ST3917B

The device ST3917B consists of thespeech and the dialer functions. It provides the DCline interfacecircuit that terminates the telephone line, analog amplifier for speech transmission and necessary signals for either DTMF and pulse ialing.When mated with a tone ringer, a completetelephonecanbe producedwith just two ICs. The DC lineinter face circuit develop sitsown line voltage across the device andit is adjus tablebyexternal resistor to suit different country's specification. A built-in LED driver can deliver excess line current to external LED indicator(s) during dialing and speech mode. The LED(s) can be used either for keyboard illumination purpose or for tone mode indication during softswitch andmixedmode dialing by connectingMU/MFI pin to VDDandGNDoranyrowrespectively.TheLEDcurrent is limited to 17mA(typical).

The ST3917B speechnetwork provides the two to fourwires interface, electronic switching between dialing and speech and automatic gaincontrol ontransmit andreceive.

The ST3917B dialing network buffersup to32 digits into the LND memorythatcanbelaterredialedwithasinglekeyinput. Users can store all 13 signalling keys and access severaluniquefunctionswith single key entries.These functions include : Pause, Last NumberDialled (LND), Softswitch and Flash. (see Figure 1).

The ST3917B FLASHkey simulatesa hookflashto transfercalls or to activate other special features provided by the PABXor central office.

The ST3917B PAUSE key stores a timed pause in the number sequence.Redial is then delayeduntil an outside line can be accessed or some other activities occur before normal signalling resumes. A LND key automatically redials the last number dialed.

ST3917B AdedicatedpinMU/MFI is used toselect themutingfor transmit amplifier and lighted dial LED for keyboard illumination ora LEDindicator for tonemode indication. The SEL pin allows selection of any one of the four possible Flash duration options.