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STMicroelectronics Confidential
In the TDA9112A only, the S and C corrections are very different (see description below).
5.1.7
Output Stage and Vertical Shift
The vertical sawtooth undergoes certain modifications before reaching the vertical output on pin 23:
To begin with, a high-impedance voltage follower transmits the signal without disturbing the
oscillator.
Considering the oscillator, the exponential shape of discharge waveform is ideal for detecting the
end-of-discharge threshold because its slope is not too steep. Nevertheless, the following stages
have different requirements:
●
a steep edge is needed on vertical booster input in order to safely trigger the flyback,
●
the vertical sawtooth is also used to generate the E/W correction waveforms. If no precautions
were taken, an E/W parabola would be generated during the oscillator discharge period. It
would cause a fast transient at the beginning of the next vertical scanning and disturb the top of
the screen.
For these reasons and before transmission to the following stages, the exponential discharge
waveform is removed from vertical sawtooth and replaced by a steep edge. The signal is then
maintained at a constant value until the rising of the positive slope as shown in
.
After this operation, the signal is sent to a variable gain amplifier with a typical adjustment range of
4.4dB. The signal with adjustable amplitude is sent to pin 23 for vertical scanning and to the
geometry control stages.
The mean value of the sawtooth on pin 23 is 3.5V. In order to allow vertical position adjustments, it
can be offset by ±0.3V through I²C bus (Register 08). Obviously, this only can work if the booster
inputs and output are DC coupled. In the section referring to “Geometry corrections”, we will
examine how this is used for geometry tracking. For details on coupling to Booster, see Application
Hints below.
The TDA9112A offers more sophisticated settings.
Figure 9: C Correction Circuit
22
3.5V
Io-
I'o
k(
Io
-I
'o
)
Co
I²C
