TEA2028 datasheet(6/47 Pages) STMICROELECTRONICS

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V - FUNCTIONAL DESCRIPTION

Majority of the on-chip analog functions were com-

puter simulated and results such as temperature

variation, technological characteristic dispersion

and stability, have led to the enhancement and

implementation of actually employed structures. A

parallel in-depth study of the device implemented

in form of integrated sub-sections is provided to

analyze the overall performance in a TV set.

V.1 - Internal Voltage and Current References

V.1.1 - 1.26V Voltage reference

For optimum operation of the device, an accurate

and temperature-stable voltage generator inde-

pendent from VCC variations is used (Band-gap

type generator).

The generated 1.26V is particularly used as refer-

ence setting on input comparators.

V.1.1.1 - Generator block diagram

VBE

GENERATOR

-2mV/

°C

+0.086mV/

°C

dVO

VBE

=+ A .

= 1.26V

Figure 3

with

λ =

⋅ T

= 25.7mV at + 25

= + 0.086mV/

dVBE

VBE(25’) − 1.26

= - 2mV/

if A

λ = 1.26 - VBE

Then : VO = 1.26V (temperature-independant)

In practice, maximum drift due to temperature can

be + 0.23mV/

i.e.,

± 1.5% for a ∆Tof 80oC.

BE1

BE2

1.26V

IREF

I14

R EXT

VCC

Band

Gap

3.32k

Ω

Figure 4

V.1.2 - Current reference

This is implemented using the 1.26V generator in

combination with an external resistor.

IREF

≈ I14 =

V14

REXT

1.26

+ VBE1 − VBE2

REXY

Let’s I14 = I and VEB1 =VBE2

then : IREF =

1.26

REXT

= 380

µA

Thus, it follows that IREF is accurate and inde-

pendent of both VCC and temperature.

A set of current generators proportional to IREF

current are used in various circuit blocks.

V.2 - Line Sync. Extraction

Horizontal and vertical time bases should be syn-

chronized with corresponding sync. pulses trans-

mitted inside the infra-black portion of video signal.

The duty of this stage is to extract these sync

pulses. The output signal, called composite sync,

contains the vertical sync which is transmitted by

simple inversion of line sync. pulses.

The vertical sync pulse is then extracted from this

composite signal.

The main advantage of this arrangement is its

ability to operate at video input signal levels falling

within 0.2V to 3V peak-to-peak range and at any

average value.

The operating principle is to lock the black level of

the input signal (Pin 27) onto internaly fixed voltage

(VN) and then memorize the average voltage of the

sync pulse by using an integrating capacitor con-

nected to Pin 26.

Finally, the composite sync signal is delivered by a

comparator the inputs of which are driven by V50%

and video signals.

TEA2028 - TEA2029 APPLICATION NOTE

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