TE
would be subjected to. The inductor will not only help protect
the device but it will also help minimize rise and fall times as
well as minimize EMI. For proper arc protection, it is impor-
tant to not omit any of the arc protection components shown
in Figure 13.
EFFECT OF LOAD CAPACITANCE
Figure 7 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.
EFFECT OF OFFSET
Figure 8 shows the variation in rise and fall times when the
output offset of the device is varied from 105 to 115V
DC. The
rise time shows a variation of less than 7% relative to the
center data point (110V
DC). The fall time shows a variation of
less than 2% relative to the center data point.
THERMAL CONSIDERATIONS
Figure 9 shows the performance of the LM2426TE in the test
circuit shown in Figure 3 as a function of case temperature.
The figure shows that the rise and fall times of the
LM2426TE increase by approximately 10% and 4%, respec-
tively, as the case temperature increases from 50˚C to 90˚C.
This corresponds to a speed degradation of 2.5% and 1% for
every 10˚C rise in case temperature.
Figure 10 shows the power dissipation of the LM2426TE vs.
Frequency when all three channels of the device are driving
an 8pF load with a 110V
PP alternating one pixel on, one pixel
off signal. The graph assumes a 72% active time (device
operating at the specified frequency) which is typical in a TV
application. The other 28% of the time the device is assumed
to be sitting at the black level (165V in this case). Table 1
also shows the typical power dissipation of the LM2426TE
for various video patterns in the 480i, 480p, 720p, and 1080i
video formats.
Figure 10, Figure 11, and Table 1 give the designer the
information needed to determine the heatsink requirement
for the LM2426TE. For example, if an HDTV application
uses the 720p format and "Vertical Lines 2 On 2 Off" is
assumed to be the worst-case pattern to be displayed, then
the power dissipated will be 14.4W (from Table 1). Figure 11
shows that the maximum allowed case temperature is 108˚C
when 14.4W is dissipated. If the maximum expected ambient
temperature is 70˚C, then a maximum heatsink thermal re-
sistance can be calculated:
This example assumes a capacitive load of 8pF 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.
Note: An LM126X preamplifier, with rise and fall times of
about 2 ns, was used to drive the LM2426TE for these power
measurements. Using a preamplifier with rise and fall times
slower than the LM126X will cause the LM2426TE to dissi-
pate less power than shown in Table 1.
OPTIMIZING TRANSIENT RESPONSE
In Figure 13, 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. In-
creasing 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 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 14 and Figure 15 can be used as a good starting point
for the evaluation of the LM2426TE. Using a variable resistor
for R1 will simplify finding the value needed for optimum
performance in a given application. Once the optimum value
is determined, the variable resistor can be replaced with a
fixed value.
20066410
FIGURE 13. One Channel of the LM2426TE with the
Recommended Application Circuit
www.national.com
7


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