AN-113
www.clare.com 3
Rev. 1
Clare’s Intergrated Telecom circuit (ITC117P) features combined circuitry in one 16 Pin SOIC package for:
•1-Form-A Solid State Relay for use as Hookswitch
•Bridge Rectifier
•Darlington Transistor
•Optocoupler that can function as a ring detector or loop current detector
Typical applications for the ITC117P include:
•PCMCIA Designs
•Modem
•Fax
•Voicemail Systems
•Telephone Sets
•Computer Telephony
DESCRIPTION
The ITC series integrates the major components found in a typical Data Access Arrangement (DAA), in a 16
lead SOIC package. As highlighted in figure 1, the 1-Form-A MOSFET SSR,Darlington transistor, bridge rec-
tifier, and optical isolator comprise this integral design. Following is the functional explanation for each
device:
Hookswitch
The ITC contains optically-coupled MOSFETs that function as a hookswitch in the DAA circuit. The hook-
switch has a blocking voltage up to 350V, isolation voltageto 3750VRMS, 15ΩRDS(on), and a maximum
switching current of 120mA, making it ideal for tip and ring switching. The hookswitch is controlled by an
LED that requires only 5mA to operate. This makes it an attractive device for battery-powered applicatoins
where power consumtion needs to be minimized for prolonged battery life.
Optocoupler
An optocoupler is included in the ITC series package that can be used as a ring detector or loop current
detector with the addition of a few passive components. the optocoupleris available with a standard photo-
transistor or a high gain Darlington transistor.
Bridge/Darlington
Referring to figure 2, a bridge rectifier (D2) and Darlington transistor (Q1) arrangement is inclded in the pack-
age for use in “dry” transformer and optical DAA designs. The bridge provides the function of current steer-
ingto maintain DAA operation and protect the Darlington during polarity reversals of a few passive
components, functions as an electronic iductor that has the effect of presenting a low resistance to the DC
curretn across the telephone line, and a relatively hig empedance for AC signals on the line. For a trans-
former based design, this enables the designer t use a small coupling transformer (T1) since the telephone
loop current is diverted through the Darlington instead passive components, functions as an electronic iduc-
tor that has the effect of presenting a low resistance to the DC curretn across the telephone line, and a rel-
atively hig empedance for AC signals on the line. For a transformer based design, this enables the designer
t use a small coupling transformer (T1) since the telephone loop current is diverted through the Darlington
instead of the transformer windings (“dry transformer”). Without the electronic inductor, the loop current
would have to flow through the transformer (“wet transformer”), however since the telehone loop current can
be as high as 120mA, the transformer would saturate, causing signal degradation unless the geopety of the
transformer becomes much larger. This is especially true for high speed modems such as V.34bis, where
return loss must meet or exceed 25dB. Return loss of 25dB is usually not attainable witha wet transformer,
and if it is, the transformer is too large and expensive for the application. *The best way to overcome this
sauration and return loss problem is to “rerout” the loop current through the electronic inductor and AC cou-
ple and modem signal via C2 to the transformer, such that no DC current flows through the transformer.
Return loss is a measure of mismatch