U4084B TEMIC [TEMIC Semiconductors], U4084B Datasheet - Page 19

• Manufacturer Part Number: U4084B
• Description: Low-Voltage, Voice-Switched Circuit for Hands-Free Operation
• Manufacturer: TEMIC [TEMIC Semiconductors]

TELEFUNKEN Semiconductors
Rev. A1, 31-Jan-97
I)
The total loop gain (of figure 21) must add up to a value
< 0 dB to obtain a stable circuit. This can be expressed as:
G
Using the typical numbers mentioned above, and using
the equation G
pling can be determined:
G
An acoustic loss of at least 23 dB is necessary to prevent
instability and oscillations, commonly referred to as
“singing”. However, the following equations show that
greater values of acoustic loss are necessary to obtain
proper level detection and switching.
II) Switching Thresholds
To switch comparator C1, the currents I
be determined. When a receive signal V
Tip/Ring, a current I
figure 21) according to the following equation:
where the terms in the brackets are in V/V gain terms. The
speaker amplifier gain is divided by two since G
differential gain of the amplifier, and V
one side of that output. The current I
microphone circuit, is defined by:
where V
threshold occurs when I
equations yields:
This is the general equation defining the microphone volt-
age necessary to switch comparator C1 when a receive
signal V
receive attenuator is at maximum gain (+ 6.0 dB). Using
the typical values of equation 5 results in:
V
To switch comparator C2, the currents I
be determined. When sound is applied to the microphone,
a voltage V
a current I
I
I
V
MA
AC
M
3
1
M
= 0.52 V
+
+
<– 31 + 20.1 + (–15) + 0 + (–40) + 26.8 + –22.9
+ G
+
Loop Gain
V
V
R
M
L
T
V
L
3
M
2
is the microphone voltage. Since the switching
is present. The highest V
+ G
L
M
into TLI1:
R
L
T
is created by the microphone, resulting in
EXT
1
G
+ G
G
MA
R
R
EXR
+ G
R
1
3
3
. . . . . . 6
= –40dB, the required acoustic cou-
ST
flows through R3 into RLI2 (see
. . . . . . 4
1
[G
+ G
G
= I
EXR
R
3
EXR
, combining the above two
G
G
MA
+ G
2
. . . . . . 1
. . . . . . 2
SA
G
Preliminary Information
M
R
1
R
, coming from the
3
. . . . . . 3
+ G
occurs when the
2
1
is obtained from
2
L
and I
and I
G
SA
is applied to
SA
+ G
]
3
4
SA
have to
need to
. . . 5
AC
is the
< 0
Since G
amplifiers, it is divided by two to obtain the voltage V
applied to R
defined by:
Setting I
in:
This equation defines the line voltage at Tip/Ring neces-
sary to switch comparator C2 in the presence of a
microphone voltage. The highest V
circuit is in transmit mode (G
typical values for equation 9 yields:
V
At idle mode, where the gain of the two attenuators is –20
dB (0.1 V/V), equations 6 and 10 yield the same result:
V
Equations 6, 10, and 11 define the thresholds for switch-
ing, and are represented in figure 22.
The “M” terms are the slopes of the lines (0.52, 0.024, and
0.0019) which are the coefficients of the three equations.
The M
in that it defines the microphone signal level necessary to
switch to transmit in the presence of a given receive signal
level. The M
threshold. The M
defines the threshold level on one side (transmit or
receive) necessary to overcome noise on the other.
I
I
V
L
M
2
4
L
V
= 840 V
12678
+
+
= 0.024 V
M
+
R
EXT
V
V
R
R
4
line represents the receive to transmit threshold
V
M
L
4
= I
2
M
M
2
is the differential gain of the external transmit
[G
2
. Comparator C2 switches when I
, and combining the above equations results
Figure 22. Switching thresholds
T
(or V
L
EXR
line represents the transmit to receive
G
I
M
. . . . . . 11
R
R
MA
]
line represents the idle condition, and
M
R
4
2
= 0.0019 V
[G
. . . . . . 8
G
T
MA
T
G
G
EXR
= + 6.0 dB). Using the
L
M
2
) . . . . . . 10
EXT
G
U4084B
I
T
L
occurs when the
2
. . . . . . 7
G
V
M
EXT
L
T
4
]
= I
19 (26)
. . . 9
2
. I
4
is
2