TS34118CS28C8 TSC [Taiwan Semiconductor Company, Ltd], TS34118CS28C8 Datasheet - Page 16

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

Design Equations - Continue
This is the general equation defining the microphone voltage necessary to switch comparator C1 when a received
signal VL is present. The highest VM occurs when the receive attenuator is at maximum gain (+6.0dB). Using the
typical numbers for Equation 6 yields:
To switch comparator C2, currents I2 and I4 need to be determined. With sound applied to the
microphone, a voltage VM is creased by the microphone, resulting in a current I2 into TLI1:
Since GHA is the differential gain of the hybrid amplifiers, it is divided by two to obtain the voltage V2 applied to R2.
Comparator C2 switches when I4=I2. I4 is defined by:
Setting I4=I2, and combining the above equations results in:
This equation defines the line voltage at Tip/Ring necessary to switch comparator C2 in the presence of a microphone
voltage. The highest VL occurs when the circuit is in the transmit mode (GTX = +6.0dB). Using the typical numbers for
Equation 10 yields:
At idle, where the gain of the two attenuators is –20dB (0.1V/V), Equations 6 and 10 yield the same result:
Equations 7, 11 and 12 define the thresholds for switching, and are represented in the following graph:
V
V
M
L
I2 =
I4 =
V
V
=0.024VL
=840VM (or VM=0.0019 VL)
L
M
= V
= 0.52VL
V
R4
V
R2
M
M
L
×
[

 
R4
R2
G
G
HR
(Equation 12)
MA
×
(Equation 7)
×
[
×
G
G
[
G
G
MA
FO
TX
HR
]
×
×
×
G
G
G
Figure 10. Basic Block Diagram For Design Purpose
TX
2
HA
FO
×
×

 
G
2
HA
]
(Equation 11)
]
(Equation 8)
(Equation 9)
(Equation 10)
16/23
Voice Switched Speakerphone Circuit
TS34118
Version: A08