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LM1279 데이터시트(PDF) 11 Page - Texas Instruments

부품명 LM1279
상세설명  110 MHz RGB Video Amplifier System with OSD
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OBSOLETE
LM1279
www.ti.com
SNOS020B – SEPTEMBER 1998 – REVISED APRIL 2013
FUNCTIONAL DESCRIPTION
Figure 1 on the front page shows the block diagram of the LM1279 along with the pinout of the IC. Each channel
receives a video input signal at its input amplifier (-A1). The output of the input amplifier goes to the contrast
attenuator stage. For easy interfacing to 5V DACs all controls inputs, including the contrast control, use a 0V to
4V range. The contrast control has no attenuation with an input of 4V, and has full attenuation (over
−40 dB) with
a 0V input. All three channels will accurately track the contrast control setting at pin 10. Each channel will have
the same amount of attenuation for a given input voltage typically to within ±0.3 dB. All channels will track
because the contrast control is the first stage of attenuation and the internal control voltage generated from the
input voltage is common to all three channels.
The output of the contrast attenuator goes to the drive attenuator. This stage has a 12 dB control range. This
stage is used for color balance, so the adjustment range has been limited to 12 dB for a more accurate color
balance. Each channel has its own independent control pin with the 0V to 4V control range. An input of 4V give
no attenuation, and an input of 0V gives the full 12 dB attenuation.
The output of the drive attenuator stage goes to the inverting input of A2. Since this is the second inversion
stage, the output of A2 will be the non-inverted video signal. Note that the output of gm1 goes to the non-
inverting input of A2. Also note that the output of A2 goes to the inverting input of gm1. This is the feedback for
the clamp circuitry. The output stage of A2 is an exact duplicate of the video output through A3. If a 390
Ω load
impedance is used at the video output, then the black level at the output stage will accurately track the output of
A2. The other input to gm1 is the desired black level output of the LM1279. Since the LM1279 has a fixed black
level output, the non-inverting inputs to gm1 in all three channels go to a fixed 1.35V internal reference. This sets
the black level output to a nominal 1.35V. gm1 acts like a sample and hold amplifier. Once the sandcastle sync
exceeds 3.6V gm1 is activated, driving the input of A2 to a level where the video output will be 1.35V. For proper
DC restoration it is important that gm1 be activated only during the horizontal flyback time when the video is at
the black level. gm1 also charges the clamp cap to the correct voltage to maintain a 1.35V black level at the
video output. When gm1 is turned off the voltage stored on the clamp cap will maintain the correct black level
during the active video, thus restoring the DC level for a 1.35V black level.
The input of A3 receives the output from A2. The video channel of A3 is a duplication of the output stage to A2.
As mentioned in the previous paragraph this is done so that the DC restoration can be done at the A2 stage. A3
also receives the OSD input and a sandcastle input for blanking. By doing DC restoration at the A2 stage, OSD
or blanking can be activated at the output stage during the time DC restoration is being done at A2. There is an
interface circuit between the sandcastle input and the A3 output stages. This interface circuit will activate the
blanking if the sandcastle sync input is between 1.7V and 6.0V. The blanking mode will force the output down to
a level of about 0.1V. This is a blacker-than-black level and can be used for blanking at the cathodes of the CRT.
Once the sandcastle exceeds 6.5V, then the output will no longer be in the blanked mode, but DC restoration is
still being done on the video signal.
The OSD signal goes into a special interface circuit. The output of this circuit will drive the output of A3 to either
an OSD black level or to about 2.4V above the video black level (OSD white level). The OSD black level is about
300 mV below the video black level. This ensures that if the OSD signal is not activated for a particular channel,
then its output will be slightly below the cutoff level. If an OSD input is received in a particular channel, then the
video output will be at the OSD white level. The OSD mode is automatically activated if there is only one OSD
signal to any of the video channels. This OSD control circuit will allow any color, except black, during the OSD
mode. This also saves the need for a special signal to switch into the OSD mode. Remember that at least one
OSD input must be high to enable the OSD mode, therefore black can't be used in the OSD window.
Sandcastle Sync
This special sync signal is used to allow for a 20-pin OSD video pre-amp with all the desired controls. By using a
sandcastle sync, both clamping and blanking can be activated from the same pin. Figure 4 shows the sandcastle
sync signal. There are four possible modes of operation with the Sandcastle pulse. These modes are:
1. Inactive Region
2. Blanking and no Clamping
3. Blanking and Clamping
4. Clamping and no Blanking
Copyright © 1998–2013, Texas Instruments Incorporated
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