LS656B 查看數據表（PDF） - STMicroelectronics
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LS656B Datasheet PDF : 15 Pages
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LS656
The corresponding current is
Therefore
∆I
=
∆V7
R3
ZM
=
∆V6
∆I
=
R3
1
+
Ra
Rb
The total impedance across the line connections
(pin 11 and 9) is given by
ZML = R1 + ZM//(R2 + ZB)
By choosing ZM ≥ R1 and ZB ≥ ZM
ZML ≅ ZM = R3
1 +
Ra
Rb
The received signal amplitude across pin 11 and 10
can be changed using different value of R1 (of
course the relationship ZL/ZB = R1/R2 must be al-
ways valid).
The received signal is related to R1 value according
to the approximated relationship :
VR
=
2
VRI
R1
R1 + ZM
Note that by changing the value of R1, the transmis-
sion signal current is not changed, being the micro-
phone amplifier a transconductance amplifier.
3. Automatic Gain Control
The LS656 automatically adjusts the gain of the
sending and receiving amplifiers to compensate for
line attenuation.
This function is performed by the circuit of fig. 9.
The differential stage is progressively unbalanced
by changing VG in the range 1 to 2 V (VREFG is an
internal reference voltage, temperature compen-
sated).
It changes the current IG, and this current is used as
a control quantity for the variable gain stages (am-
plifier (4) and (5) in the block diagram). The voltage
VG can be taken :
a) from the LS656 itself (both in variable and in fixed
mode) and.
b) from a resistive divider, directly at the end of the
line.
a) In the first case, connecting VG (pin 8) to the regu-
lator bypass (pin 5) it is possible to obtain a gain
characteristic depending on the current.
In fact (see fig. 6)
V5 = VB + V7 ≅ VB = (IL - Io) R3
Thestarting point of theautomaticlevel controlis ob-
tained at IL = 25 mA when the drain current
Io = 7.5 mA.
Minimum gain is reached for a line current of about
50 mA for the same drain current Io = 7.5 mA. When
Io isincreased by means of the external resistor con-
nected to pin 4, the two above mentioned values of
the line current for the starting point and for the mini-
mum gain increase accordingly.
It is also possibleto changethe starting point without
changing Io by connecting pin 8 to the centre of a re-
sistive divider placed betweenpin 5 and ground (the
total resistance seen by pin 5 must be at least 100
KΩ). In this case, the AGC range increases too; for
example using a division 1 : 1 (50 K/50 K) the AGC
starting point shifts toaboutIL =40 mA, and the mini-
mum gain is obtained at IL = 95 mA. In addition to
this operation mode, the VG voltage can be main-
tained constant thus fixing the gain value (Rx, Tx) in-
dependently of the line conditions.
For this purpose the VDD voltage, available for sup-
plying the MF generator, can be used.
b) When gains have to be related to the voltage at
the line terminals of the telephone set, it is neces-
sary to obtainVG froma resistive divider directly con-
nected to the end of the line.
This type of operationmeets the requirements of the
French standard. (See the application circuit of
fig. 13).
4. Transducer Interfacing
The microphone amplifier (3) has a differential input
stage with high impedance (≅ 40kΩ) so allowing a
good matching to the microphone by means of ex-
ternal resistor without affecting the sending gain.
The receiving output stage (6) is particularly in-
tended to drive dynamic capsules. (Low output im-
pedance (100Ω max) ; high current capability 3
mAp).
When a piezoceramic capsule is used, it is useful to
increase the receiving gain by increasing R1 value
(see the relationship for VR).
Whit very low impedance transducer, DC decou-
pling by an external capacitor must be provided to
prevent a large DC current flow across the
transducer itself due to the receiving output stage
offset.
5. Multifrequency Interfacing
The LS656 acts as a linear interface for the Multifre-
quency synthesizer M761 according to a logical sig-
nal (mute function) present on pin 3.
When no key of the keyboard is pressed the mute
state is low and the LS656 feeds the M761 through
pin 15 with low voltage and low current (standby op-
eration of the M761). The oscillator of the M761 is
not operating.
When one key is pressed, the M761 sends a ”high
state” mute condition to the LS656. A voltage com-
7/15
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