LM1246AAA National Semiconductor, LM1246AAA Datasheet - Page 7

• Manufacturer Part Number: LM1246AAA
• Description: 150 MHz I2C Compatible RGB Preamplifier with Internal 512 Character OSD ROM/ 512 Character RAM and 4 DACs
• Manufacturer: National Semiconductor

Compatibility with LM1237 and
LM1247
The Compatibility of the LM1246 to the LM1237 and
LM1253A is the same as that of the LM1247. Please refer to
the LM1247 datasheet for details.
OSD vs Video Intensity
The OSD amplitude has been increased over the LM1237
level. During monitor alignment, the three gain registers are
used to achieve the desired front of screen color balance.
This also causes the OSD channels to be adjusted accord-
ingly, since these are inserted into the video channels prior
to the gain attenuators. This provides the means to fine tune
the intensity of the OSD relative to the video as follows. If a
typical starting point for the alignment is to have the gains at
maximum (0x7F) and the contrast at 0x55, the resultant
OSD intensity will be higher than if the starting point is with
the gains at 0x55 and the contrast at maximum (0x7F). This
(Continued)
LM1246 Pin:
Assignment:
TABLE 2. LM1253A/LM1237 Compatibility
DAC 1
Blue
DAC Bias Outputs
DAC 2
Green
7
In order to maintain register compatibility with the LM1253A
and LM1237 preamplifier datasheet assignments for bias
and brightness, the color assignments are recommended as
shown in Table 2. LM1253A/LM1237 Compatibility. If
datasheet compatibility is not required, then the DAC assign-
ments can be arbitrary.
tradeoff allows fine tuning the final OSD intensity relative to
the video. In addition, the OSD contrast register, 0x8438
[4:3], provides 4 major increments of intensity. Together,
these allow setting the OSD intensity to the most pleasing
level.
ESD Protection
The LM1246 features a 3.0 kV ESD protection level (see
(Notes 4, 13)). This is provided by special internal circuitry
which activates when the voltage at any pin goes beyond the
supply rails by a preset amount. At that time, the protection is
applied to all the pins, including SDA and SCL.
DAC 3
Red
Brightness
DAC 4
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