Compact Switching Step-Down Converter Schematic Circuit Diagram

Switch-mode power supplies are used in electronic circuits to increase (step up) or reduce (step-down) voltage levels in the most efficient manner possible. Compared to linear voltage regulators, switch-mode supplies convert relatively little energy into heat. Their efficiency is thus high. This is a major advantage with compact power supplies in particular since it sometimes makes forced-air cooling unnecessary.

Building a switch-mode supply is considerably easier if you use components that have been specially developed for this application. One example of such an integrated step-down converter is the Maxim MAX639. This is designed for a fixed output voltage of +5 V, with an input voltage ranging between +5.5 and +11.5 V. Although this IC is primarily designed for a fixed output voltage, the output voltage can be tailored using a simple feedback network. With the given component values, resistors R3 and R4 determine the output voltage, with

R3 = R4 [(Vout / 1.28) – 1]

The value of R4 may lie between 10 kΩ and 10 MΩ, but a value of 100 kΩ is a good choice for most applications. The maximum output current is 100 mA. If desired, a different type of Schottky diode with similar specifications can be used in place of D1 (a 1N5817). Inductor L1 must be suitable for a maximum current of 500 mA.

The MAX639/MAX640/MAX653 step-down switching regulators provide high efficiency over a wide range of load currents, delivering up to 225mA. A current-limiting pulse-frequency-modulated (PFM) control scheme gives the devices the benefits of pulse-width-modulated (PWM) converters (high efficiency at heavy loads), while using only 10µA of supply current (vs. 2mA to 10mA for PWM converters). The result is high efficiency over a wide range of loads.

Applications

• 9V Battery to 5V, 3.3V, or 3V Conversion
• High-Efficiency Linear Regulator Replacement
• Portable Instruments and Handy-Terminals
• 5V-to-3.3V Converters

Features

• High Efficiency for a Wide Range of Load Currents
• 10µA Quiescent Current
• Output Currents Up to 225mA
• Preset or Adjustable Output Voltage:
  5.0V (MAX639)
  3.3V (MAX640)
  3.0V (MAX653)
• Low-Battery Detection Comparator
• Current-Limiting PFM Control Scheme