ia3211 integration, ia3211 Datasheet - Page 2

• Manufacturer Part Number: ia3211
• Description: Optical Silicon Daa Modem/lineside Chipset
• Manufacturer: integration

DETAILED DESCRIPTION
A DAA circuit known formally as the “Digital Access
Arrangement” or “line interface” circuit is the physical
connection between any electronic equipment and the
telephone line. Most countries enforce specific requirements
when connecting devices to their public switched telephone
network (PSTN). These requirements are primarily developed to
prevent any damage to the network by improper equipment
and to protect the end user from any potential danger, such as
a lightning surge, coming from the network.
The DAA performs four critical functions:
- Line termination
- Isolation
- Hybrid
- Ring Detection
The following is a description of how the IA3211 and IA3212
chipset optical DAA circuit fulfills these requirements, as well
as providing other useful features. The IA3211 and IA3212
chipset can fulfill a wide variety of telephone interface
applications including voice, FAX, and data.
General Architecture
The circuit is broken into two parts: the line side circuit and the
modem side circuit. Telephone regulations require a high
voltage barrier between these two sides, which is provided by
the dual linear analog opto-isolators.
The line side circuit derives its operating power from the
telephone line directly. A bridge rectifier allows operation from
either battery polarity. A fuse and a high voltage transient
protection device are used to preserve telephone line integrity
in the event of a lightning strike, or other high energy pulse
coming in from the loop.
The IA3212 line side chip includes a HYBRID circuit that
separates the TX and RX signals by doing a 2-wire to 4-wire
conversion. The success of this circuit (transhybrid balance)
depends on the line termination impedance shown in the block
diagram as a simple resistor. In fact, the termination
impedance is a combination of an external resistor and an
external capacitor. The value of these components depends on
the telephone regulations of the country concerned.
The RX audio signal is transmitted across the high voltage
barrier with an analog opto-isolator. This circuit is capable of
very good linearity and broad bandwidth by using a novel
feedback technique. In a similar circuit, the TX audio is passed
across the barrier with a second analog opto-isolator.
On-hook and off-hook functions are taken care of by turning
on/off the series switch. In the DAA circuit, this is a DMOS
transistor capable of withstanding at least 350 volts. When in
the "on-hook" condition, the switch is open and the DAA draws
no current from the loop. The DAA can be commanded to go
"off-hook" by asserting the "hook command" input to the
IA3211. This command is communicated to the IA3212 via the
dual analog opto-isolator. An internal multiplexing operation
takes place, which combines this command, the Caller ID
command, and the actual TX audio signals. When the off-hook
command is given, the IA3212 switches on the hook switch by
fully saturating the DMOS transistor.
The Caller-ID function is sometimes referred to as the "snoop"
mode. When this command is given, the IA3212 turns on
slightly the hook switch. In this mode, the loop current is kept
low enough to keep the central office (CO) from seeing an off-
hook situation, yet it allows the IA3212 to transmit the RX
audio signal over to the IA3211.
When in the "off-hook" mode, the IA3212 passes the full loop
current through itself. Power for the chip is derived by an
internal circuit, which acts like a zener diode. The voltage
across the chip from Vdd to Vss is maintained at about 5 volts.
With the hook switch fully "on” loop current is determined by
the external loop resistance. In some countries the current
must be limited to 60mA maximum by the DAA. In those cases,
an external current limiting circuit must be added.
To prevent damage when the DAA is accidentally connected to
a digital PBX line with unlimited current capability, the DAA has
an internal shutdown circuit. This circuit is sensitive to total
power dissipated by both hook switch and the IA3212, and
internal temperature. When power from the line exceeds about
3 to 4 watts at room temperatures, the IA3212 turns off the
hook switch. In this situation the hook switch itself can
generate significant heat. Therefore the switch should be
thermally connected to the IA3212 with ground plane
techniques. This shutdown function is linearly de-rated to zero
line current at +70°C.
A ring detector is built into the IA3212, which can be configured
as either a "full wave" detector or a "half wave" detector. In the
full wave mode, a pulse is generated al the IA3211 "ring
detect" output pin for each half cycle of the ring signal. In the
half wave mode, one pulse is generated for each full cycle of
the ring signal.
Isolation Barrier
For many years, the only practical solution to provide high
voltage isolation between the CO loop and the modem (or any
telephone device) has been the use of a transformer. In
modern equipment design, space, weight, and cost are all
prime concerns. The transformer does not always fit into
today's solutions. Optical isolators can provide excellent high
voltage isolation in a very compact size. The dual opto devices
have a minimum of 2500 V breakdown voltage rating.
IA3211 & IA3212
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