Data Sheet
May 2001
L8576B Dual Ringing SLIC
Tip/Ring Drivers
Functional Description
The L8576B has two tip/ring drivers whose outputs are
PT and PR. Each driver operates as a current source
capable of sinking or sourcing adequate ac signal bias
current. In the normal talk operating mode, these driv-
ers are current-limited at a nominal 24 mA to minimize
the power dissipation of short loops. These amplifiers
are also used to drive balanced ringing. During ringing,
the current limit is raised to approximately 85 mA.
Refer to the architectural and application diagrams
(Figures 1 and 2, respectively).
General
The L8576B is a dual subscriber line interface circuit
with each half of the device providing battery feed,
supervision, and balanced ringing. It is designed to
support short loops, typically on customer premises.
The use of a single battery for both battery feed and
ringing makes this device particularly advantageous
where it is desirable to minimize power supply costs in
small systems, such as terminal adapters. The tip and
ring drive amplifiers are used with a very relaxed cur-
rent limit to develop a trapezoidal, balanced ringing sig-
nal. Use of a nominal –65 V power supply allows for
ringing of normal phones, whether equipped with a
mechanical ringer, or a peak-detection type of ringing
detector. While balanced ringing is not the norm world-
wide, its use in short, customer premises loops is gain-
ing popularity.
The external resistor connected to the RPWR pin is used
to dissipate power externally and also to drop the bat-
tery voltage which is higher than normal in order to
support balanced ringing. Note that this external power
dissipation is present during both ringing or normal bat-
tery feed operation. Power limitations restrict the dual
device to actively ringing only one channel at a time;
thus, ringing cadence must be used to ensure that only
one channel is actively ringing at any given instant of
time. In other words, to ring both channels at the same
time, ring each channel during the quiet interval of the
other channel.
Battery Operation
In addition to the ringing and battery functions, the
L8576B device provides the ac receive and transmit
paths. Also, integral within the device is an off-hook
detection circuit and a ring trip detection circuit that
have their outputs multiplexed on a single lead.
There are two VBAT inputs to the device. Pin 1 (VBAT)
provides voltage to the entire SLIC and pins 9 and 37
(RPWR) provide voltage to the individual tip and ring
amplifiers of each channel through RPWR resistors. A
shared current sourcing scheme is employed within the
device. For loop currents below 20 mA, the VBAT
applied to pin 1 sources all of the loop current in addi-
tion to driving internal circuitry. For loop currents
greater than 20 mA, loop current is primarily provided
through the RPWR resistors and the pin 1 VBAT mainly
powers internal circuitry. The RPWR resistors can be
replaced by a lower-voltage auxiliary battery. Operation
with an auxiliary battery is described in the Applications
section of this document.
Thermal protection within the device is also provided,
and an external resistor is used to drop the high battery
voltage before applying loop current, thus allowing a
significant portion of the power to be dissipated outside
of the device. Removing much of this power makes it
possible to incorporate two complete circuits in a
44-pin, surface-mount package.
Protection
The L8576B contains some overvoltage protection in
addition to the thermal protection within the device.
This protection, along with the associated tip and ring
protection resistors, may be sufficient in some benign
environments. However, if power line cross or lightning
protection is desired, the use of an external protection
circuit (such as the L7591 device from Agere) is highly
recommended.
Transmit and Receive Interface
The interface is suitable for direct coupling to a ±5 V
only codec. When interfacing a 5 V only codec, cou-
pling capacitors are required.
The transmit interface circuitry couples the differential
voltage on tip and ring to transmit output VITR. The
inverting input of the driving amplifier is available on
lead TG1, so connecting a resistance between VITR
and TG1 allows adjustment of the transmit gain
(transconductance).
The integrated thermal protection consists of a thermal
shutdown circuit which places the tip/ring drivers in a
high-impedance state when the temperature of the die
exceeds 160 °C. In thermal shutdown, all supervision
states are undefined.
Agere Systems Inc.
7
AGERE [ AGERE SYSTEMS ]
