PRELIMINARY
May 1998
PBL 388 14
Voice - switched 2-channel Circuit with
loudspeaker amplifier
Description.
The PBL 388 14 contains all the necessary circuitry, amplifiers, detectors,
comparator and control functions to implement a high performance voice switched
handsfree two- way communication system. The gain dynamics (attenuation between
channels) is selectable (25dB or 50dB) via a separate pin. A background noise
detector in the transmitting channel reduces the influence of continuous external noise
signals to the switching .
The PBL 388 14 is designed for mains powered handsfree telephones, vehicular
mobile telephone handsfree systems and handsfree intercom systems. Automatic
volume attenuation in the power amplifier extends the operating range at low supply
currents. The circuit has two special features, the power amplifiers volume control can
be implemented either as an ac. potentiometer control or as a digital control by a
µ-
processor (dc. control) and that the feedback loop of the power amplifier is accessible
thus making it possible to add a simple external power stage driving low impedance
loudspeakers up to several watts.
Filtering is possible of both, the audio and the speech switching control signals,
in both transmitter and receiver channels.
•
Key Features
Minimum of external components
needed for function.
Selectable gain dynamics. (25 or 50
dB)
Low power consumption:
≈1mA
at 3.3V
(typical) for speech switching, audio
power amplifier quiscent current
≈1mA.
Drives an 25 - 50 ohm loudspeaker
without a transformer.
Background noise compensation in the
transmitting channel with hold function
at receive.
Input amplifiers of both channels have
balanced inputs.
Exellent noise performance.
Encapsulated in 24 pin plastic ”skinny”
DIP and 24 pin SO .
•
•
•
•
•
18
19
21
22
20
•
23
•
4
24
16
PBL 388 14
+
F3
F6
17
12
3
8
8
1
4
3
5
13
11
P
Control
B
L
24 pin SO
F2
1
–
2
F1
+
8
6
9
10
Ref.
7
–
F4
14
+
15
P
B
L
3
8
24 pin DIP
Figure 1. Block diagram.
1
8
F5
1
4
ERICSSON [ ERICSSON ]
