APPENDIX A

Battery voltage autoset

The STLC30R80 shows a line voltage depending on the voltage applied to Vbat pin. In particular in the

On-Hook the line voltage is Vbat if the SLIC is put in HI-Z mode or Vbat -7.8V if the SLIC is put in Active

mode.

If the battery voltage applied to the Vbat pin is always -70V (necessary to generate the proper ringing

signal), during the On-Hook the line voltage is higher than 60V.

A simple circuit to generate the proper Off-Hook battery voltage can be used starting from the -70V as

shown in the below figure A1.

The RING command (active low) is used to switch on the NPN transistor and apply the battery voltage

directly to the Vbat pin. When the RING command is high the NPN transistor is off and the zener diode

reduces the voltage applied to the Vbat pin.

STLC30R80

24V

BATTERY VOLTAGE (-70V)

D00TL461

BC558

BC556

RING

VCC

470K

Ω

47K

Ω

470K

Ω

68K

Ω

Figure A1.

STLC30R80

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