U4082B Datasheet, PDF (8/13 Pages) ATMEL Corporation – Low-voltage Voice-switched IC for Hands-free Operation

UNISONIC TECHNOLOGIES CO., LTD
U4082B
LINEAR INTEGRATED CIRCUIT
„ FUNCTIONAL DESCRIPTION
INTRODUCTION
Between operation of a speakerphone and a handset, the fundamental difference is that of half-duplex versus
full-duplex. The UTC U4082B includes the necessary level detectors, attenuators, and switching control for a
properly operating speakerphone. The detection sensitivity and timing can be externally controlled. The UTC
U4082B also provides background noise monitors to make the circuit insensitive to room and line noise, hybrid
amplifier to interface to Tip and Ring, the microphone amplifier, and other associated functions.
ATTENUATORS
The transmit and receive attenuators are complementary in function, and they are never both fully on or both
fully off. For example, when one is at maximum gain: +6dB, the other is at maximum attenuation: -46dB, and vice
versa. But sum of their gains remains constant at -40dB typically. To control the transmit and receive paths to
provide the half-duplex operation required in speakerphone is the attenuators’ purpose. The attenuators are
non-inverting, have a -3dB (from max gain) frequency of ~100kHz, their input impedance of each one is nominally
10kΩ, and to prevent distortion, their input signal should be limited to 350 mVrms(same as 990mvp-p). After the
output current limit (typically 2.5mA) is reached, the output impedance is<10Ω.
VB
TXI/RXI
10kΩ
4.0kΩ
To Attenuator Input
96KΩ
These attenuators can be controlled by the Control Block’s output signal. When the CT pin which measures the
signal is at +240mV with respect to VB, the circuit is in the receive mode and so receive attenuator is at +6dB. When
the CT voltage is equal to VB, the circuit is in an idle mode, and it also causing the attenuators’ gains to be halfway
between their fully on and fully off positions (typical -20 dB each). The most direct method to monitor the circuit’s
mode is monitoring the CT voltage (with respect to VB).
LEVEL DETECTORS
There are two level detectors on the receive side and two level detectors on the transmit side. The terms in
parentheses form one system, and the other terms form the second system. All the four level detectors are high
amplifiers which have back-to-back diodes in the feedback path, and then it is resulting in non-linear gain, which
permits operation over a wide dynamic range of speech levels. TLI1, TLI2, RLI1, and RLI2, the external resistor and
capacitor determine the sensitivity of each level detector. Through a diode each output charges an external capacitor
and limiting resistor, thus providing a DC representation of the input AC signal level. The capacitor and an internal
350Ω resistor provide the outputs’ quick rise time. And an internal current source and the capacitor set a slow decay
time. On the receive side, RLI1 is at the receive input receiving the same signal as at Tip and Ring, and RLI2 is at
the output of the speaker amplifier. On the transmit side, TLI2 is at the microphone amplifier, while TLI1 is at the
hybrid output.
UNISONIC TECHNOLOGIES CO., LTD
www.unisonic.com.tw
8 of 13
QW-R108-020.A