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LM1282 データシート(PDF) 11 Page - National Semiconductor (TI) |
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LM1282 データシート(HTML) 11 Page - National Semiconductor (TI) |
11 / 14 page Pin Descriptions (Continued) Pin Pin Schematic Description No. Name 18 20 23 Blue Video Out Green Video Out Red Video Out Video output. For proper black level the output must drive 390 Ω impedance. 22 V CC2 Power supply pin for the output stage. There are no internal connections to V CC1. 25 RGB Cutoff Adjust 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. Functional Description Figure 1 on the front page shows the block diagram of the LM1282 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 LM1282. 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 LM1282 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 LM1282. 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 LM1282 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 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 LM1282 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 LM1282 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 LM1282 must be terminated with a 390 Ω impedance. 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 LM1282 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 11 www.national.com |
同様の部品番号 - LM1282 |
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同様の説明 - LM1282 |
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