TEA2029 STMICROELECTRONICS [STMicroelectronics], TEA2029 Datasheet - Page 40

• Manufacturer Part Number: TEA2029
• Description: APPLICATION NOTE
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

TEA2028 - TEA2029 APPLICATION NOTE
VII.7 - AGC Key Pulse
Figure 79
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 ” 1”
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 ”C
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”
40/46
VIDEO SIGNAL
(Pin 27)
AGC SIGNAL
(Pin 20)
0V
12V
4.7 s
1.3 s
1
”
Burst
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”
- Power thyristor soft ”turn-off”
- Excellent efficiency of power stage due to very
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.
The secondary output current begins decreasing
and falls to 0 at the middle of retrace. The thyristor
is thus automatically ”turned-off”.
low ”turn-on” and ”turn-off” switching losses.
2.3 s
Without
Video Signal