TEA2028 datasheet(40/47 Pages) STMICROELECTRONICS

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4.7

µs

Burst

VIDEO SIGNAL

(Pin 27)

2.3

µs

1.3

µs

12V

AGC SIGNAL

(Pin 20)

Without

Video Signal

Figure 79

VII.7 - AGC Key Pulse

As illustrated below, this signal is used in some TV

sets to perform sampling window for Automatic

Gain Control of picture demodulation network.

This system is called ”clamped” AGC, and locks the

demodulated line sync amplitude and hence sets

the video signal amplitude.

This signal generated by line logic circuitry is cor-

rectly positioned by the first phase locked loop ”

φ1”

and includes the line sync pulse of the video signal.

This is an open-collector output.

VIII - APPLICATION INFORMATION ON FRAME

SCANNING IN SWITCHED MODE

(TEA2029 ONLY)

VIII.1 - Fundamentals (see Figure 80)

The secondary winding of EHT transformer pro-

vides the energy required by frame yoke.

The frame current modulation is achieved by

modulating the horizontal saw-tooth current and

subsequent integration by a ”L.C” network to reject

the horizontal frequency component.

VIII.2 - General Description

The basic circuit is the phase comparator ”C1”

which compares the horizontal saw-tooth and the

output voltage of Error Amplifier ”A”.

The comparator output will go ”high” when the

horizontal saw-tooth voltage is higher than the ”A”

output voltage. Thus, the Pin 4 output signal is

switched in synchronization with the horizontal fre-

quency and the duty cycle is modulated at frame

frequency.

A driver stage delivers the current required by the

external power switch.

The external thyristor provides for energy transfer

between transformer and frame yoke.

The thyristor will conduct during the last portion of

horizontal trace phase and for half of the horizontal

retrace.

The inverse parallel-connected diode ”D” conducts

during the second portion of horizontal retrace and

at the beginning of horizontal trace phase.

Main advantages of this system are :

- Power thyristor soft ”turn-on”

Once the thyristor has been triggered, the current

gradually rises from 0 to IP, where IP will reach

the maximum value at the end of horizontal trace.

The slope current is determined by, the current

available through the secondary winding, the

yoke impedance and the ”L.C.” filter charac-

teristics.

- Power thyristor soft ”turn-off”

The secondary output current begins decreasing

and falls to 0 at the middle of retrace. The thyristor

is thus automatically ”turned-off”.

- Excellent efficiency of power stage due to very

low ”turn-on” and ”turn-off” switching losses.

TEA2028 - TEA2029 APPLICATION NOTE

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