TDA9112 datasheet(30/51 Pages) STMICROELECTRONICS

Part #	TDA9112

Description	LOW-COST I2C CONTROLLED DEFLECTION PROCESSOR FOR MULTISYNC MONITOR

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Figure 6. Horizontal oscillator (VCO) schematic diagram

9.3.3 - Voltage controlled oscillator

The VCO makes part of both PLL1 and PLL2

loops, being an “output” to PLL1 and “input” to

PLL2. It delivers a linear sawtooth. Figure 6 ex-

plains its principle of operation. The linears are ob-

tained by charging and discharging an external ca-

pacitor on pin CO, with currents proportional to the

current forced through an external resistor on pin

RO, which itself depends on the input tuning volt-

age VHO (filtered charge pump output). The rising

and falling linears are limited by VHOThrLo and

VHOThrHi thresholds filtered through HOscF pin.

At no signal condition, the VHO tuning voltage is

clamped to its minimum (see chapter ELECTRI-

CAL PARAMETERS AND OPERATING CONDI-

TIONS, part horizontal section), which corre-

sponds to the free-running VCO frequency fHO(0).

Refer to Note 1 for formula to calculate this fre-

quency using external components values. The ra-

tio between the frequency corresponding to maxi-

mum VHO and the one corresponding to minimum

VHO (free-running frequency) is about 4.5. This

range can easily be increased in the application.

The PLL1 can only lock to input frequencies falling

inside these two limits.

9.3.4 - PLL2

The goal of the PLL2 is, by means of phasing the

signal driving the power deflection transistor, to

lock the middle of the horizontal flyback to a cer-

tain threshold of the VCO sawtooth. This internal

threshold is affected by geometry phase correc-

tions, like e.g., parallelogram. The PLL2 is much

faster than PLL1 to be able to follow the dynamism

of phase modulation. The PLL2 control current

(see Figure 7) is significantly increased during dis-

charge of vertical oscillator (during vertical retrace

period) to be able to make up for the difference of

dynamic phase at the bottom and at the top of the

picture. The PLL2 control current is integrated on

the external filter on pin HPLL2C to obtain

smoothed voltage, used, in comparison with VCO

ramp, as a threshold for H-drive rising edge gener-

ation.

As both leading and trailing edges of the H-drive

signal in the Figure 7 must fall inside the rising part

of the VCO ramp, an optimum middle position of

the threshold has been found to provide enough

margin for horizontal output transistor storage time

as well as for the trailing edge of H-drive signal

with maximum duty cycle. Yet, the constraints

thereof must be taken into account while consider-

ing the application frequency range and H-flyback

duration. The Figure 7 also shows regions for ris-

ing and falling edges of the H-drive signal on HOut

pin. As it is forced high during the H-flyback pulse

and low during the VCO discharge period, no edge

during these two events takes effect.

The flyback input configuration is in Figure 8.

9.3.5 - Dynamic PLL2 phase control

The dynamic phase control of PLL2 is used to

compensate for picture asymmetry versus vertical

axis across the middle of the picture. It is done by

modulating the phase of the horizontal deflection

with respect to the incoming video (synchroniza-

tion). Inside the device, the threshold VS(0) is com-

pared with the VCO ramp, the PLL2 locking the

middle of H-flyback to the moment of their match.

The dynamic phase is obtained by modulation of

the threshold by correction waveforms. Refer to

Figure 12 and to chapter TYPICAL OUTPUT

WAVEFORMS. The correction waveforms have

no effect in vertical middle of the screen (for mid-

dle vertical position). As they are summed, their ef-

fect on the phase tends to reach maximum span at

top and bottom of the picture. As all the compo-

nents of the resulting correction waveform (linear

for parallelogram correction, parabola of 2nd order

for Pin cushion asymmetry correction and half-pa-

RS

Flip-Flop

CO

VHOThrHi

4I0

I0

2

HPLL1F

I0

RO

9

+

-

(PLL1 filter)

6

HOscF

+

-

+

-

4

VHOThrLo

VHOThrHi

8

from charge pump

VHO

VCO discharge

control

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