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參數(shù)資料
| 型號： | LM2457 |
| 文件頁數(shù)： | 7/14頁 |
| 文件大?。?/td> | 823K |
| 代理商： | LM2457 |
Application Hints
(Continued)
IMPORTANT INFORMATION
The LM2457 performance is targeted for monitor applica-
tions with pixel clock frequencies up to 95MHz. It is a part of
the AC2DC new product family that is being developed by
National Semiconductor. Since this device is significantly
different from all our previous CRT drivers, it is very impor-
tant to read all the application information supplied in this
document. 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
discussion.
POWER SUPPLY BYPASS
Since the LM2457 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. Addi-
tionally, 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 LM2457 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 18 ). A 300
resistor should also be used
between the V
REF
pin of the LM2457 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 LM2457. 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
Figures 17, 18, 19for 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 LM2457. They are also
ineffective at limiting the peak voltage of the initial high
frequency burst. This fast, high voltage, high energy pulse
can damage the LM2457 video output stages. The applica-
tion circuit shown in Figure 14 is designed to help clamp the
voltage at the output of the LM2457 to a safe level. The
clamp diodes should have a fast transient response, high
peak current rating, low series impedance and low shunt
capacitance. FDH400 or equivalent diodes are recom-
mended. 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 capacitor (C3 in Figure 14). The ground connection
of the diode and the decoupling capacitor should be very
close to the LM2457 ground and should be of as low imped-
ance as possible. This will significantly reduce the high
frequency voltage transients that the LM2457 would be sub-
jected to during an arcover condition. Resistor R2 limits the
arcover current that is seen by the diodes while R1 limits the
current into the LM2457 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 levels that the LM2457 would be sub-
jected 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 optimize rise and fall times as well as minimize
EMI. For proper arc protection, it is important to not omit any
of the arc protection components shown in Figure 14 Omit-
ting 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
Magnetics (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 18exhibited 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 LM2457 in a new monitor
application. Using a variable resistor for R1 in series with a
fixed value inductor is a great way to help dial in the values
needed for optimum performance in a given application.
Once the optimum values are determined the variable resis-
tors 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 LM2457 inputs.
DS101379-14
FIGURE 14. One Video Channel of the LM2457 with the
Recommended Application Circuit
L
www.national.com
7
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