TS34118

11-20

2004/09 rev. B

Attenuator Control Block

4) The circuit will switch to the slow idle mode when a) both talkers are quiet (no speech present), or b) when one talker’s

speech level is continuously overridden by noise at the other speaker’s location.

The time required to switch the circuit between transmit, receive, fast idle and slow idle is determined in part by the

components at the CT pin (pin 14). (see the section on Switch Time for a more complete explanation of the switching

time components). A schematic of the CT circuitry is shown in Figure 6, and operates as follows:

-RT is typically 120KΩ, and CT is typically 5.0µF.

-To switch to the receive mode, I1 is turned on (I2 is off), charging the external capacitor to +240mV above VB. (An

internal clamp prevents further charging of the capacitor.)

-To switch to the transmit mode, I2 is turned on (I1 is off) bringing down the voltage on the capacitor to –240mV with

respect to VB.

-To switch to idle quickly (fast idle), the current sources are turned off, and the internal 2.0KΩ resistor is switched in,

discharging the capacitor to VB with a time constant = 2.0K×CT.

-To switch to idle slowly (slowly idle), the current sources are turned off, the switch at the internal 2.0KΩ resistor is

open, and the capacitor discharges to VB through the external resistor RT with a time constant = RT × CT.

Figure 6. CT Attenuator Control Block Circuit

Microphone Amplifier

the output are pinned out. Unlike most op-amps, the amplifier has an all-NPN output stage, which maximizes phase

margin and gain-bandwidth. This feature ensures stability at gains less than unity, as well as with a wide range of reactive

loads. The open loop gain is typically 80dB (f<100Hz), and the gain-bandwidth is typically 1.0MHz (see Figure 16). The

maximum p-p output swing is typically 1.0 volt less than Vcc with an output impedance of <10

Ω until current limited is

reached (typically 1.5mA).Input bias current at MCI is typically 40nA out of the pin.

The muting function (Pin 12), when activated, will reduce the gain of the amplifier to ≈-39dB (with RMI=5.0KΩ) by shorting

output to the inverting input (see Figure 7). The mute input has a threshold of ≈1.5 volts, and the voltage at this pin must

be kept within the range of ground and Vcc (see Figure 17). If the mute function is not used, the pin should be grounded.