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参数资料
| 型号： | LM1283N |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 音频／视频放大 |
| 英文描述： | 140 MHz RGB Video Amplifier System with On Screen Display (OSD) |
| 中文描述： | 3 CHANNEL, VIDEO AMPLIFIER, PDIP28 |
| 封装： | 0.600 IINCH, PLASTIC, DIP-28 |
| 文件页数： | 12/13页 |
| 文件大小： | 401K |
| 代理商： | LM1283N |
Applications of the LM1283
(Continued)
ommended; one to V
and one to ground. Normally a de-
signer may want to increase the value of the 33
resistor at
pins 5, 8, and 11 for additional ESD protection at the video
inputs. Remember that the input capacitor to the video inputs
is also part of the DC restoration circuit. This circuit is de-
pending on a maximum circuit resistance of about 110
. The
33
resistors should not be increased in value. The designer
does have the option of decreasing the 10 μF video input ca-
pacitors down to 1 μF. The internal ESD protection and the
external clamp diodes, one to +12V and the other to ground,
will provide excellent ESD protection.
Interfacing to the OSD inputs is quite easy since the signal
processing necessary to match the OSD signals to the video
levels is done internally by the LM1283. However, proper de-
sign techniques must be followed in assuring that a good
TTL signal is received at the LM1283. Ground bounce in the
TTL signal can cause improper switching times, possibly
with multiple switching. Such affects will result in degradation
in the quality of the displayed OSD window. The final TTL
stage needs to be located near the LM1283 to assure clean
TTL signals. Propagation delay is another source capable of
degrading the OSD display. The optimum condition is to
have all OSD signals originate from one register, keeping the
variation in the propagation delays under 5 ns. If the OSD
feature is not used, or the lines may be disconnected for
some testing operations, then the Video/OSD Switch pin (pin
4) must have a pull down resistor to ground to insure opera-
tion in the video mode. Using a 47k pull down resistor will
keep this pin low, and provide enough resistance to where
the pin can still be driven directly by a TTL signal. Pins 1
through 3 should also be terminated the same way, eliminat-
ing the potential to switch logic levels just from the noise at
the open pins.
Figure 2 through Figure 4 show the timing diagrams for the
LM1283. When measuring propagation delays all TTL sig-
nals are measured at the time they cross 1.3V. The video
output is set to 4 V
. Propagation delay is measured when
the output is half way in its transition (changed by 2V). Rise
and fall times of the video output are measured between the
10% and 90% points of the transitions.
Board layout is always critical in a high frequency application
such as using the LM1283.Apoor layout can result in ringing
of the video waveform after sudden transitions, or the part
could actually oscillate. A good ground plane and proper
routing of the +12V are important steps to a good PCB lay-
out. The LM1283 can operate on a single sided board with a
good layout.Aground plane is recommended and it is best to
isolate the output stage grounds from the rest of the circuit.
Also the two grounds should be connected together only at
one point, ideally where the ground cable is connected to the
board ground. Yes, all grounds are connected internally, but
trace resistance can still allow for ground bounce, giving
enough feedback for oscillations. The output stage power
supply pin, pin 22, does not have an internal connection to
the other power supply pins. This pin must be connected to
the +12V supply, preferably with high frequency isolation.
This is easily done with a ferrite bead between pin 22 and the
+12V supply. Figure 8 and Figure 9 show the waveform ob-
tained with the LM1283 using the single sided demo board
designed for this part.
References
Zahid Rahim, “Guide to CRT Video Design,” Application
Note 861, National Semiconductor Corp., Jan. 1993
Ott, Henry W. Noise Reduction Techniques in Electronic sys-
tems , John Wiley & Sons, New York, 1976
DS012684-22
FIGURE 8. LM1283 Rise Time
DS012684-23
FIGURE 9. LM1283 Fall Time
www.national.com
12
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