Subscriber Line Interface Circuit (SLIC) Including A Transient Output Current Limit Circuit And Related Method

Abstract

A subscriber line interface circuit (SLIC) for a subscriber loop includes a pair of output amplifiers for connection to the subscriber loop and a transient output current limit circuit. The transient output current limit circuit may have at least one programmable output current limit and limit respective output currents from the pair of output amplifiers based thereon. The transient output current limit circuit may limit both source and sink currents for each of the pair of output amplifiers.

1 . A subscriber line interface circuit (SLIC) for a subscriber loop, the SLIC comprising: 
 a pair of output amplifiers for connection to the subscriber loop; and    a transient output current limit circuit having at least one programmable output current limit and for limiting respective output currents from said pair of output amplifiers based thereon.    
   
   
       2 . The SLIC of  claim 1  wherein said transient output current limit circuit limits both source and sink currents for each of said pair of output amplifiers.  
   
   
       3 . The SLIC of  claim 1  wherein said transient output current limit circuit generates a source current limit and a sink current limit based upon the at least one programmable current limit.  
   
   
       4 . The SLIC of  claim 3  wherein the sink current limit is offset a predetermined amount higher than the source current limit.  
   
   
       5 . The SLIC of  claim 4  wherein the predetermined amount is less than about 40% higher than the source current limit.  
   
   
       6 . The SLIC of  claim 1  wherein the at least one programmable output current limit comprises a programmable sink current limit and a programmable source current limit.  
   
   
       7 . The SLIC of  claim 1  wherein said transient output current limit circuit comprises: 
 a reference current generator for generating a reference current based upon the at least one programmable output current limit; and    source and sink comparators for comparing sensed source and sink output currents for each of said output amplifiers with the reference current.    
   
   
       8 . The SLIC of  claim 7  wherein said transient output current limit circuit further comprises at least one transconductance stage connected between said source and sink comparators and said pair of output amplifiers.  
   
   
       9 . The SLIC of  claim 7  further comprising respective source and sink prescalers connected to said source and sink comparators.  
   
   
       10 . The SLIC of  claim 7  wherein the at least one programmable output current limit is programmed by connecting a programming resistor between a reference voltage and said reference current generator.  
   
   
       11 . The SLIC of  claim 7  wherein said reference current generator comprises a temperature stabilized voltage reference source.  
   
   
       12 . The SLIC of  claim 1  further comprising a direct current (DC) loop current limit circuit for limiting current on the subscriber loop to a DC current limit.  
   
   
       13 . The SLIC of  claim 12  wherein the at least one programmable output current limit is higher than the DC current limit.  
   
   
       14 . The SLIC of  claim 13  wherein the at least one programmable output current limit is less than about 50% higher than the DC current limit.  
   
   
       15 . A subscriber line interface circuit (SLIC) for a subscriber loop, the SLIC comprising: 
 a pair of output amplifiers for connection to the subscriber loop;    a direct current (DC) loop current limit circuit for limiting current on the subscriber loop to a DC current limit; and    a transient output current limit circuit having at least one programmable output current limit and for limiting both source and sink currents from said pair of output amplifiers based on the at least one programmable output current limit.    
   
   
       16 . The SLIC of  claim 15  wherein said transient output current limit circuit generates a source current limit and a sink current limit based upon the at least one programmable current limit.  
   
   
       17 . The SLIC of  claim 16  wherein the sink current limit is offset a predetermined amount higher than the source current limit.  
   
   
       18 . The SLIC of  claim 17  wherein the predetermined amount is less than about 40% higher than the source current limit.  
   
   
       19 . The SLIC of  claim 15  wherein the at least one programmable output current limit comprises a programable sink current limit and a programable source current limit.  
   
   
       20 . The SLIC of  claim 15  wherein said transient output current limit circuit comprises: 
 a reference current generator for generating a reference current based upon the at least one programmable output current limit; and    source and sink comparators for comparing sensed source and sink output currents for each of said output amplifiers with the reference current.    
   
   
       21 . The SLIC of  claim 20  wherein said transient output current limit circuit further comprises at least one transconductance stage connected between said source and sink comparators and said pair of output amplifiers.  
   
   
       22 . The SLIC of  claim 20  further comprising respective source and sink prescalers connected to said source and sink comparators.  
   
   
       23 . The SLIC of  claim 20  wherein the at least one programmable output current limit is programmed by connecting a programming resistor between a reference voltage and said reference current generator.  
   
   
       24 . The SLIC of  claim 20  wherein said reference current generator comprises a temperature stabilized voltage reference source.  
   
   
       25 . The SLIC of  claim 15  wherein the at least one programmable output current limit is higher than the DC current limit.  
   
   
       26 . The SLIC of  claim 25  wherein the at least one programmable output current limit is less than about 50% higher than the DC current limit.  
   
   
       27 . A transient output current limit circuit for limiting current from an amplifier comprising: 
 a reference current generator for generating at least one reference current based upon at least one programmable output current limit;    respective source and sink prescalers for sensing source and sink output currents for the amplifier; and    respective source and sink comparators for comparing the sensed source and sink output currents with the at least one reference current and for limiting current from the amplifier based thereon.    
   
   
       28 . The transient output current limit circuit of  claim 27  further comprising at least one transconductance stage connected between said source and sink comparators and the amplifier.  
   
   
       29 . The transient output current limit circuit of  claim 27  wherein the at least one reference current comprises a source reference current and a sink reference current.  
   
   
       30 . The transient output current limit circuit of  claim 29  wherein the sink reference current is offset a predetermined amount higher than the source reference current.  
   
   
       31 . The transient output current limit circuit of  claim 30  wherein the predetermined amount is less than about 40% higher than the source reference current.  
   
   
       32 . The transient output current limit circuit  claim 27  wherein the at least one programmable output current limit comprises a programmable sink current limit and a programmable source current limit.  
   
   
       33 . The transient output current limit circuit of  claim 27  wherein the at least one programmable output current limit is programmed by connecting a programming resistor between a reference voltage and said reference current generator.  
   
   
       34 . The transient output current limit circuit of  claim 27  wherein said reference current generator comprises a temperature stabilized voltage reference source.  
   
   
       35 . A method for limiting current on a subscriber loop comprising: 
 connecting a pair of output amplifiers to the subscriber loop;    programming at least one output current limit; and    limiting currents from the pair of output amplifiers based upon the at least one programmable output current limit.    
   
   
       36 . The method of  claim 35  wherein limiting currents comprises limiting both source and sink currents for each of the pair of output amplifiers.  
   
   
       37 . The method of  claim 36  further comprising generating a source current limit and a sink current limit based upon the at least one programmable current limit; and wherein limiting both source and sink currents comprises limiting both source and sink currents based upon the source current limit and the sink current limit.  
   
   
       38 . The method of  claim 37  wherein the sink current limit is offset a predetermined amount higher than the source current limit.  
   
   
       39 . The method of  claim 38  wherein the predetermined amount is less than about 40% higher than the source current limit.  
   
   
       40 . The method of  claim 35  wherein programming the at least one output current limit comprises programming the at least one output current limit using a programming resistor.  
   
   
       41 . The method of  claim 35  further comprising limiting current on the subscriber loop to a direct current (DC) current limit.  
   
   
       42 . The method of  claim 41  wherein the at least one programmable output current limit is higher than the DC current limit.  
   
   
       43 . The method of  claim 42  wherein the at least one programmable output current limit is less than about 50% higher than the DC current limit.