ST TDA911 Series, Application Note, Page: 23 / 62

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AN1290 Vertical Section
5 Vertical Section
5.1 Theory of Operation
5.1.1 Structure of V Section
The vertical section of the TDA9112 remains basically the same as in the TDA9111. It includes a
sync detector, a V ramp generator with Free-running and Synchronized modes, an AGC system to
maintain the amplitude of the V ramp constant, and S- and C-correction generators.
Vertical amplitude and position as well as S- and C-correction are controlled through I²C (Registers
07 to 0A). All important voltages for the Vertical section are derived from the 8V reference voltage
(same as for the Horizontal section), and are defined as fractions of this voltage.
5.1.2 Ramp Generator and AGC Loop
The structure of the complete ramp generator is represented in Figure 6 .
There are two different operating modes, depending on whether a sync pulse is present:
When there is no sync pulse, switch NoS (= No Sync) is closed, and switch S ( = Sample) is open.
Comparison voltage for discharging C
o
is set to 5V (or more precisely to 5/8 of reference voltage).
Because 5V are forced on C
s
, current I'
o
is null or very small, and only current I
o
is mirrored to
oscillator capacitor C
o
.
Combined with an “end of discharge” comparator with a 2V threshold, this circuitry provides a linear
sawtooth, taking place between the 2V and 5V levels. Discharge takes place through a 500
Ω
resistor switched to ground, and a simple calculation provides:
With C
o
= 0.15 µF, V
1
= 5 V and V
2
= 2 V, the discharge time is 69 µs.
Figure 6: Vertical Oscillator and AGC
1.6V
5V or
7V
2 22 20
VSync
5V
Co
Cs
5V
Io
I'o
Io-I'o
NoS
V
syn
c
p
roc
es
so
r
2V
S Rs
R
R1
R2
(A)
VA
RxCxLog
V
1
V
2
-------
è ø
ç ÷
æ ö
Discharge Time =