Cable and xDSL modems are experiencing increasing popularity. For this reason, many designs are now required to interface with existing telephones at the subscriber's location. The subscriber line interface circuit (SLIC) within the modem has the additional burden of ringing the phone as well as providing loop current while a conversation is taking place.
While the phone is ringing and "on-hook," it appears as an 8k resistance in series with another 1k of capacitive reactance. Typically, the SLIC must be capable of driving this impedance with a 45-VRMS, 20-Hz sinewave with a negative dc-offset in order to ring the phone. Therefore, a high voltage of between −50 and −105 V is usually mandated by the SLIC. Once the handset is lifted, the phone places a much lower impedance across the phone terminals, and the SLIC goes into a 20-mA constant-current mode. As a result, the SLIC only needs a power supply of −24 V.
Figure 1 shows the schematic of the power supply for the SLIC portion of the modem. This circuit provides multiple outputs from a single power switch and control IC. It also uses an efficient n-channel MOSFET with low voltage stress. A low input voltage powers a flyback topology. The input source could be either a widely differentiating (generally 2.5 to 1) output from an ac adapter, or a regulated supply used by some other portion of the system.
U2 is the brain of the power supply, as it modulates Q2's duty factor to control the output voltage. Also, it produces a reference voltage inverting amplifier U1 uses to generate an error signal. The control IC can perform either in current-mode or voltage-mode control.
A key advantage of the multi-winding approach is that a single control circuit and single MOSFET can supply two telephony voltages. Good cross-regulation can be achieved even though the SLIC places large current swings on both outputs (Fig. 2). With the −24-V output, variations range from 3 mA of load current (when all phones are idle) to 80 to 100 mA of current (when all phones are off-hook). The −72-V output current can vary from 1 mA to a peak of 100 mA. Changes in the −24-V output voltage occur due to the operational amplifier's offset voltages, which are typically 3%, as well as divider and reference tolerances. Capable of such tolerances, the −72-V output also offers cross regulation of the transformer and the fluctuating diode voltage drops. Even with all of these variables, however, the −72-V output's worst-case variation is less than 4 V, way below the 14-V requirement.
Because of its low conduction and switching losses, the n-MOSFET, multi-output flyback configuration provides excellent efficiency. The efficiency measurements with the −72-V output loaded to full current are given over a wide input voltage range (Fig. 3). This design has been optimized with approximately 90% efficiency for a 12-V input. For the low input voltages, the efficiency is somewhat lower due to the increased conduction losses in the MOSFET. But at high voltages, the higher switching losses cause the lower efficiency.