TDA9109 datasheet(21/30 Pages) STMICROELECTRONICS

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OPERATING DESCRIPTION (continued)

III - VERTICAL PART

III.1 - Function

23 VOUT

18 BREATH

VERT_AMP

SUB05/7bits

VMOIRE

SUB0C/5bits

VPOSITION

SUB06/7bits

SYNCHRO

OSCILLATOR

OSC

CAP

DISCH.

VSYNCIN

POLARITY

SAMPLING

CAPACITANCE

Vlow

Sawth.

Disch.

REF

TRANSCONDUCTANCE

AMPLIFIER

CHARGE CURRENT

VS_AMP

SUB07/6bits

COR_C

SUB08/6bits

S CORRECTION

C CORRECTION

Figure 15 : AGC Loop Block Diagram

When the synchronization pulse is not present, an

internal current source sets the free running fre-

quency. For an external capacitor, COSC = 150nF,

the typical free running frequency is 100Hz.

The typical free running frequency can be calcu-

lated by :

(Hz)

= 1.5 ⋅ 10−5 ⋅ 1

COSC

A negative or positive TTL level pulse applied on

Pin 2 (VSYNC) as well as a TTL composite sync on

Pin 1 can synchronize the ramp in the range

[fmin , fmax]. This frequency range depends on the

ext er nal capac it or connect ed on Pin 22.

A 150nF (

±5%) capacitor is recommended for

50Hz to 185Hz applications.

The typical maximum and minimum frequency, at

oC and without any correction (S correction or

C correction), can be calculated by :

f(Max.) = 3.5 x f0 and f(Min.) = 0.33 x f0

If S or C corrections are applied, these values are

slighty affected.

If a synchronization pulse is applied, the internal

oscillator is synchonized immediately but its

amplitude changes. An internal correction then

adjusts it in less than half a second. The top

value of the ramp (Pin 22) is sampled on the

AGC capacitor (Pin 20) at each clock pulse and

a transconductance amplifier modifies the

charge current of the capacitor in such a way to

make the amplitude again constant.

The read status register provides the vertical Lock-

Unlock and the vertical sync polarity information.

We recommend the use of an AGC capacitor with

low leakage current. A value lower than 100nA is

mandatory.

A good stability of the internal closed loop is

reached by a 470nF

± 5% capacitor value on

Pin 20 (VAGC).

TDA9109/S

21/30

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