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參數(shù)資料
| 型號： | LM246N |
| 廠商： | 意法半導(dǎo)體 |
| 元件分類： | 運(yùn)動(dòng)控制電子 |
| 英文描述： | PROGRAMMABLE QUAD BIPOLAR OPERATIONAL AMPLIFIERS |
| 中文描述： | 可編程四雙極運(yùn)算放大器 |
| 文件頁數(shù)： | 5/12頁 |
| 文件大?。?/td> | 423K |
| 代理商： | LM246N |
Theory of Operation
The LM2465 is a high voltage monolithic three channel CRT
driver suitable for high resolution display applications. The
LM2465 operates with 80V and 12V power supplies. The
part is housed in the industry standard 9-lead TO-220
molded plastic power package.
The circuit diagram of the LM2465 is shown in Figure 1 The
PNP emitter follower, Q5, provides input buffering. Q1 and
Q2 form a fixed gain cascode amplifier with resistors R1 and
R2 setting the gain at 20. Emitter followers Q3 and Q4
isolate
the
high
output
impedance
decreasing the sensitivity of the device to changes in load
capacitance. Q6 provides biasing to the output emitter
follower stage to reduce crossover distortion at low signal
levels.
Figure 2 shows a typical test circuit for evaluation of the
LM2465. This circuit is designed to allow testing of the
LM2465 in a 50
environment without the use of an
expensive FET probe. In this test circuit, two low inductance
resistors in series totaling 4.95k
form a 200:1 wideband,
low capacitance probe when connected to a 50
coaxial
cable and a 50
load (such as a 50
oscilloscope input).
The input signal from the generator is ac coupled to the base
of Q5.
of
the
amplifier,
Application Hints
INTRODUCTION
National Semiconductor (NSC) is committed to provide
application information that assists our customers in
obtaining the best performance possible from our products.
The following information is provided in order to support this
commitment.
The
reader
should
optimization of performance was done using a specific
printed circuit board designed at NSC. Variations in
performance can be realized due to physical changes in the
printed circuit board and the application. Therefore, the
designer should know that component value changes may
be required in order to optimize performance in a given
application. The values shown in this document can be used
as a starting point for evaluation purposes. When working
with high bandwidth circuits, good layout practices are also
critical to achieving maximum performance.
be
aware
that
the
IMPORTANT INFORMATION
The LM2465 performance is targeted for the 17“ and low end
19“ monitor market with resolutions up to 1280 X 1024 and a
75Hz refresh rate. It is designed to be a replacement for
discrete CRT drivers. The application circuits shown in this
document to optimize performance and to protect against
damage from CRT arcover are designed specifically for the
LM2465. If another member of the LM246X family is used,
please refer to its datasheet.
POWER SUPPLY BYPASS
Since the LM2465 is a wide bandwidth amplifier, proper
power supply bypassing is critical for optimum performance.
Improper power supply bypassing can result in large
overshoot, ringing or oscillation.A0.1 μF capacitor should be
connected from the supply pin, V
and V
, to ground, as
close to the LM2465 as is practical. Additionally, a 47μF or
larger electrolytic capacitor should be connected from both
supply pins to ground reasonably close to the LM2465.
ARC PROTECTION
During
normal
occasionally occur. 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 LM2465. This fast, high voltage, high
energy pulse can damage the LM2465 output stage. The
application circuit shown in Figure 9 is designed to help
clamp the voltage at the output of the LM2465 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. FDH400 or equivalent diodes
are recommended. Do not use 1N4148 diodes for the clamp
diodes. 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 9). The ground connection of
D2 and the decoupling capacitor should be very close to the
LM2465 ground. This will significantly reduce the high
frequency voltage transients that the LM2465 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 LM2465 as well as the voltage stress at
the outputs of the device. R2 should be a
1
2
W solid carbon
type resistor. R1 can be a
1
4
W 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 LM2465 would be
subjected to. 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 9
CRT
operation,
internal
arcing
may
L
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