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LM1204V データシート(PDF) 10 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
部品番号 LM1204V
部品情報  150 MHz RGB Video Amplifier System
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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)

LM1204V データシート(HTML) 10 Page - National Semiconductor (TI)

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Applications Hints
The LM1204 is a wideband video amplifier system designed
specifically for high resolution RGB CRT monitors The de-
vice includes circuitry for DC restoration of video signals
and also allows contrast and brightness control DC restora-
tion is done during the back porch interval of the video sig-
nal An internal sync separator generates a back porch
clamp signal either from a ‘‘Sync on Green’’ signal applied
to the composite video input (pin 18) or from an externally
supplied gH Sync signal
The LM1204 first looks at the
g
H Sync input (pin 23) if an external horizontal sync signal
is not present then the device syncs off the composite video
input The internally generated back porch clamp pulse
width is user adjustable
A blanking function is also included This allows the user to
cutoff the beam current in the CRT’s guns during the blank-
ing interval thereby preventing horizontal retrace lines from
being visible Normally blanking is done by applying a high
voltage pulse at the grid However blanking at the cathode
using the LM1204 leads to ease of design and lowered cost
Figure 2 shows the block diagram of the green video chan-
nel and the control logic The two modes of operation with
and without blanking are described below in detail
10 Operation without Blanking
For operation without blanking the blank gate input (pin 24)
should be connected to a4V This causes the blank com-
parator to connect switch S2 to position Y (See
Figure 2 )
Furthermore the brightness control input pin (pin 20) should
be biased at a potential between 1V (Min) and 38V (Max) it
is best to bias this pin at 2V The video signal is AC coupled
to the input of the LM1204 as shown for the green channel
in
Figure 2 During the back porch interval of the video sig-
nal (See
Figure 3 ) the internally generated back porch
clamping pulse goes low causing switches S1A and S1B to
be closed The closure of S1A causes gm1 to charge capac-
itor C2 to a potential determined by the DC voltage at pin
20 This allows gm1 to set up an average DC bias for the AC
coupled video signal at the input of A1 When the back
porch clamping pulse is high S1A and S1B are opened
With S1A open gm1 is effectively disconnected from C2 C2
now holds the DC bias voltage The transconductance
stage gm1 therefore functions as a sample and hold device
and holds the input of A1 at the desired DC bias
The LM1204 uses black level clamping at the back porch of
the video signal to accomplish DC restoration The trans-
conductance stage gm2 is enabled during the back porch
clamp period to provide a sample and hold function During
the back porch clamp period DC feedback from LM1204’s
video output is compared with the voltage set by potentiom-
eter R9 Depending on A2’s output voltage C6 is either
charged or discharged so that the feedback loop consisting
of gm2 and A2 is stabilized and the output is clamped to the
black level All this occurs during the back porch clamp peri-
od During the video portion of the signal gm2 is disabled
and C6 holds the fixed black level reference voltage The
beginning of each new line on the raster always starts from
a fixed reference black level thus restoring the DC compo-
nent of each line
A2 is a summing amplifier that adds a DC offset component
from gm2 to the video signal from the multiplier Adjusting
R9 will DC offset the output signals of all three channels
thus providing brightness control Individual cutoff adjust-
ment for each channel is done by varying the feedback volt-
age at each of the R G and B feedback inputs (Pins 38 28
and 33) For example cutoff adjustment for the green chan-
nel is done by potentiometer R8 shown in
Figure 2
Adjusting the contrast control (potentiometer R3 in
Figure
2 ) varies the peak to peak amplitude (includes sync tip if
present) of all three video output signals relative to their
black reference level The D Gain adjust (pins 9 15 and 13
for R G and B channels respectively) allows the user to
individually adjust the AC gain of each channel For example
the AC gain of the green channel is adjusted using potenti-
ometer R5 as shown in
Figure 2 Normally the red channel
is set for maximum gain and the gains of the blue and green
channels are reduced until white balance is achieved on the
CRT monitor’s screen
Figure 4 shows the adjustments for
operation without blanking
20 Operation with Blanking
Much of what was discussed in Section 10 also applies
when the LM1204 is used with the blanking function How-
ever there are notable differences as described herein For
operation with blanking a TTL compatible blanking signal
must be applied to the blank gate input (pin 24)
During the blanking period the blanking comparator con-
nects switch S2 to position X (See
Figure 2 ) This causes
the LM1204 to level shift the video output signal to the blank
level Adjusting R9 will adjust the blank level of all three
channels Individual blank level adjustment for each chan-
nel is done by varying the feedback voltage at each of the
R G and B feedback inputs (pin 38 28 and 33) In
Figure 2
this is done by adjusting potentiometer R8 for the green
channel
During the video portion of the video signal S2 is connected
to position Y Brightness control is now accomplished by
varying the potential at the brightness control pin (pin 20)
Adjusting R6 offsets the video portion of all three output
signals relative to the fixed blank level restoring the DC
level of the video signal
Figure 5 shows the adjustments for
operation with blanking
30 Stability Considerations
For optimum performance and stable operation a double
sided PC board with adequate ground plane is essential
Moreover soldering the LM1204 on to the PC board will
yield best results Each supply pin (pins 2 4 6 19 31 36
41 and 44) should be bypassed with a 001 mF and a 01 mF
capacitor connected as close to the supply pin as is possi-
ble
When driving the LM1204 from a 75X video source the
cable is terminated with 75X to minimize reflections caused
by transmission line effects However the input impedance
of LM1204 is capacitive and is also affected by the stray
capacitance of the PC board Thus the input impedance is a
function of frequency This changes the impedance of the
cable termination This can introduce overshoot and ringing
in LM1204’s pulse response A 100X resistor in series with
the blocking capacitor at the video input will minimize over-
shoot and ringing (see
Figure 8 ) The value of the resistor is
empirically determined 100X is a good starting value
Since the LM1204 is a wide bandwidth amplifier with high
gain at high frequencies the device may oscillate when driv-
ing a large capacitiveinductive load To prevent oscillation
the amplifier’s gain is rolled off at high frequencies This is
accomplished by an RC network comprised of a resistor in
10


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