STV6889 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'STV6889' PDF : 56 Pages View PDF
STV6889
9.4 Vertical section
9.4.1 General
The goal of the vertical section is to drive vertical
deflection output stage. It delivers a sawtooth
waveform with an amplitude independent of de-
flection frequency, on which vertical linearity cor-
rections of C- and S-type are superimposed (see
Chapter 7 - page 21).
Block diagram is in Figure 12. The sawtooth is ob-
tained by charging an external capacitor on pin
VCap with controlled current and by discharging it
via transistor Q1. This is controlled by the CON-
TROLLER. The charging starts when the voltage
across the capacitor drops below VVOB threshold.
The discharging starts either when it exceeds VVOT
threshold (free run mode) or a short time after ar-
rival of synchronization pulse. This time is neces-
sary for the AGC loop to sample the voltage at the
top of the sawtooth. The VVOB reference is routed
out onto VOscF pin in order to allow for further filtra-
tion.
The charging current influences amplitude of the
sawtooth. Just before the discharge, the voltage
across the capacitor on pin VCap is sampled and
compared to VVOTref. The comparison error voltage
is stored on a storage capacitor connected on pin
VAGCCap. This voltage tunes gain of the transcon-
ductance amplifier providing the charging current
in the next vertical period. Speed of this AGC loop
depends on the storage capacitance on pin
VAGCCap. The VLock I²C-bus flag is set to 1 when
the loop is stabilized, i.e. when the tops of saw
tooth on pin VCap match VVOT value. On the
screen, this corresponds to stabilized vertical size
of picture. After a change of frequency on the
sync. input, the stabilization time depends on the
frequency difference and on the capacitor value.
The lower its value, the shorter the stabilization
time, but on the other hand, the lower the loop sta-
bility. A practical compromise is a capacitance of
470nF. The leakage current of this capacitor re-
sults in difference in amplitude between low and
high frequencies. The higher its parallel resistance
RL(VAGCCap), the lower this difference.
When the synchronization pulse is not present, the
charging current is fixed. As a consequence, the
free-running frequency fVO(0) only depends on the
value of the capacitor on pin VCap. It can be rough-
ly calculated using the following formula
fVO(0) =
150nF . 100Hz
C(VCap)
The frequency range in which the AGC loop can
regulate the amplitude also depends on this ca-
pacitor.
The vertical sawtooth with regulated amplitude is
lead to amplitude control stage. The discharge ex-
ponential is replaced by VVOB level, which, under
control of the CONTROLLER, creates a rapid fall-
ing edge and a flat part before beginning of new
ramp.
The AGC output signal passes through gain and
position adjustment stages controlled through
VSIZE and VPOS I²C-bus registers. The resulting
signal serves as input to all geometry correction
circuitry including EW-drive signal, horizontal
phase modulation and dynamic correction outputs.
9.4.2 S and C corrections
For the sake of vertical picture linearity, the S- and
C-corrections are now superimposed on the linear
ramp signal. They both track with VSIZE and
VPOS adjustments to ensure unchanged linearity
on the screen at changes of vertical size or vertical
position. As these corrections are not included in
the AGC loop, their adjustment via CCOR and
SCOR I²C-bus registers, controlling shape of verti-
cal output sawtooth affects by principle its peak-to-
peak amplitude. However, this stage is conceived
in a way that the amplitude be independent of
these adjustments if VSIZE and VPOS registers
are set to their medium values.
9.4.3 Vertical breathing compensation
The signal provided with the linearity corrections is
amplitude affected in a gain control stage, ruled by
the voltage on VEHTIn input and its I²C-bus control
VEHTG.
9.4.4 Vertical after-gain and offset control
Another gain control is applied via VSAG I²C-bus
register. Then an offset is added, its amount corre-
sponding to VPOF I²C-bus register value. These
two controls result in size and position changes
with no effect on shape of output vertical sawtooth
or any geometry correction signal.
37/56
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