TS34118CS28C8 TSC [Taiwan Semiconductor Company, Ltd], TS34118CS28C8 Datasheet - Page 18

• Manufacturer Part Number: TS34118CS28C8
• Description: Voice Switched Speakerphone Circuit
• Manufacturer: TSC [Taiwan Semiconductor Company, Ltd]

Application Information
Dial Tone Detector
The threshold for the dial tone detector is internally set at 15mV (10mVrms) below VB (see Figure 5). That threshold
can be reduced by connecting a resistor from RXI to ground. The resistor value is calculated from:
Where VB is the voltage at Pin 15, and ΔV is the amount of threshold reduction. By connecting a resistor from Vcc to
RXI, the threshold can be increased. The resistor value is calculated from:
Where ΔV is the amount of threshold increase.
Background Noise Monitors
For testing or circuit analysis purposes, the transmit or receive attenuators can be set to the “on” position, by disabling
the background noise monitors, and applying a signal so as to activate the level detectors. Grounding the CPR pin will
disable the receive background noise monitor, thereby indicating the “presence of speech” to the attenuator control
block. Grounding CPT does the same for the transmit path.
Additionally, the receive background noise monitor is automatically disabled by the dial tone detector whenever the
receive signal exceeds the detector’s threshold.
Transmit / Receive Detection Priority
Although the TS34118 was designed to have an idle mode such that the attenuators are halfway between their full on
and full off positions, the idle mode can be biased towards the transmit or the receive side. With this done, gaining
control of the circuit from idle will be easier for that side towards which it is biased since that path will have less
attenuation at idle.
By connecting a resistor from CT (pin 14) to ground, the circuit will be biased towards the transmit side. The resistor
value is calculated from:
Where R is the added resistor, RT is the resistor normally between pins 14 and 15 (typically 120Ω), and ΔV is the
difference between VB and the voltage at C7 at idle C refer to (Figure 10)
By connecting a resistor from CT (pin 14) to Vcc, the circuit will be biased towards the transmit side. The resistor value
is calculated from:
R, RT, and ΔV are the same as above. Switching time will be somewhat affected in each case due to the different
voltage excursions required to get to transmit and receive from idle. For practical considerations, the ΔV shift should
not exceed 100mV.
R =
R =
R =
R =
10K
10K
R
R
T
T

 

 

 

 
VB
ÄV
V
ÄV
V
V
CC
CC
ÄV
B
ÄV
−1
−
−1
−
V

 
V

 
B
B
−
−
1
1

 

 
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Voice Switched Speakerphone Circuit
TS34118
Version: A08