UTC L388 12
LINEAR INTEGRATED CIRCUIT
VOICE-SWITCH CIRCUIT FOR
HANDSFREE SPEAKERPHONE
TAM
DESCRIPTION
The UTC L388 12 contains all the necessary circuitry,
amplifiers, detectors, comparators and control functions to
implement a high performance, voice-switched, ”hands-free”
function in an answering machine.
The UTC L 388 12 is designed for answering machines
that are either powered from the telephone line or from a
mains powered dc. supply.
Filtering of both the audio and control signals in both
transmitter and receiver channels possible.
An external loudspeaker amplifier has to be used, normally
the same as used for the answering machine.
SOP-16
DIP-18
FEATURES
*Settable gain dynamics (25 or 50dB)
*Low power consumption, totally 1.0mA at 3.3V typical.
*Background noise compensation in the transmitting channel
*with
hold function.
*Excellent noise performance.
*Both channel input amplifiers have balanced inputs.
*Minimum of external components needed for function.
PIN CONFIGURATION
16-pin SO package
CTR
-Txin
+Txin
F2out
Txout
TxDetin
TxDetout
NDet
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
GND
+V
-Rxin
F5out
Rxout
RxDetin
RxDetout
CMP
F2out
Txout
TxDetin
TxDetout
N Det
CMP
RxDetout
Rx Detin
Rxout
1
2
3
4
5
6
7
8
9
18-pin DIP package
18
17
16
15
14
13
12
+Txin
-Txin
CTR
NC
GND
+V
-Rxin
11 +Rxin
10
F5out
UTC
UNISONIC TECHNOLOGIES CO. LTD
1
QW-R108-013,A
UTC L388 12
PIN DESCRIPTIONS
16-pin
SO
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
LINEAR INTEGRATED CIRCUIT
DESCRIPTION
Control input for gain dynamics (25 or 50dB), mute and disable.
Transmitter channel negative input. Input impedance 3kΩ.
Transmitter channel positive input. Input impedance 100kΩ.
Output of the second amplifier in the transmitter channel.
Transmitter channel output. Min. ac load impedance 10kΩ.
Input of the transmitter channel signal detector. Input impedance 13kΩ.
Output of the transmitter channel signal detector. Goes negative referred
to the internal ref. voltage of app.2V when a transmitter signal is present.
Background noise detector output. Goes positive referred to the internal
ref. Voltage of app.2V when a background noise signal is present.
Comparator input..
Summing point to the different Detector outputs.
Output of the receiver channel signal detector. Goes positive referred to
the internal ref. Voltage of app.2V when a receiver signal is present
Input of the receiver channel signal detector. Input impedance 13kΩ.
Receiver channel output. Min. ac load impedance 10kΩ.
Output of the second amplifier in the receiver channel.
Receiver channel positive input. Input impedance 140kΩ.
Receiver channel negative input,Input impedance 20kΩ.
Supply of the speech switching circuitry. A shunt regulator, voltage
apprx. 3.3V at 1.0mA.
System ground.
Not connected.
18-pin
DIP
16
17
18
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
SYMBOL
CTR
-Txin
+Txin
F2out
Txout
TxDetin
TxDetout
NDet
CMP
RxDetout
RxDetin
Rxout
F5out
+Rxin
-Rxin
V+
GND
NC
BLOCK DIAGRAM(16-pin SO package)
1
5
F3
Control
F6
12
16
13
11
4
6
15
+
F2
F5
2
3
+
-
F1
8
7
9
10
Ref.
