TDA9115 datasheet(28/45 Pages) STMICROELECTRONICS

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TDA9115

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Figure 10. Control of HOut and BOut at start/stop at nominal Vcc

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 geometry cor-

rections of C- and S-type are superimposed (see

chapter TYPICAL OUTPUT WAVEFORMS).

Block diagram is in Figure 11. 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 or a short time after arrival of synchroni-

zation pulse. This time is necessary for the AGC

loop to sample the voltage at the top of the saw-

tooth. The VVOB reference is routed out onto VO-

scF pin in order to allow for further filtration.

The charging current influences amplitude and

shape of the sawtooth. Just before the discharge,

the voltage across the capacitor on pin VCap is

sampled and stored on a storage capacitor con-

nected on pin VAGCCap. During the following ver-

tical period, this voltage is compared to internal

reference REF (VVOT), the result thereof control-

ling the gain of the transconductance amplifier pro-

viding the charging current. Speed of this AGC

loop depends on the storage capacitance on pin

VAGCCap. On the screen, this corresponds to sta-

bilized 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 stability. A practical compromise is

a capacitance of 470nF. The leakage current of

this capacitor results in difference in amplitude be-

tween low and high frequencies. The higher its

parallel resistance RL(VAGCCap), the lower this dif-

ference.

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

roughly 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 C- and S-corrections of shape serve to com-

pensate for the vertical deflection system non-line-

arity. They are controlled via

CCOR and SCOR

I2C bus controls.

Shape-corrected sawtooth with regulated ampli-

tude is lead to amplitude control stage. The dis-

V(HPosF)

Soft start

Soft stop

Normal operation

Start

HOut

Start

BOut

Stop

HOut

Stop

BOut

HOut

H-duty cycle

BOut (positive)

B-duty cycle

100%

VHOn

VBOn

VHBNorm

VHPosMax

VHPosMin

HPOS (I2C)

range

minimum value

maximum value

C(VCap)

. 100Hz

150nF

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