TDA9108 datasheet(7/9 Pages) STMICROELECTRONICS

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APPLICATION INFORMATION

Sync Extractor and Polarity Detection

This circuit is able to handle both positive or nega-

tive TTL input signal on Pin 9. The voltage on Pin

10 drives an internal inverter providing a constant

sync polarity to the 1st phase comparator.

When using a RC network between Pin 9 and 10

(see Typical Application), the IC will adapt itself

automatically to positive or negative sync. On an

other hand, and in order to simplified the applica-

tion, the Pin 10 can be connected to ground or

supply (through a resistor), in this case the IC will

work only with one sync polarity.

1st PLL

It is composed by a phase comparator, the oscilla-

tor and an external loop filter (see Figure 2)

- The phase comparator receives the H-sync sig-

nal (with positive polarity) and a signal coming

from the internal current controlled oscillator. The

loop is closed through an external resistor be-

tween Pin 11 and 13.

- The oscillatorgenerates a sawtooth waveform on

Pin 12 by charging and discharging the external

capacitor. The capacitor is discharging by the

current flowing Pin 13 and charged by two times

this latter (see Figure 3).

The sawtooth is used internally to generate all the

required timings.

It is possible to DC control the frequency by adding

or substracting a DC current on Pin 13 (see Fig-

ure 2).

VCO

PHASE

COMPARATOR

LOOP

FILTER

H-sync

Figure 2

Pin 12

1/3T

VREF

Figure 3

2nd Phase Locked Loop

To compensate the delay introduced by the hori-

zontal final stage, the flyback pulse (Pin 6) and the

oscillator waveform (Pin 12) are compared in the

2nd phase comparator. The result of the compari-

son is a control current which, after it has been

filtered by the external capacitor on Pin 5, is sent

to a phase shifter which adequately regulates the

horizontal output pulses phase.

The maximum phase shift allowed is td =tp -tf

where tf is the flyback duration (see Figure 4).

If td >tp -tf, then the horizontal output transistor will

be tunned on during flyback distroying it.

HORIZONTAL

FLYBACK

HORIZONTAL

DRIVE

t f

H P

t p

t d

BU ON

BU OFF

Figure 4

TDA9108

7/9

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