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PDF SAN2202a Data sheet ( Hoja de datos )

Número de pieza SAN2202a
Descripción SINGLE CHIP SPEAKERPHONE WITH 8 - OHM LOUDSPEAKER AMPLIFIER
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SAN2202
APPLICATION NOTE
SINGLE CHIP “SPEAKERPHONE” WITH
8 - OHM LOUDSPEAKER AMPLIFIER
1 Scope
This application note describes a low-cost add-on circuit for driving an 8-Loudspeaker to get a simple loudhearing
(“Speakerphone”) Telephone.
To possible ways of supplying the amplifier are shown:
SAN2202a:
SAN2202b:
Amplifier supply in shunt transistor path
Amplifier supply by power extraction circuit (SAN3020)
It can be added to any SA253x application without interfering with PTT-relevant parameters, like AC-impedance,
Sidetone cancellation or DC-mask.
2 Key Features
Low cost additional circuit enables use of loudhearing function
Full duplex “handsfree” operation via handset microphone and loudspeaker (at distances <2m between
telephone and user)
Can be added to any SA253x- application
Enables use of low cost 8- Loudspeaker
High output power with low distortion
Entirely powered from the telephone line
No influence of loudspeaker amplifier in AC-impedance, sidetone cancellation or DC-mask
Loudspeaker volume adjustable by potentiometer
Excellent sidetone cancellation of SAS253x circuits enables high loudspeaker volume without howling between
handset microphone and loudspeaker
SAN2202
PDS038-SA2531/2-001 Rev. B 21-03-00

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SAN2202a pdf
SAN2202
7 Circuit description
7.1 8loudspeaker amplifier
The amplifier is a simple Class-A type amplifier. Gain is set by R6, the bandwidth is imited by CC4 (to avoid
howling). Q4 and Q6 should be matched in gain (complementary types).
Nearly all the line current will flow through these transistors, independent of the signal output amplitude. The
maximum power dissipation of Q4 and Q6 is
Ptot = VCE * (ILine - IIC1)
where:
VCE,Q4 = VBE,Q4+UD2 and
VCE,Q6 = VBE,Q6+UD1
VBE(Q4,Q6) 0.7V, VTH(D2,D1) 0.52V.
ISA2531/2= typ. 3mA (see data sheet)
Power dissipation at Iline =100mA:
Ptot = (0.7+0.52) * (0.1-0.003) = 118mW
In order to maintain no interference of the circuit to AC-impedance, DC-mask, maximum sending level, etc..., the
amplifiers´ supply voltage VCC must be limited to <2.44V 1) .
This is accomplished by
VCC = VBE(Q6) + VTH(D1)+ VTH(D2)+VBE(Q4) .
VCC = 0.68V + 0.5V + 0.5V + 0.68V = 2.36V
1) Critical for SAN2202a configuration, not critical for SAN2202b configuration.
VLI can range between 2.5...6.5V 4.5V ± 2Vpeak(max. sending level),
the C-E saturation voltage of Q3 is assumed >60mV
To maintain full transmit swing (± 2Vpeak ) VC(Q3) = VCC =(2.5V-60mV) < 2.44V
Transistor Q1,R1,R2 and R3 forms an impedance matching circuit to provide better matching between the receive
output of the IC and the output amplifier stage.
7.2 Amplifier supply in shunt transistor path
This configuration requires no extra components. However, the maximum voltage of VPP must be considered: If VPP
rises >2.44V, the maximum transmit level will be slightly distorted at the negative half-wave.
Additionally, the “Make”-resistance in pulse dialing will be higher, because with CS being pulled to VSS, VLI cannot
be shorted to VBE (as in the standard application AN1500 with the collector of Q3 being connected to VSS).
During pulse dialing, VLI will be 2.5V, thus increasing the total “ON”-resistance by
RON
=
2.5V 0.6V
ILine 3mA
= 111(20mA)....19.6(100mA)
7.3 Amplifier supply by power extraction circuit SAN3020
This power extraction can be used to supply any external load by the available line current without affecting the
performance of the single chip telephone. A detailed description of this circuit is given in application note SAN3020.
“ON”-resistance in pulse dialing (compared to the standard application AN1500) is only increased by 10(see.
Fig. 6).
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