14
F4+
UTC
UNISONIC TECHNOLOGIES CO. LTD
2
QW-R108-013,A
UTC L388 12
MAXIMUM RATINGS
PARAMETER
Speech switch supply current
Voltage Pin 1-14
Operating temperature
Storage temperature
LINEAR INTEGRATED CIRCUIT
SYMBOL
I
D
T
opr
Tstg
RATINGS
10
-0,5 ~ Vpin
15
+0.5
-20 ~ +70
-55 ~ +125
UNIT
mA
V
℃
℃
TEST CIRCUIT
V
+
+
V
Txout
RxDetin 11
I
D
100nF
15 V
+
R
Txout
10μF
10μF
+
I
D
100μF/16V
15 V+
5 Txout
6 Tx
Detin
GND 16
10μF
Rxout 12 +
Rx
Detin
11
V
Rout
R
Rout
C
RX 10μF
+
L388 12
GND RxDetout 10
16
V
Ref
C
Tx
F2out
RF2out
+
4 F2
out
+
L388 12
F5out 13
V
+
I
Txin
4.7μF
V
Txin
1μF
-Rx in + I
Rxin
3 +Tx
in
Rx
Detout
CTR 14
N
2
-Txin
8
Det
Tx
Detout
CMP
7
9
10
1
+
CTxDet
1μF
0.1μF
C
RxDet
V
Rxin
+
+
R
CTR
F5out
R
F5out
ISOLATION AND MEASUREMENT OF V
R
ef
I
NDet
V
NDet
I
TxDet
V
TxDet
I
RxDet
I
CTR
V
CMP
V
RxDet
V
CTR
ELECTRICAL CHARACTERISTICS
(Ta=25℃,f=1kHz, R
CTR
=0,C
TxDet
=0,R
Tx
out=
∞,R
RXout
=
∞,R
F2
out=
∞,
R
F5
out=
∞,R
TX
=0,R
RX
=0,C
RxDet
=0 and
I
D
=1.0mA unless otherwise noted)
PARAMETER
Speech control section
Terminal voltage, V
+
Internal reference voltage, V
R
ef
Frequency response for all amplifiers
Transmit gain, 20·Iog(V
Txout
/V
Txin
)
TEST CONDITIONS
I
D
=1.0mA
MIN
TYP
3.3
1.96
MAX
UNIT
V
V
dB
dB
dB
dB
dB
dB
dB
dB
dB
dB
dB
dB
dB
dB
200-3400Hz,Relative 1 kHz
V
CMP
=V
R
ef
-0.1V
V
CMP
=V
R
ef
+0.1V
V
CMP
=V
R
ef
-0.1V R
CTR
=100k,V
CTR
=V+
V
CMP
=V
R
ef
+0.1V R
CTR
=100k,V
CTR
=V+
Receive gain, 20·Iog(V
R
xout/V
R
xin)
V
CMP
=V
R
ef
+0.1V
V
CMP
=V
R
ef
-0.1V
V
CMP
=V
R
ef
+0.1V R
CTR
=100k,V
CTR
=V+
V
CMP
=V
R
ef
-0.1V R
CTR
=100k,V
CTR
=V+
Max transmit detector gain,
V
TxDet
<200mV
P
,C
TX
=100nF
20·Iog(V
T
xdet/V
T
xin)
V
CMP
=V
R
ef
+0.1V
V
CMP
=V
R
ef
-0.1V
Max transmit detector gain,
V
RxDet
<200mV
P
,C
TX
=100nF
20·Iog(V
R
xdet/V
R
xin)
V
CMP
=V
R
ef
+0.1V
V
CMP
=V
R
ef
-0.1V
Background noise rectifier gain,(note 1) V
CMP
=V
R
ef
-0.1V,C
txdet
=1μF
V
CMP
=V
R
ef+
0.1V,C
txdet
=1μF
-1
41.5
41.5
26.5
26.5
1
44
-6
44
19
29
-21
29
4
67.5
42.5
53
28
6.0
Hold
-3.5
21.5
-18.5
6.5
37
22.5
UTC
UNISONIC TECHNOLOGIES CO. LTD
3
QW-R108-013,A
UTC L388 12
PARAMETER
LINEAR INTEGRATED CIRCUIT
TEST CONDITIONS
MIN
80
2.4
16
10
10
10
10
2% distortion, R
Txout
=R
Rxout
=25 kΩ
2% distortion, R
Txout
=R
Rxout
=25 kΩ
V
CMP
=V
R
ef
-0.1V,v
TXIn
=0V
V
CMP
=V
R
ef
+0.1V,v
RXIn
=0V
V
txDetIn
=V
R
ef
+0.1V
V
RxIn
=V
R
ef
-0.1V
V
CMP
=V
R
ef
