TCA3388 Datasheet, PDF (9/16 Pages) Motorola, Inc – TELEPHONE LINE INTERFACE

TCA3388
Figure 7. Derived Network for Z21 in Case of
Complex Set Impedance
The microphone signal current is derived from the
microphone signal according to the schematic in Figure 9.
Rv
SAO
Ra
Rb
Cb
Rv
LAI
Rw
Cw
Gnd
ǒ Ǔ ^{+ Rv}
R1 x Z0
) 2 Ra Rb – R1
+ _ǒ ^{ǒ Ǔ} _Ǔ R_w
R1 x Z0 Ra – R1
) 4 Rb Ra Rb – R1
+ C_w
4 R_b² x C_b
R1 x Z0
TRANSMIT
When a current is injected on Pin LAI, via the loop
depicted in Figure 6, a signal is created on the line. In this
way the microphone signals and DTMF signals (from an
external source) are transmitted. It can be derived that the
signal voltage on the line (VLine) depends on the signal
current injected in LAI (ILAI) according to:
+ ₎ VLine
–ILAI
x
Z0
Zin
x
ZLine
ZLine
With this relation, a simplified replacement circuit can be
made for the transmit amplifier (see Figure 8). Here the
product of ILAI and Z0 is replaced by one voltage source.
Figure 8. Replacement Diagram for
the Transmit Amplifier
Zin
–ILAI*Z0
VLine _ZLine
Figure 9. Microphone Amplifier Input Stage
VCC
R _Mic
C _Mic
Ru ^Cu
Iu
TXI
TCA3388
RTXI
Ku
Line AGC
ILAI
Handset
Mic
Microphone
Mute
The input stage of Figure 9 consists of a current amplifier
with transfer Ku, an input impedance of 1.0 k (RTXI), plus an
attenuator which reduces the signal current at high line
currents (AGC). This attenuator can be switched on/off via
the microcontroller. The input current Iu within the telephony
speech band is derived from the microphone signal according
+ _{) )} [ Iu
RMic
Vu
Ru
RTXI
Vu
Ru
With: Vu = signal of the microphone only loaded with RMic
The overall gain from microphone to line (ATX) now follows
as
+ + ₎ ATX
VLine
Vu
Ku
Ru
x
Z0
Zin
x
ZLine
ZLine
Practically, the gain can be varied only with Z0, Ru and
RMic.
The TCA3388 offers the possibility to mute the
microphone, also called privacy mode, by making the MUT
Pin high. During pulse–dialing, the microphone bias is
switched off. Pin Mic will be made high impedance, shutting
off the microphone dc current. This reduces the current
consumption of the circuit during pulse–dialing.
MOTOROLA ANALOG IC DEVICE DATA
9