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TEA7540STMN/a2000avaiHANDSFREE CONTROLLER
TEA7540. |TEA7540STN/a4avaiHANDSFREE CONTROLLER


TEA7540. ,HANDSFREE CONTROLLERBLOCK DIAGRAM2/12TEA7540PIN FUNCTIONN° Name Function1 GND Ground2 CS Chip Select3 OUTE Transmit Att ..
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TEA7540-TEA7540.
HANDSFREE CONTROLLER
TEA7540
HANDSFREE CONTROLLER
NOISE/SPEECH DISCRIMINATIONIN EMIS-
SION AND RECEPTION
INTEGRATED SIGNAL GAIN COMPRESSOR BOTH MODES
PROGRAMMABLE ATTENUATORSIN BOTH
MODES
ADAPTED TO ACOUSTIC PARAMETERS OF
ALL CABINETS
LOW OPERATING VOLTAGE 2.5V
LOW OPERATING CURRENT 2.1mA
CHIP SELECT BETWEEN HANDSFREE AND
MONITORING MODES
DESCRIPTION

This28 pinsICisan innovative approachto quality
handsfreetelephonesets.It results fromanextensive
researchon speechsignal.
July 1998
HYST2
TIM
CCE
CE3
CE2
MICOUT
CE1
INE
VREF
INR
RECOUT
CR1
CR3
CR2
CCR
AMPIN
AMP SUP
VOL23
D93TL005
GND
OUTE
ATTE
HYST1 ATR
ATTR
OUTR
IREF
PIN CONNECTION
(top view)
SO28
ORDERING NUMBERS:
TEA7540FP
1/12
BLOCK DIAGRAM
TEA7540

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FUNCTIONAL DESCRIPTION
SWITCHED ATTENUATORS
Fig.A representsa block diagramofa handsfree
subset with attenuatorsin signal mode. To pre-
vent the system from howling, the total loop gain,
including acoustic feedback through the housing
and sidetone coupling, must be less than 0dB.
For this purpose, two switched attenuators arein-
sertedin each mode (emission and reception).
The attenuationis shifted from one modeto the
other, resulting from the speech level comparison
between each way. prevent the circuitto switch continuouslyin
one way, the operationof the IC must be fully
symetricalin both ways. This involves signal com-
parison, attenuationvalue.
GAIN COMPRESSOR TEA7540, two signal compressors are inserted each mode before the signal comparison, so
the signal coming from each end has the same
level (100mV peak), the lossesin each way (for
instance losses resulting from the line lengthin
receiving mode) are compensated and the signal
comparisonis fully symetrical. The time constant each signal compressor decreases 80 times
more quickly thanit increasesto prevent from
noise increasing between words. The compress-
ing depthis 38dB.
BACKGROUND NOISE DISCRIMINATION additional feature providedin TEA7540is
background noise level discriminationin each
way. The IC stores the background sound level
witha long time constant(3to5 seconds depend-
ing on an external RC) and comparesit with the
incoming signalin orderto distinguisha useful
signal (speech) from the background noise. This
background noise memorizationis also usedto
compensatethe noisein each mode before signal
comparison: the noise levelin each modeis sub- Name Function GND Ground CS Chip Select OUTE Transmit Attenuator Output ATTE Transmit Attenuator Input HYST1 Transmit Channel Hysteresis HYST2 Receive Channel Hysteresis TIM RC Timer CCE Time Constantof the Transmit Signal Compressor CE3 Transmit Background Noise Memorization Output CE2 Transmit Peak DetectorOutput CE1 Transmit Rectifier Input MICOUT Transmit Signal Compressor Output INE Transmit Signal Compressor Input Vref V+/2- Reference Voltage INR Receive Signal Compressor Input RECOUT Receive Signal Compressor Output CR1 Receive Rectifier Input CR2 Receive Peak Detector Output CR3 Receive Background Noise Memorization Output CCR Time Constantof the Receive Signal Compressor AMPIN Handset Preamplifier Input AMP SUP Handset Preamplifier Power Supply VOL Volume Control ATR Attenuation Value ATTR Receive Attenuator Input OUTR Receive Attenuator Output Iref Reference Current Source V+
PIN FUNCTION
TEA7540

