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PULSE_DIALING_TELEPHONE

2014-11-05 11:07  
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PULSE_DIALING_TELEPHONE
The TP5700 or TP5710 can reduce the number of components required to build a pulse-dialing telephone, as shown. The usual current source can be eliminated by using the VREG1 output to power a TP50982A low-voltage (1.7 V) pulse dialer via a blocking diode. A low forward-voltage drop diode such as a Schottky type is necessary because VREG1 is used in its nonregulated mode and its output voltage might fall to 2 V on a 20-mA loop. A 100-μF decoupling capacitor is required to hold up the pulse dialer supply voltage during dialing. This capacitor will take about one second to charge up when the telephone is first connected to the line, but thereafter, the 20-MΩ resistor, required to retain the last-number dialed memory, will keep this capacitor charged. Partial muting is obtained by directly connecting the N-channel opendrain mute output of the pulse dialer to the RCV in pin on the speech circuit. A fully muted pulse dialer design requires the use of a shunt-mode dialer, such as the TP50981A or TP50985A.