A new multiwinding transformer (configurable by the user for a variety of applications) enables an inverting controller to produce the high negative voltages required by an ISDN board or other telephone line card. Such line cards use a subscriber-line interface circuit (SLIC), such as the 79R79 ringing SLIC from AMD. This IC generates the off-hook and on-hook signal transmission, ring-tone generation, and ring-tip detection that constitute an analog telephone interface. For off-hook signal transmission, it requires a tightly regulated -24 or -48 V; to generate ring tones, it requires a loosely regulated -70 V. The "five -ringer-equivalent" requirement demands 9 to 10 W from the -70-V output, which translates to a full-load lout of about 150 mA. IC1 is an inverting switching regulator that usually converts a 3- to 16-V input to a fixed output of -5 V or an adjustable output. In the circuit shown, three pairs of windings in series (provided by a single off-the-shelf multiwinding transformer) enable IC1 to generate the high voltages needed by a SLIC IC1 (D1). Connecting a diode and output capacitors (C7 and C9) at the first or second pair of windings produces -24 V (as shown) or -48 V, respectively. Feedback to the IC via R1 and R2 achieves tight regulation at this output. The trans-former turns ratios establish a loose regulation at the -70-V output. The circuit can service a five telephone load (10 W) from an input of 12 V, ±10 percent. It operates down to 3 V and produces about 2.4 W at 3.3 V and 3.9 W at 5 V. The -70-V output depends on cross-regulation with respect to the -24-V output. It is, therefore, affected by relative loading on the two outputs (i.e., whether one is heavily loaded and the other lightly loaded, or vice versa).
Required Field 