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stracted from the incoming signal before the com-
parison. So very high noise levelin one mode
cannot trouble the comparison between the useful
signals.
The resultof the comparison manages the attenu-
atorsin the following way: The maximum attenuationis switched on
the mode where the speech signalis the
lowest. The maximum attenuationis fixed two external resistor (maximum 52dB).
The time constantof the switchis fixedby
the timer viaan external capacitor. When neither partyis talking both attenu-
ators are settoa medium attenuation.
Thus each modeisin idle mode. The time
constantof the switch from active modeto
idle mode mustbe long enoughto prevent
from switchingto idle mode between two
words (see fig.B). This time constantis
fixedbyan external RC.
FigureA
Rec
SPEECH/NOISE maxdB
0dB
IDLE(A max/2)
D93TL009A
FigureB
TEA7540

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TEA7540 OPERATION
TEA7540is powered through an external shunt
regulator (for instance the shunt regulatorof the
monitor amplifier TEA7532)or an external zener
diode. can workata very low voltage (2.5V) over the
circuit andit hasa low current consumption
(2.1mA).
It’s also possible via the chip select pin (CS)to
put the handsfree functionin standbyto use the
circuitin monitoringmode with the handset micro-
phone.
TEA7540is designedto work withall kindof mi-
crophone, including Electret.
TEA7540 also handles the handset microphone
signal (AMP IN) when the systemis setto normal
conversation mode.
FigureC:
Application Diagram (Exampleof high range telephone set using TEA7540).
Symbol Parameter Value Unit
+ Supply Voltage 12 V
Top Operating Temperature -20to70 °C
Tstg Storage Temperature Range -65to 125 °C
ABSOLUTE MAXIMUM RATINGS
TEA7540

5/12
Symbol Parameter Test Condition Min. Typ. Max. Unit Fig.
SUPPLY SECTION
+ Supply Voltage 2.5 7.0 V 1 Current Consumption Vcs=0
Handsfree mode
Vcs= N.C.
Monitoring mode
TRANSMIT SECTION
COMPRESSOR
Rine Compressor input impedance PIN13 7.5 10.0 14.5 KΩ 1 Compressor Range 16.5 dB
G1max Maximum Gain 40.5 41.5 42.5 dB 2
G1min Minimum Gain 24.0 25.0 26.0 dB 3 Output Voltage PIN12 compressingrange 160 200 240 mVpp 4
Tdh Transmit Distortion 3.0 % 4
Icce1 Compressor dacay timecurrent Increasing gain 1.0 1.25 1.5 μA5
Icce2 Compressor rise time current Decreasing gain 65 85 105 μA6
Vcce1 Voltage drop PIN8
Vcce= Vref- Vpin8
G1max 0 20 mV 2
Vcce2 G1min 175 225 275 mV 3
PEAK DETECTOR
Rinpd Input impedance PIN11 7.5 10.0 14.5 KΩ 1
Ice2 Rise time current 16 20 24 μA7
NOISE MEMORIZATION Max voltage droponpin9 33 36 45 mV 8
ATTENUATOR
Rina Input impedance PIN4 7.5 10 14.5 KΩ 1
ATE- Attenuation=
20log(Voute/Vatte)
Mode: Inactive Tx,Rx
Comp. Max Gain
Rpin24= 11KΩ
Rpin24= 15KΩ
ATE-active Mode: Active Rpin24= 11KΩ 0 1.5 3 dB 10
ATE-IDLE Mode: Noise Tx,Rx Comp.
Max Gain
Rpin24= 11KΩ 23 dB 11
RECEIVE SECTION

COMPRESSOR
Rinr Input impedance PIN15 7.5 10.0 14.5 KΩ 1 Compressor Range 20.5 dB
G2max Maximum Gain 35.5 36.5 37.5 dB 2
G2min Minimum Gain 15.0 16.0 17.0 dB 3
Vmicout Compressor output voltage PIN16 compressingrange 160 200 240 mVpp 4
Rdh Receive Distortion 3.0 % 4
Iccr1 Compressor decaytime
current
Increasing gain 1.0 1.25 1.5 μA5
Iccr2 Compressor rise time current Decreasing gain 65 85 105 μA6
Vccr1 Voltage drop PIN20
Vccr= Vref- Vpin20
G2max 0 20 mV 2
Vccr2 G2min 175 225 275 mV 3
ELECTRICALCHARACTERISTICS
(Referto testcircuits,Tamb =25°C;V+= 3V;f =1KHzunless otherwise
specified).
TEA7540

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