For systems that require 150 mW or less, the power supply in Fig 1 draws power from ordinary phone jacks without interrupting the voice signal, thereby eliminating the need for batteries and ac adapters. Built into peripheral equipment such as PCMCIA modem cards, this circuit can spare the battery in a host computer. The circuit is useful in portable systems that connect to subscriber (household) telephone lines, such as modems and telephone test sets.

The line current available to a telephone in the off-hook state is limited not by regulations or code, but by only the sum of impedances in the central-office battery and intervening phone lines. These line impedances vary greatly in proportion to distance from the central office, so the customary practice of matching impedances for maximum power transfer is impractical.

However, D₁'s zener-clamp termination works well for line impedances to 1-Kohm and for worst-case conditions. This termination also meets the one condition imposed on line current by the phone system: Off-hook current must exceed 20 mA to ensure activation of a network-access relay in the central office.

D₁ provides approximately 6.8V to the center tap of T₁ and 5V to the V_CC terminal of IC₁, a 400-kHz oscillator driving an internal flip-flop. IC₁ generates two push-pull, 50%-duty-cycle, 200-kHz square waves that drive internal, ground-referenced switches connected to the primary of T₁. On the secondary side, Schottky diodes D₂ and D₃ rectify the isolated power, which IC₂'s low-dropout regulator then regulates to 5V.

T₁'s primary is a center-tapped winding whose ET product (a voltage-time product of 25V µsec) is sufficient to prevent saturation under worst-case conditions. Similarly, T₁'s turns ratio should provide the minimum required output voltage for maximum load and minimum input voltage. The calculation for the turns ratio should also assume worst-case losses in D₂ and D₃.

The resultant turns ratio produces a much higher secondary voltage for best-case conditions, which is acceptable for some applications. If not, you can add a linear regulator (IC₂). For isolated 5V outputs, the ideal turns ratio is 1.2CT:1.0CT (CT=Center Tapped). Wind the transformer on Magnetics Inc (Butler, PA, (412) 282-8282) "W," Fair-Rite "76," or other high-permeability magnetic material. To minimize radiated noise, choose a pot core, E/I/U core, toroid, or other geometry with closed magnetic paths.

Consider a typical toroid, such as the 40603-TC, which is 0.125 in. thick with a 0.230-in. outside diameter. For 6.8V inputs, this core should have a 48-turn primary (24 turns on either side of the CT), which yields a nominal end-to-end primary inductance of 8 mH. You can scale the secondary for any reasonable output voltage. For example, 40 turns (20 turns on either side of the CT) delivers 5.2V min, as required by the linear regulator for maintaining a regulated 5V output.

For isolated 3.3V applications, the minimum voltage to ICis 3.5V. T₁'s turns ratio should be 2.0CT:1.0CT, with a primary ET product of at least 25V msec. If you use the same 48-turn primary as for 5V applications, the required number of secondary turns is 24 (12 on either side of the CT). In addition, you must add a resistive divider for setting IC₂'s regulated output to 3.3V.

Q₁, Q₂, and the associated resistors ensure a low-power shutdown mode for IC₁until its supply voltage can sustain a full power-up. IC₁'s supply current is fairly constant, so light filtering (by L₁ and C₃) is sufficient to prevent noise from entering the hybrid transformer (not shown). (DI #1573)