LM1253A National Semiconductor, LM1253A Datasheet - Page 17

• Manufacturer Part Number: LM1253A
• Description: Monolithic Triple 180 MHz I2C CRT Pre-amp With Integrated Analog On Screen Display (OSD) Generator
• Manufacturer: National Semiconductor

ESD and Arc-Over Protection
The I
pins should be protected from arc-over. This is done by add-
ing a resistor in series with each pin. Figure 27 shows how
the resistors are connected to the SDA and SCL lines.
FIGURE 26. Recommended Video Input ESD Protection
(Continued)
FIGURE 27. Recommended Arc-Over Protection for
2
C specification recommends that the SDA and SCL
SDA and SCL Pins
DS101265-30
DS101265-29
17
Pre-Amp Functional Description
Figure 1 shows the block diagram of the LM1253A. The
video signal is input to the LM1253A through the circuit
shown in Figure 26 . Black level clamping is carried out di-
rectly on the AC coupled input signal at the input of the high
impedance buffer amplifier, thus eliminating the need for
black level clamp capacitors. The following sections then
modify the input video signal:
The above sections use the registers shown in Table 2 . Fig-
ure 28 shows which section of the output video and clamp
signal each register controls.
• Contrast Attenuator — Sets the contrast level of the video
• Auto Beam Limit — Reduces the gain of all three video
• Gain Attenuator — Separately sets the gain for each
• Brightness — Sets the brightness for all three channels.
• Bias — Adds an offset to the brightness controls for all
• Pedestal — Adds a dc voltage offset to the brightness
• Source Select — Switches between the input video signal
• DC Offset — Adds a dc voltage offset to all three video
signal for all three channels.
amplifiers in response to a control voltage proportional to
the CRT beam current.
video channel.
three video channels.
control, in order that bi-directional control of the bright-
ness control is always possible at all bias voltage set-
tings.
and the analog OSD signal generated by the 3 bit pallet
DAC control block.
outputs.
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