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LM1283N データシート(PDF) 11 Page - National Semiconductor (TI) |
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LM1283N データシート(HTML) 11 Page - National Semiconductor (TI) |
11 / 13 page Pin Descriptions (Continued) Pin No. Pin Name Schematic Description 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 LM1283 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 LM1283. 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 LM1283 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 LM1283. 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 LM1283 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 LM1283 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 LM1283 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 LM1283 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 LM1283 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 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 givesa4to1 range in the output levels between the three channels. Applications of the LM1283 A schematic for a demonstration board is shown in Figure 7. This board was used for the characterization of the LM1283. Note that a 33 Ω resistor is in series with all inputs to the IC that receive external signals. These resistors are necessary 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 de- signer must include these resistors in his design. If additional protection against ESD at the video inputs is necessary, then adding clamp diodes on the IC side of the 33 Ω resistor is rec- www.national.com 11 |
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