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PDF L38812 Data sheet ( Hoja de datos )
| Número de pieza | L38812 | |
| Descripción | LINEAR INTEGRATED CIRCUIT | |
| Fabricantes | Unisonic Technologies | |
| Logotipo |  |
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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.
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 1
-Txin 2
+Txin 3
F2out 4
Txout 5
TxDetin 6
TxDetout 7
NDet 8
16 GND
15 +V
14 -Rxin
13 F5out
12 Rxout
11 RxDetin
10 RxDetout
9 CMP
SOP-16
DIP-18
18-pin DIP package
F2out 1
Txout 2
TxDetin 3
TxDetout 4
N Det 5
CMP 6
RxDetout 7
Rx Detin 8
Rxout 9
18 +Txin
17 -Txin
16 CTR
15 NC
14 GND
13 +V
12 -Rxin
11 +Rxin
10 F5out
UTC
UNISONIC TECHNOLOGIES CO. LTD 1
QW-R108-013,A
1 page  
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
5
QW-R108-013,A
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