-0.1V
V
RxDetIn
=V
R
ef
+0.1V
V
TxDetIn
=V
R
ef
+0.1V
V
RxDetIn
=V
R
ef
-0.1V
V
TxDetIn
=V
R
ef
-0.1V
V
CMP
=V
R
ef
-0.1V
V
TxDetIn
=V
R
ef
+0.1V
V
CMP
=V
R
ef
+0.1V
V
TxDetIn
=V
R
ef
-0.1V
V
CMP
=V
R
ef
+0.1V
V
CMP
=V
R
ef
-0.1V
V
TxDetIn
=V
R
ef
+0.1V
Tx mode=max Tx gain,
Rx mode=max Rx gain
V
CMP
=V
R
ef
+0.35V,R
CTR
=100kΩ
V
CMP
=V
R
ef
+0.35V
0.55
TYP
100
3.0
20
MAX
120
3.6
24
UNIT
kΩ
kΩ
kΩ
kΩ
kΩ
kΩ
kΩ
mV
p
mV
p
dB
psof
dB
A
mA
mA
μA
μA
V
V
mA
μA
nA
V
+Tx
In
input impedance
-Tx
In
input impedance
-Rx
In
input impedance
Tx
out
ac, load impedance
Rx
out
ac, load impedance
F2
out
ac, load impedance
F5
out
ac, load impedance
Transmitter channel output swing, V
TXOut
Receiver channel output swing, V
RxOut
Transmitter output noise, V
TxOut
Receiver output noise, V
RxOut
Tx
Det
sink current, I
TxDetOut
Rx
Det
source current, I
RxDetOut
T
xDet
source current, I
TxDet
Rx
Det
sink current, I
RxDetOut
Tx
Det
swing relative to V
Ref
, V
TxDetOut
Rx
Det
swing relative to V
Ref
, V
RxDetOut
N
Det
sink current (fast charge), I
NDet
N
Det
source current, I
NDet
N
Det
Ieakage current(hold),I
NDet
N
Det
swing relative to V
Ref
,V
NDet
CMP(comparator) sensitivity,
Transmit(Tx) mode to receive
(Rx) mode or vice versa
CTR voltage for 25 dB dynamics,V
CTR
CTR voltage for mute,I
CTR
CTR voltage for disable,V
CTR
2.5
-30
(note 2)
(note 2)
500
500
-75
-75
-6.0
6.0
-2.5
30
-0.7
+0.7
-4.5
5
-100
-0.45
50
-1.5
7
3
100
mV
V
+
V
μA
V
Notes:
V
NDet
-V
R
ef
1.
20· log
(
)
V
TxDet
-V
TxDetO
V
NDet
=voltage at noise detector output
V
R
ef
=reference voltage (about 1.9V)
V
TxDet
=Voltage at transmit detector output
V
TxDetO
=voltage at transmit detector at the point
when the voltage at the noise detector starts
moving when a signal at transmit channel input is
gradually increased (threshold, typical value 30 mV)
2.
Depends on V
+.
Channels are tracking.
UTC
UNISONIC TECHNOLOGIES CO. LTD
4
QW-R108-013,A
UTC L388 12
LINEAR INTEGRATED CIRCUIT
FUNCTIONAL DESCRIPTION SPEECH CONTROL SECTION
Transmitter and Receiver Channels
The transmitter and receiver channels consist of three amplifying stages each, F1,F2,F3 and F4,F5,F6.The inputs
of the amplifiers must be ac. coupled because they are dc. vise at the internal reference voltage(≈2V) level. F1 and
F4 are fixed gain amplifiers of 30,5 dB and 15.5 dB respectively, while the rest of them are of controlled gain type.
