U4030B Datasheet, PDF (4/14 Pages) TEMIC Semiconductors – Telephone Speech Circuit

U4030B
20
R_swamp = 82 68 47
15
10
5
0
0
93 7622 e
RDC = 10 kW
20 40 60 80 100 120
Line current I_L ( mA )
Figure 5. Typical DC characteristics for various
SWAMP resistors
Charge Up Circuit
By OFF HOOK the handset, an integrated charge up
circuit provides V_D with the whole line current. When V_D
reaches 2.2 V, the charge up circuit is automatically
switched off.
The specifications for the German “Deutsche Telekom”
(speech readiness, start time) will be fulfilled even with
1000 mF at Pin 20.
Figure 6 illustrates the transient behavior of the circuit at
I_L = 20 mA.
V_L
charge-up
off
V_D
93 7623 e
time: 50 ms / div
Figure 6. Charge up characteristics at I_L = 20 mA
Electronic Inductance
The AC resistance ( Rimp 1 kW ) of the telephone should
be much higher than the DC resistance ( 325 W ), the latter
being decoupled via an electronic inductance.
The value of L is given by:
L = CVC RDC 30 kW || R
where
RDC = 10 W
CVC = 10 mF
R30 = 30 kW
RC = infinite
L =3H
Transmit
Microphone amplifier
The microphone amplifier of U4030B has symmetrical
inputs ( MIC1 and MIC2 ) with an input resistance of
60 kW ( typical). It has a gain of 29 dB which is adjustable
with resistances RS1 and RS2 as follows:
G_T = V_MICO / V _{( MIC1, MIC2 )}
= 20 log [ (RS1 / RS2 ) + 1 ] dB
A low pass function can be realised with CSLP. The
corner frequency is given by:
fC = 1 / ( 2 p RS1 CSLP )
When the AC level on V_L is very high, the amplification
of the microphone is reduced by the limiter function. The
threshold of the limiter is fixed at 5.5 dBm ( typical).
DTMF
The amplification of the DTMF signal is determined by
the ratio of RS1 and RS2 as follows:
G_D = V_MICO / V_DTMF
= 20 log {0.19 [ ( RS1 / RS2 ) + 1 ] }
An external voltage divider is used to adjust the proper
DTMF level at line.
For monitoring the dialing procedure, an attenuated
DTMF signal is sent to the earpiece. The Deutsche Tele-
kom specification is fulfilled with the nominal value of
the transmit and receive gain.
4 (14)
Rev. A2, 27-Jan-98