TS34118CS28C8 TSC [Taiwan Semiconductor Company, Ltd], TS34118CS28C8 Datasheet - Page 13

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

TS34118
Voice Switched Speakerphone Circuit
Filter – Continue
As a low pass filter (Figure 9), it can be used to roll off the high end frequencies in the receive circuit, which aids in
protecting against acoustic feedback problems. With an appropriate choice of an input coupling capacitor to the low
pass filter is formed
Figure 9. Low Pass Filter
Power Supply, V
, and Chip Disable
B
The power supply voltage at Vcc (pin 4) is to be between 3.5 and 6.5 volts for normal operation, with reduced operation
possible down to 2.8 volts. The power supply current is shown in Figure 18 for both the power-up and power-down
mode. The output voltage at VB (pin 15) is ≈ (Vcc-0.7)/2, and provides the ac ground for the system. The output
impedance at VB is ≈400Ω and in conjunction with the external capacitor at VB, forms a low pass filter for power supply
rejection with different capacitors. The choice of capacitor is application dependent base on whether the circuit is
powered by the telephone line or a power supply. Since VB biases the microphone and hybrid amplifiers, the amount
of supply rejection at their outputs is directly related to the rejection at VB, as well as their respective gains. Depicts
this graphically.
The Chip Disable (pin 3) permits powering down the IC to conserve power and/or for muting purposes. With CD≤0.8
volts, normal operation is in effect. With CD ≥ 2.0 volts and ≤Vcc, the IC is powered down. In the powered down mode,
the microphone and the hybrid amplifiers are disable, and their outputs go to a high impedance state. Additionally, the
bias is removed from the level detectors. The bias is not removed from the filter (pins 1,2). The attenuators (pin 8,9, 21,
22), or from pin 13,14, and 15 (the attenuators are disabled, however, and will not pass a signal). The input impedance
at CD is typically 90KΩ, has a threshold of ≈ 1.5 volts, and the voltage at this pin must be kept within the range of
ground and Vcc. If CD is not used, the pin should be grounded.
Switching Time
The switching time of the TS34118 circuit is dominated by the components at CT (pin 14, refer to Figure 6), and
secondarily by the capacitors at the level detector outputs (RLO1, RLO2, TLO1, TLO2).
The time to switch to receive or to transmit from idle is determined by the capacitor at CT, together with the internal
current sources (refer to Figure 6). The switching time is:
ΔT=ΔV × CT / I
For the typical cause where ΔV=240mV, I=60µA. And CT is 50µF, ΔT=20ms. If the circuit switches directly from
receive to transmit (or vice-versa), the total switching time would be 40ms.
The switching time from either receive or transmit to idle depends on which type of idle mode is in effect. If the circuit is
going to “fast idle”, the time constant is determined by the CT capacitor, and the internal 2.0KΩ resistor (Figure 6). With
CT = 5.0µF, the time constant is ≈30ms (for 95% change). Fast idle is an infrequent occurrence, however, occurring
when both speakers are talking and competing for control of the circuit. The switching time from idle back to either
transmit or receive is described above.
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Version: A08