Electronic Design

Low-cost, isolated 5-V supply for 3.3-V logic

Occasionally, a 3.3-V logic system will need an isolated supply for a peripheral function. There are some dc-dc converters available. However, if spare inverters are present in the circuit, a low-cost solution can be produced using only four inverters and a few discrete components.

The design presented here employs four inverters to produce a squarewave RC oscillator with complimentary outputs (see the figure). The output inverters drive switching transistors (ZTX451), which in turn drive an off-the-shelf, low-power transformer. The transformer (76253/35) is designed for use in 3-V to 5-V supply systems with the MAX253 IC. In the circuit shown here, the discrete implementation of inverters and transistors imitates the basic operation of the IC and produces an oscillator frequency of 200 kHz.

The transformer’s output is unregulated and supplies approximately 250 mW of useful power at 4.80-V output from the circuit. This is often sufficient for peripheral functions such as slow ADCs and as a reference for 3-V to 5-V logic translators. If the circuit is planned for logic-translator applications, then the isolation may not be required and input and output ground can be connected together.

The circuit can be improved for higher current loads by reducing the 1k base resistors to the ZTX451 driver transistors. The overall circuit efficiency is 50%. Improvements can be made by using better Schottky diode rectifiers and lower Vcesat transistors (e.g., ZTX489).

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