MICROPROCESSOR SUPERVISOR CIRCUITS
Jul 29, 2002
Low-Cost AC Supply
The following application note discusses a low-cost solution to implement a
3V, 3.3V or 5V logic supply from AC power. It uses the MAX6330 shunt
regulator which can supply up to 50mA as well as offering a reset output
(power-on reset, POR).
One often needs a low-cost logic supply for powering the MCU and related control circuitry in
"white goods" products such as industrial controllers and sensors. Those applications usually
include 24VAC, 115VAC, or higher levels of AC power for conversion to 3.3V or 5.0V. (The
following circuit suits applications that include 24VAC. With proper safety precautions, it can also
be applied to double-insulated white goods and other products that require a logic supply for
control or monitoring functions.)
The simplest approach for generating low-current logic levels from an AC source is to apply the
rectified and filtered AC input to a high-input-voltage linear regulator. But, power dissipation in
the regulator can be considerable, even for modest load currents. A standard shunt regulator
also dissipates notable power in the limiting resistor. A switching regulator minimizes power
dissipation, but that type may not be practical for cost-sensitive designs.
As an alternative, consider an AC-coupled approach to reduce the power dissipation in linear or
shunt-regulator designs. To transfer energy to the regulator with negligible power loss, use a
coupling capacitor in conjunction with an IC containing a shunt regulator and power-on reset
(Figure 1). Available with 50mA maximum output-current capability in 3.0V, 3.3V, and 5.0V
shunt-voltage versions, IC1 also includes a power-on-reset (POR) function. Because IC1 is an
active shunt (vs. a passive zener diode) one must rectify the AC voltage before applying it.
Typically, a capacitor follows the rectifier to hold the charge during off cycles. As shown, a
simple half wave rectifier is used to save cost.