LM1281N National Semiconductor, LM1281N Datasheet - Page 11

• Manufacturer Part Number: LM1281N
• Description: IC VIDEO AMP SYSTEM 28-DIP
• Manufacturer: National Semiconductor

Specifications of LM1281N

• Applications: General Purpose
• Number Of Circuits: 3
• -3db Bandwidth: 85MHz
• Current - Supply: 95mA
• Current - Output / Channel: 28mA
• Voltage - Supply, Single/dual (±): 11.4 V ~ 12.6 V
• Mounting Type: Through Hole
• Package / Case: 28-DIP (0.600", 15.24mm)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Other names: *LM1281N

Pin No.
Pin Descriptions
Functional Description
Figure 1 on the front page shows the block diagram of the
LM1281 along with the pinout of the IC. Each channel re-
ceives both a video signal and an OSD signal at its input am-
plifier (−A1). The Video/OSD Switch signal also goes to the
input amplifiers, controlling whether the video or the OSD
signal passes through the LM1281. Both the OSD inputs and
the Video/OSD Switch accept standard TTL signals. If video
is selected then a TTL low is applied to pin 4, for OSD a TTL
high needs to be applied. When the OSD feature is not used,
then pin 4 needs to be connected to ground via a 47k resis-
tor. Although the OSD input signal is a TTL signal, the input
amplifier processes this signal to match the video levels. A
TTL high signal will be at the video white level and a TTL low
signal will typically be within 100 mV of the video black level.
Note that by using the LM1281 the monitor designer con-
nects the OSD input signals directly to the IC with NO signal
processing.
DC restoration is performed on the video inputs to the
LM1281. Remember video inputs are always AC coupled to
the video pre-amp. There is no DC standard for the video in-
put, therefore AC coupling the video inputs is necessary for
proper operation of the monitor. A minimum capacitance of
1 µF is recommended at the video input pins. The preferred
value is 10 µF. Part of the signal processing of the TTL OSD
inputs is matching the black level of the OSD signal (TTL
low) to the black level of the video signal. With AC coupling
of the video inputs, DC restoration must be done at the input
to perform the black level matching.
The next stage in the LM1281 is the Contrast Attenuation.
Both the video and OSD contrast controls go to this stage.
For easy interfacing to 5V DACs all control inputs, including
these two controls, use a 0V to 4V range. Both contrast con-
trols give no attenuation at 4V and full attenuation (over
−50 dB) at 0V. The video and OSD contrast adjustments are
completely independent of each other, allowing the user to
set the desired contrast of the OSD window without affecting
the video portion of the display. There is only one output from
18
20
23
22
25
Blue Video Out
Green Video Out
Red Video Out
V
RGB Cutoff Adjust
CC2
Pin Name
(Continued)
Schematic
11
this section, any adjustments on the signal path beyond the
contrast stage affects both the video signal and the OSD sig-
nal.
Following the Contrast Attenuation block is the Drive Attenu-
ation. By having the Drive Attenuation past the contrast
stage, any adjustment made on the video signal will equally
affect the OSD signal. This configuration simplifies the white
level adjustment. When the white level of the video is ad-
justed then the OSD white level is automatically set. The
only OSD adjustment necessary when using the LM1281 is
the OSD contrast. Note that when performing the white level
adjustments the video portion of the display must be used,
because there are minor variations between the OSD levels
and the video levels.
The output stage is the −A2 amplifier. This stage is similar to
the LM1205 output stage, where the video output can be
blanked to a level below the video black level. A blacker than
black output during blanking provides the capability to blank
at the cathodes of the CRT. This eliminates the need for us-
ing high voltage transistors at G1 of the CRT to perform the
blanking function. When the outputs are blanked the
LM1281 can still DC restore the video output signal by using
the Clamp Gate. There is an internal feedback stage that
does the DC restoration. In order to maintain the correct
video levels based on this feedback loop, the video output of
the LM1281 must be terminated with a 390
The required correction voltage for DC restoration is stored
on the clamp cap. A value of 0.1 µF is recommended for the
clamp cap. If the cap value is too small then there will be a tilt
(shift) in the DC level of the video output during the horizon-
tal scan. If the cap value is too large, then the DC restoration
circuit may not be able to maintain the proper DC level of the
video signal. Since DC restoration is also done at the video
inputs, larger clamp cap values will be less of a problem with
the LM1281 than with most other video preamps. The refer-
ence level for the DC restoration circuit is set at the RGB
Cutoff Adjust pin (pin 25). Most monitor applications AC
Video output. For proper black level the output
must drive 390
Power supply pin for the output stage. There
are no internal connections to V
Sets the black level of the video outputs to all
three channels. Range is 0V to 4V. Minimum
black level is limited to about 300 mV.
impedance.
Description
CC1
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impedance.