am79r251 Advanced Micro Devices, am79r251 Datasheet - Page 22

• Manufacturer Part Number: am79r251
• Description: Intelligent Subscriber Line Interface Circuit Islic
• Manufacturer: Advanced Micro Devices

Table 2.
22
Mode Descriptions
Disconnect
Standby
Tip Open
Active High
Battery
Active Low
Battery
Active Boosted
Battery
Active Internal
Ringing
On-Hook
Transmission
(OHT), Fixed
Longitudinal
Voltage
Operating
Mode
This mode disconnects both A and B output amplifiers from the AD and BD outputs. The
A and B amplifiers are shut down and the Am79R251 selects the low battery voltage at
the VBL pin. In the Disconnect state, the currents on IMT and ILG represent the voltages
on the SA and SB pins, respectively. These currents are scaled to produce voltages across
RMTi and RLGi of
The power amplifiers are turned off. The AD output is driven by an internal 250
resistor, which connects to ground. The BD output is driven by an internal 250
resistor, which connects to the high battery (BATH) at the VBH pin, through a clamp circuit,
which clamps at –50 V with respect to BGND. For VBH values above–55 V, the open-
circuit voltage, which appears at this output is ~VBH + 5 V. If VBH is below –55 V, the
voltage at this output is –50 V. The battery selection for the balance of the circuitry on the
chip is VBL. Line supervision remains active. Current limiting is provided on each line to
limit power dissipation under short-loop conditions as specified in the “Am79R251 Current-
Limit Behavior” section. In external ringing, the standby ISLIC state is selected.
In this mode, the AD (Tip) lead is opened and the BD (Ring) lead is connected to a clamp,
which operates from the high battery on VBH pin and clamps to approximately –50 V with
respect to BGND through a resistor of approximately 250
for the balance of the circuitry on the chip is VBL. To prevent excessive power dissipation,
the current in the Ring lead is limited by an internal current source to 30 mA.
In the Active High Battery mode, battery connections are connected as shown in
Table 1. Both output amplifiers deliver the full power level determined by the programmed
DC-feed conditions. Active High Battery mode is enabled during a call in applications
when a long loop can be encountered.
Both output amplifiers deliver the full power level determined by the programmed DC-feed
conditions. VBL, the low negative battery, is selected in the Active Low Battery mode. This
is typically used during the voice part of a call.
In the Active Boosted Battery mode, battery connections are as shown in Table 1. Both
output amplifiers deliver the full power level determined by the programmed DC-feed
conditions. Active Boosted Battery mode is enabled during a call in applications when an
extended loop can be encountered.
In the Internal Ringing mode, the Am79R251 selects the battery connections as shown
in Table 1. When using internal ringing, both the AD and BD output amplifiers deliver the
ringing signal determined by the programmed ringing level.
In the On-Hook Transmission, Fixed Longitudinal Voltage mode, battery connections are
as shown in Table 1. The longitudinal voltage is fixed at –30 V to allow compliance with
safety specifications for some classes of products.
V
--------- -
Am79R251
400
SA
and
V
--------- -
400
SB
, respectively.
Description
(typical). The battery selection
(typical)
(typical)