Electronic Design

Micropower Backup Supply Operates From Phone Line

Designers of electronic circuits for telephone systems are constantly enticed by the possibility of utilizing the “free” power provided by the telephone company. The catch involved in pulling off this feat is that this power is limited to 5 µA maximum when the phone is “on-hook” (that is, not in use). To complicate things further, the nominal 48-V dc line voltage can fluctuate over an approximate 24- to 58-V dc range.

A typical use for this on-hook power would be as a power backup source for a microprocessor. Using a battery or Supercap for the backup would require a periodic recharge. This can be accomplished when the phone is off-hook and allowed to take much more current from the line. If the phone is on-hook for a long period of time, however, the supercap or battery would discharge, possibly causing loss of data.

Use of a voltage regulator in such an application creates two basic problems. The quiescent current, even in “micropower” IC regulators, is well over the 5 µA maximum. Also, IC regulators can’t take the maximum line voltage of 58 V or more. “Micropower” and “high voltage” are generally contradictory requirements.

The circuit described here provides a regulated 3.3-V (or 5-V) supply, with at least 1.0 µA of current, from a widely varying line voltage. The circuit takes less than 4 µA from the line over a 24- to 58-V input.

Referring to the figure, Q1 and Q2 form a current source, providing about 3 µA of current to A1, which consists of an LTC1540CS8 micropower reference and comparator. A1 consumes a quiescent current of 0.6 µA max (0.3 µA typical); this includes the on-board 1.182-V reference.

R1 and R2 set the comparator to trip when the output voltage is around 3.3 V. R3 and R4 are connected to provide about 40 mV p-p of hysteresis. Therefore, the output will have about 40 mV of ripple voltage as the comparator alternately charges and discharges the output capacitor. Changing R1 to 18M would set the output voltage to 5.0 V nominal.

The current source uses three 18M resistors in series to develop a reference voltage across Q1. A single 56M resistor could be used, but pc-board leakage would be a problem. It’s also a good idea to mill a slot under each of these resistors to reduce the possibility of contamination or board leakage, which causes a higher current flow.

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