PBL 3786
March 1996
PBL 3786
Voice-switched Speakerphone
and Toneringer Circuit
Description
The PBL 3786 contains all the necessary circuitry, amplifiers, detectors, comparators
and control functions to implement a high-performance, voice-switched, loud-
speaking, “hands-free” telephone. The gain dynamics (attenuation between
channels) is continuously adjustable (0 - 50 dB) via a separate pin. A background
noise detector in the microphone channel reduces the influence of continuous noise
signals.
The PBL 3786 is designed for telephone line powered applications. The circuit
contains a transformerless power amplifier with patented current supply circuitry that
eliminates the need for inductors. Automatic gain regulation in the power amplifier
extends the operating range at low line voltages.
The PBL 3786 incorporates a high-impedance toneringer circuit with free
continuously selectable frequencies that can be set either by capacitors or controlled
by a microprocessor powered from the chip. The outputs of the toneringer and
speech power amplifier use the same loudspeaker, which is switched with a built-in
automatic change-over function.
Key Features
• Adjustable gain range in the
attenuated channel (0 - 50 dB).
• Direct telephone-line powered
(patented).
• Low power consumption, 2.2 mA at
3.2 V (typical).
• Direct drive of 25 - 50 ohm loud-
speaker.
• On-chip toneringer with common
loudspeaker, automatic switch-over.
• 22-pin dual in-line plastic and 28-pin
PLCC encapsulation.
• Background noise compensation with
hold.
• Minimum of external components.
20
18
15
17
14
13
12
1
22
19
21
10
3
–
F3
11
+
Control
16
*)
All pin numbers in this document
refer to 22-pin DIP package.
PBL 3786
+
–
F6
8
P
3 B
7 L
8
6
28 PLCC
Ref.
≈1.9V
F4
9
2
F1
5
4
6
7
P
22-pin
Figure 1. Block diagram.
1
B
L
37
F2
F5
86
ERICSSON [ ERICSSON ]