The gain of F2,F3 as well as F5 and F6 is controlled by comparators. The comparator receives its information partly
from the summing point of the transmitter, receiver and background noise detectors at CMP input and partly through
the control input, CTR, which controls the gain dynamics (25 or 50 dB). Amplifiers F2 and F3 have the maximum
gain when the transmitter channel is fully open, consequently the amplifiers F5 and F6 will have minimum gain and
vice versa. See figure.1 and figure. 9.
The positive input on transmitter and the negative input on receiver channel has a rather high input impedance. It
renders a good gain precision and noise performance when used with low signal source impedance. The differential
input of the transmitter channel can be used to suppress unwanted signals in the microphone supply, see figure. 5.
Signal Detectors and Comparator
The signal detectors sense and rectify the receiver and microphone signals to opposite polarities referenced to the
internal reference voltage of approx. 2V.The voltage at RxDet will go positive and at TxDet negative in the presence
of a signal at the respective channel input. In the idle(no signal) state, the voltages at RxDet, TxDet and CMP are
equal to the internal reference voltage. Signal at Txin will result in an decreasing level at TxDetout and hence also at
CMP input. the transmitter channel and decrease it in the receiver channel. Signal at Rxin will do vice versa. The
voltages RxDetout and TxDetout controlthus the gain setting in respective channel through the comparators using
the CMP input as a summing point with an input current of less than 1μA. The attack time in the receiver channel is
set by C2 together with C1 and either by the maximum current capability of the detector output or it with R2 added.
The transmitter channel works likewise. See figure.5.
The decay time in the receiver and transmitter channels is set by C2 and C3 respectively. The resistor in the time
constant is formed by an internal 200kΩresistor in parallel with the external resistors R3 and R4 respectively. The
influence of eventual R1 and R2 can be omitted.
The text above describes the case when only one channel is open at a time and there is a distinctive pause
between signals at receiver and transmitter channel inputs so the circuit will have time to reach its idle state. See
figure.10 A) to E). If one of the channels gets an input signal immediately after the signal has disappeared from the
other channel input the effective decay time , as the CMP input sees it, will be shorter than in the first case. See
figure.10 F) to G). The capacitor C4 at CMP -input sets the speed of the gain change in the transmitter and receiver
channels. The capacitors C2 and C3 should be dimensioned for a charging time of 0.5-10mS and for a discharge
time of 150-300mS. The question of switching times being a highly subjective proposition, is in large dependent of
the language being spoken in the system , caused by the varying sound pressure picture of the different languages.
A hysteresis effect is achieved in the switching since the level detectors sense the signals after F2 and F5
respectively (F2 and F5 are affected by the gain setting). For example: If the transmitter channel is open (maximum
gain),a smaller signal at Txin is enough to keep the channel open than would be necessary to open it when the
receiver channel is open. The output swing of the level detectors is matched for variations in the supply voltage. The
detectors have a logarithmic rectifier characteristic whereby gain and sensitivity is high at small signals. There is a
break point in the curve at a level of + 200mV from the internal reference voltage app.2V,where the sensitivity for
increasing input signals decreases with factor of 10, increasing the detectors dynamic range. See figure.8.
Background Noise Detector
The general function of the background noise detector in the transmitting channel is to create a positive signal (in
respect to the reference) so that, when coupled to the summing point at the CMP input, will counteract the signal
from the transmitter level detector representing the actual sound pressure level at the microphone. This counteracts
the noise from influencing the switching characteristics. The input signal to the background noise level detector is
taken from the output of the transmitter detector, a voltage representing the envelope of the amplified microphone
signal. The detector inverts and amplifies this signal 2 X (transmitting mode) and has on it is output a RC network
consisting of an internal resistor of 100k and an external capacitor C4.The voltage across C4 is connected to the
CMP input (summing point) via a resistor R5.The resistor R6 is important in order to keep the charging current of C4
within safe limits in regard of high charge peaks that could be audible in the system. The extent to which the NDet
UTC
UNISONIC TECHNOLOGIES CO. LTD
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QW-R108-013,A
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