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參數(shù)資料
| 型號： | LM2453TA |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 音頻／視頻放大 |
| 英文描述： | Monolithic Triple 6 nS CRT Driver With Integrated Clamp and G1 Blanking |
| 中文描述： | 3 CHANNEL, AUDIO/VIDEO AMPLIFIER, PZFM15 |
| 封裝： | PLASTIC, TO-220, 15 PIN |
| 文件頁數(shù)： | 7/14頁 |
| 文件大小： | 823K |
| 代理商： | LM2453TA |
Application Hints
(Continued)
IMPORTANT INFORMATION
The LM2453 performance is targeted for the high-resolution
market (up to 135 MHz pixel clock frequencies). It is the first
device in a new product family that is being developed by
National Semiconductor. Since this device is significantly dif-
ferent from all our previous CRT drivers, it is very important
to read all the application information supplied in this docu-
ment. Should you have additional questions, please contact
your local FAE or sales office.
Please refer to the NSC neck board schematic and layout
that are shown in Figures 17, 18, 19 during the following dis-
cussion.
POWER SUPPLY BYPASS
Since the LM2453 is a high bandwidth amplifier, proper
power supply bypassing is critical for optimum performance.
Improper power supply bypassing can result in large over-
shoot, ringing or oscillation.
V
CC
Bypassing
—A 0.1 μF (C48) capacitor should be con-
nected from the V
CC
supply pin (7) to ground, as close to the
supply pin and heatsink (device tab) ground point as pos-
sible.Additionally, a 47 μF electrolytic capacitor (C39) should
be connected through a ferrite bead (FB2 in the schematic)
to the supply pin and ground. A Fair-Rite 2743003112-TR
(100
@
100 MHz) or equivalent should be used.
V
Bypassing
—A 0.1 μF (C8) capacitor should be con-
nected from the V
supply to ground, as close as possible
to the device with as short as possible connections. Addition-
ally, a 47 μF electrolytic capacitor (C21) should be con-
nected through a ferrite bead (FB1) to the supply pin and
ground. A Fair-Rite 2743003112-TR (100
@
100 MHz) or
equivalent should be used.
REFERENCE VOLTAGE
The LM2453 receives a reference voltage from the LM1253A
preamp. It is critical that a 0.1 μF bypass cap be added from
the V
pin to ground. The ground connection should be as
close as possible to the video input side of the device (see
C30 in Figure 17 ). A 75
resistor should also be used be-
tween the V
REF
pin of the LM2453 and the V
pin of the
LM1253A. V
REF
should also be bypassed close to the
preamp.
THE VIDEO APPLICATION CIRCUIT
Figure 14 shows the recommended circuit topology for the
video amplifiers in the LM2453. This circuit is designed to
yield the best transient response and reduce radiated emis-
sions while also protecting the amplifier from damage that
can be caused by CRT Arcs. The schematic in Figures 17,
18 shows the values that yielded best performance in the
NSC reference design board. The following sections discuss
arc protection and transient response in detail. Refer to Fig-
ures 17, 18, 19 for the full application schematic and PCB
layout.
ARC Protection
During normal CRT operation, internal arcing may occasion-
ally occur. At the beginning of an arc event there is some
very high frequency ringing (
@
～
100 MHz). Spark gaps, in
the range of 200V, 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 LM2453. They are also
ineffective at limiting the peak voltage of the initial high fre-
quency burst. This fast, high voltage, high energy pulse can
damage the LM2453 video output stages. The application
circuit shown in Figure 14 is designed to help clamp the volt-
age at the output of the LM2453 to a safe level. The clamp
diodes should have a fast transient response, high peak cur-
rent rating, low series impedance and low shunt capaci-
tance. FDH400 or equivalent diodes are recommended. D1
and D2 should have short, low impedance connections to
V
and ground respectively. The cathode of D1 should be
located very close to a separately decoupled bypass capaci-
tor (C3 in Figure 14). The ground connection of the diode
and the decoupling capacitor should be very close to the
LM2453 ground and should be of as low impedance as pos-
sible. This will significantly reduce the high frequency voltage
transients that the LM2453 would be subjected to during an
arcover condition. Resistor R2 limits the arcover current that
is seen by the diodes while R1 limits the current into the
LM2453 as well as the voltage stress at the video outputs of
the device. R2 should be a 1/2W solid carbon type resistor.
R1 can be a 1/4W metal or carbon film type resistor. Inductor
L1 is critical to reduce the initial high frequency voltage lev-
els that the LM2453 would be subjected to. Having large
value resistors for R1 and R2 would be desirable, but this
has the effect of increasing rise and fall times. The inductor
will not only help protect the device but it will also help opti-
mize rise and fall times as well as minimize EMI. For proper
arc protection, it is important to not omit any of the arc pro-
tection components shown in Figure 14 Omitting any of
these components could cause serious reliability problems in
production.
Optimizing Transient Response
Referring to Figure 14 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 Mag-
netics (part # 78FXXXM, where XXX specifies the value)
were used for optimizing the performance of the device in
the NSC application board. The values shown in Figure 17
exhibited the best overall performance in a 17 inch monitor in
our lab. These can be used as a good starting point for the
evaluation of the LM2453 in a new monitor application. Us-
ing a variable resistor for R1 in series with a fixed value in-
ductor is a great way to help dial in the values needed for op-
timum performance in a given application. Once the
optimum values are determined the variable resistors can be
replaced with fixed values and corresponding inductor can
be installed. In addition to the output circuit, a series inductor
is used between the LM1253A video outputs and the
LM2453 inputs.
DS101302-14
FIGURE 14. One Video Channel of the LM2453 with the
Recommended Application Circuit
L
www.national.com
7
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