LM1282N National Semiconductor, LM1282N Datasheet - Page 12

• Manufacturer Part Number: LM1282N
• Description: IC VIDEO AMP SYSTEM 28-DIP
• Manufacturer: National Semiconductor

Specifications of LM1282N

• Applications: General Purpose
• Number Of Circuits: 3
• -3db Bandwidth: 110MHz
• 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: *LM1282N

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Functional Description
couple the preamp output to the cathode drivers. Therefore
only one cutoff adjustment is provided, this is used primarily
to optimize the operation of the cathode drivers.
Note that the Blank and Clamp Gates are active low. These
pins are normally controlled by standard TTL signals. For
video applications the Clamp Gate must be used. There are
designs where the blank function may not be required. When
the Blank Gate is not used, it must be tied high by a pullup
resistor. A resistor value of 47k is acceptable, going to either
4V or 12V.
Gain of −A2 is controlled by the Drive Adjust pins. These are
also 0V to 4V control voltages. 4V results in no attenuation at
−A2, and 0V results in a −12 dB attenuation. The 12 dB ad-
justment range should provide more than enough adjust-
ment for setting the white level. Note that a 12 dB range
gives a 4 to 1 range in the output levels between the three
channels.
Applications of the LM1282
A schematic for a demonstration board is shown in Figure 7 .
This board was used for the characterization of the LM1282.
Note that a 33
that receive external signals. These resistors are recom-
mended to protect the IC from any sudden voltage surges
that may result during the power up and power down modes,
or when connecting the monitor to other equipment. The
monitor designer should include these resistors in his de-
sign. If additional protection against ESD at the video inputs
is necessary, then adding clamp diodes on the IC side of the
33
ground. Normally a designer may want to increase the value
of the 33
protection at the video inputs. Remember that the input ca-
pacitor to the video inputs is also part of the DC restoration
circuit. This circuit is depending on a maximum circuit resis-
tance of about 110 . The 33
creased in value. The internal ESD protection and the exter-
nal 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 LM1282. However, proper de-
sign techniques must be followed in assuring that a good
TTL signal is received at the LM1282. 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 LM1282 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.
Figures 2, 3, 4 show the timing diagrams for the LM1282.
When measuring propagation delays all TTL signals are
resistor is recommended; one to V
resistor at pins 5, 8, and 11 for additional ESD
resistor is in series with all inputs to the IC
resistors should not be in-
(Continued)
CC1
and one to
12
measured at the time they cross 1.3V. The video output is set
to 4 V
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 LM1282. A poor 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 LM1282 can operate on a single sided board with a
good layout. A ground 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 LM1282 using the single sided demo board
designed for this part.
PP
. Propagation delay is measured when the output is