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L8576B 参数 Datasheet PDF下载

L8576B图片预览
型号: L8576B
PDF下载: 下载PDF文件 查看货源
内容描述: 双振铃SLIC [Dual Ringing SLIC]
分类和应用:
文件页数/大小: 26 页 / 564 K
品牌: AGERE [ AGERE SYSTEMS ]
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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