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LM1208 データシート(PDF) 10 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
部品番号 LM1208
部品情報  130 MHz/85 MHz RGB Video Amplifier System with Blanking
Download  23 Pages
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メーカー  NSC [National Semiconductor (TI)]
ホームページ  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM1208 データシート(HTML) 10 Page - National Semiconductor (TI)

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Functional Description
Figure 6 is a detailed block diagram of the green channel of
the LM1208 along with the recommended external compo-
nents. The IC pin numbers are circled and all external com-
ponents are shown outside the dashed line. The other two
video channels are identical to the green channel, only the
numbers to the pins unique to each channel are different.
The input video is normally terminated into 75
Ω. The termi-
nation resistor depends on the impedance of the coax cable
being used, 75
Ω being the most common impedance used in
video applications. The video signal is AC coupled through a
10 µF capacitor to the input, pin 6. There is no standard for
the DC level of a video signal, therefore the signal must be
AC coupled to the LM1208. Internal to the LM1208 is a 2.8V
reference, giving the input video an offset voltage of 2.8V.
This voltage was selected to give the input video enough DC
offset to guarantee that the lowest voltage of the video signal
at pin 6 is far enough above ground to keep the LM1208 in
the active region. The 200
Ω resistor at the input is for ESD
protection and for current limiting during any voltage surge
that may occur at the input, driving pin 6 above V
CC. The
input video signal is buffered by −A1. In this circuit descrip-
tion an inverting amplifier is shown with a “−” (minus sign) in
front of the amplifier designation. The output of −A1 goes to
the contrast and drive attenuator sections.
The contrast and drive control sections are virtually identical.
Both sections take a 0V to 4V input voltage, 4V giving the
maximum gain for either the contrast or the drive. This is a
high impedance input, allowing for an easy interface to 5V
DACs. One may also use 100k potentiometers with no deg-
radation in performance. The contrast control section is com-
mon to all three channels. It converts the input voltage at pin
12 to a couple of internal DC voltages that control the gain of
the contrast attenuator. Referring to the Attenuation vs Con-
trast Voltage under typical performance characteristics note
that a 4V control voltage results in no attenuation of the
video signal. A 0.25V control voltage results in an attenuation
of 40 dB. Again note that these internal control voltages are
common to all three channels. To minimize crosstalk, these
voltages go to pins 1 and 2. Minimizing crosstalk is done by
adding the RC network shown in the block diagram
(
Figure 6).
The 0V to 4V drive control signal comes in on pin 18. Each
channel has its own drive section, therefore the crosstalk
compensation needed for the contrast control voltages is not
required for the drive control, thus no external pins for the
drive control. The drive attenuator features a full range gain
control over 40 dB. This gives no attenuation of the video
signal with a 4V control voltage. A 0.25V control voltage
results in an attenuation of 40 dB.
The output of the drive attenuator stage goes to A2, the
amplifier in the DC restoration section. The video signal goes
to the non-inverting input of A2. The inverting side of A2 goes
to the output of gm1, the clamp comparator, and the clamp
capacitor at pin 8.
During the back porch period of the video signal a negative
going clamp pulse from pin 14 is applied to the clamp
comparator, turning on the comparator. This period is where
the black level of the video signal at the output of the
LM1208 is compared to the desired black level which is set
at pin 19.
Figure 7 shows the timing of the clamp pulse
relative to the video signal. The clamp capacitor is charged
or discharged by gm1, generating the correction voltage
needed at the inverting input of A2 to set the video output to
the correct DC level. Removing the clamp pulse turns off
gm1 with the correction voltage being maintained by the
clamp capacitor during active video. Both the clamp pulse
and the blank pulse at pin 13 are TTL voltage levels.
There are actually two output sections, −A3 and −A4. Both
sections have been designed to be identical, except −A4 has
more current drive capability. The output transistor shown is
part of −A4, but has been shown separately so the user
knows the configuration of the output stage. −A3 does not go
to the outside world, it is used for feeding back the video
signal for DC restoration. Its output goes directly to the
inverting input of the clamp comparator via the voltage di-
vider formed by the 500
Ω and 4k resistors. −A4 will be close
to the same output as −A3 and will temperature track due to
the similar design of the two output stages. However, the
current at the output of −A4 will be ten times the current at
the output of −A3. To balance both outputs, a load resistance
of 390
Ω needs to be connected from pin 20, the green video
output pin, to ground. Another input to −A4 is the blank
pulse. When a negative going blank pulse is applied to pin
13, the output of the LM1208 is driven to less than 0.1V
above ground. Using the timing shown in
Figure 7 for the
blank pulse, the output of the LM1208 will be less than 0.1V
during the inactive portion of the video signal. This is a
“blacker than black” condition, blanking the CRT at the cath-
odes. By using the blank function of the LM1208 no grid
blanking is necessary. Note that the DC restoration is done
by feeding back the video signal from −A3, but blanking is
done at −A4. By using the two output stages, blanking can
be done at the CRT cathodes, and at the same time activate
the DC restoration loop.
V
CC1 goes to pins 3, 11, and 25 (see Figure 1 ). These three
pins are all internally connected. For proper operation of the
LM1208 it is necessary to connect all the V
CC1 pins to the
input power to the PCB and bypass each pin with a 0.1 µF
capacitor. V
CC2 is the input power at pins 22 and 23 for the
three output stages. This is a separate power input from
V
CC1, there are no internal connections between the two
different power inputs. There must be a connection on the
PCB between V
CC1 and VCC2. Pins 22 and 23 must be
bypassed by a parallel connection of a 10 µF and 0.1 µF
capacitors. The ground connections for the LM1208 are at
pins 7, 21, and 24. All three ground pins are internally
connected, and these pins must also be connected exter-
nally to a good ground plane for proper operation of the
LM1208.
www.national.com
10


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