TS34118CS datasheet(16/20 Pages) TSC

16 / 20 page

TS34118

16-20

2004/09 rev. B

Design Equations

Figure 10. Basic Block Diagram For Design Purpos

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:

VM = 0.52VL

(Equation 7)

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:

I2 =

⎥⎦

⎤

⎢⎣

⎡

(Equation 8)

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:

I4 =

[]

(Equation 9)

Setting I4=I2, and combining the above equations results in:

VL = VM ×

[]

(Equation 10)

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:

VL=840VM (or VM=0.0019 VL)

(Equation 11)

At idle, where the gain of the two attenuators is –20dB (0.1V/V), Equations 6 and 10 yield the same result:

VM=0.024VL

(Equation 12)

Equations 7, 11 and 12 define the thresholds for switching, and are represented in the following graph:

TS34118CS Datasheet(PDF) 16 Page - Taiwan Semiconductor Company, Ltd