STV6889 datasheet(37/56 Pages) STMICROELECTRONICS

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STV6889

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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) =

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

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.

C(VCap)

. 100Hz

150nF

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