ST TDA911 Series, Примечание к применению

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Obsolete Product(s) - Obsolete Product(s)
Vertical Section AN1290
26/62 STMicroelectronics Confidential
5.1.5 AGC Loop Stability
Like all sampled feedback systems, the AGC loop has a particular instability mode, which require
special attention:
At a first sampling time, the vertical sawtooth peak voltage is compared to the pre-set 5V value. A
corrective current proportional to the difference is added to the charge current of the oscillator
capacitor with the appropriate sign to decrease the voltage difference at the next sampling time.
Nevertheless, if the correction is too large, the next voltage difference may present a higher value
with the opposite sign, this will lead to instability.
Referring to Fig. 3, let
∆
V
o
be the initial voltage error on capacitor C
o
,
∆
V
o
amplified by gain A
causes a voltage change
∆
V
s
on the sampling capacitor:
(t
s
is the sampling time; charge current is almost constant during t
s
because t
s
= 13 µs
≤
R
s
x C
s
)
This entails a change -
∆
V
s
/ R in total charge current, so that the next sampled voltage will be
changed by:
where T = vertical period.
If this value is higher than the initial
∆
V
0
, there will be permanent amplitude oscillation. The
condition for stability is then:
With internal values A = 20, t
s
= 13µs, R
s
= 6k
Ω
, R = 18k
Ω
, and recommended values C
s
= 470 nF
and C
0
= 150 nF, this leads to T
≤
29.3 ms (34.6 Hz).
Although this seems a comfortable safety margin compared to the usual 50 or 60Hz in display
appliances, one must remember that all parameters (excluding T) in the formula possibly have a
spread. For stability, it is better to stick to the recommended component values.
5.1.6 S and C Correction (TDA9112 to TDA9116)
In the TDA9112, S and C corrections are independent. The circuits are similar to the ones in the
TDA9111 and are controlled by I²C programming (Registers 09 and 0A).
The same control principle is used for S and C corrections. Considering the V-oscillator schematic
diagram shown in
Figure 6 , capacitor C
0
receives currents I
0
and I'
0
and also two extra currents: one
for S correction, one for C correction.
∆
V
s
Ax
∆
V
0
R
s
-------------------x
t
s
C
s
------- =
∆
V
s
R
-----------x
T
C
0
-------
Ax
∆
V
0
xt
s
xT
R
s
xC
s
xRxC
0
--------------------------------------
–
è ø
ç ÷
æ ö
=
–
Ax t
s
xT
( )
R
s
xC
s
xRxC
0
--------------------------------------
1
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