be added to reduce the level at the speech circuit DTMF

Input For application requiring higher DTMF level the signal

swing across the device can be raised by using 620X in

10 mf capacitor in order to maintain same AC off hook im-

pedance

PULSE DIALING TELEPHONE

The TP5700A can reduce the number of components re-

quired to build a pulse dialing telephone as shown in

Figure

5 The usual current source can be eliminated by using the

pulse dialer via a blocking diode A low forward-voltage drop

diode such as a Schottky type is necessary because

age may fall to 2V on a 20 mA loop A 100 mF decoupling

capacitor is required to hold up the pulse dialer supply volt-

age 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 MX resistor required to retain the

last-number dialed memory will keep this capacitor charged

Partial muting is obtained by directly connecting the N-chan-

nel open-drain 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

TLH5201 – 9

Low voltage drop diode (eg Schottky)

Low leakage type

Indicates National Semiconductor discrete transistor process number

FIGURE 5 Typical Pulse Dialing Telephone

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