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TEA2028 Scheda tecnica(PDF) 8 Page - STMicroelectronics

Il numero della parte TEA2028
Spiegazioni elettronici  APPLICATION NOTE
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Produttore elettronici  STMICROELECTRONICS [STMicroelectronics]
Homepage  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

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- In practice, at low amplitude video signals, it is
recommended to insert a low-pass filter before
the ”C27” capacitor so as to attenuate the chromi-
nance sub-carrier and the noise components.
The aim is to reduce the phase variations of the
detected sync pulse and thus enhance the hori-
zontal scanning stability.
27
220nF
1k
CC
V
+
Chroma
Burst
Figure 8
V.2.2 - Memorizing the sync pulse 50% value
The objective is to memorize the voltage corre-
sponding to 50% of the line sync pulse VS1 by using
an external capacitor connected to Pin 26 (see
Figure 9).
The overall arrangement comprises two compara-
tors.
- Comparator C2 : delivers an output voltage ”V1”
by comparing VS1 +VD,V26 and the voltage drop
across two resistors.
- Comparator C3 : which delivers a constant output
current thereby maintaining on capacitor ”C26”,
the voltage V50% corresponding to 50% of peak
to peak sync pulse.
During the line scanning, diode ”D” is reverse bi-
ased : VS1 +VD =V1 <V26 and C3 will deliver a
current ID which will discharge the capacitor.
During sync pulse interval, VS1 +VD =VP +VD,
diode ”D” begins conducting and thus :
V1 =(VP +VD)- (2 R i1). Since the capacitor has
been slightly discharged
⇒ V1 >V26, comparator
C3 begins charging the capacitor until C2 is
brought to equilibrium.
At this time, I1 =
i
2
where i =
V26 − VD − VN
R
thus V1 =VP +VD =2
i
2
=VP +VN +2 VD -V26
and V1 =V26
⇔ V26 =
VP
+ VN
2
+VD =V50%
A high value C26 capacitor will thus memorize the
voltage level correspondingto 50% of the line sync.
pulse.
V.2.2.1 -
IC
ID
Ratio calculation
During the line scanning period (TH -TS), the
capacitor C26 will loose a charge equivalent to :
ID (TH -TS).
This energy must be recovered before the end of
sync pulse such that : IC
⋅ tS >ID (TH -TS)
therefore
IC
ID
>
TH tS
tS
IC
ID
> 12.6
In practice, for C26 = 100nF, ID =25
µAand
IC = 800
µA.
i
R
VN
CC
V
+
V50%
IC
ID
V1
V
+
V
S1
D
i 1
C2
2R
xK
CC
V
+
26
C26
t
V
+
V
S1
D
V50%
V
+
V
ND
0
I26
V26
IC
ID
t S
V1
V
+
V
P
D
D
t
t
t
V50%
0
0
V50%
Figure 9
TEA2028 - TEA2029 APPLICATION NOTE
8/46


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