If you wish to convert a range of possible input voltages into an output voltage that lies somewhere in the middle of that range, a regulator that can automatically switch between step-up and step-down modes is required. Such a device is the MAX1759 from Maxim (www.maxim-ic.com), which can take in an input voltage between +1.6 V and +5.5 V and generate an output between +2.5 V and +5.5 V. It is based around a switched capacitor (C2).
A further special feature of the MAX1759 is the automatic detection of the potential divider R3/R4. This allows it to produce an externally settable output voltage between +2.5 V and +5.5 V, as given by the following formula:
VOUT = 1.235 V (1 + R3/R4)
The resistors should be chosen with values of the order of 100 kΩ.
If the feedback input FB is tied to ground, the MAX1759 switches over to an internal voltage reference giving a fixed output voltage of 3.3 V.
The open-drain ‘power OK’ output POK goes low when the regulator control loop is not stabilized and is pulled high when the output voltage is stable and at the desired value.
The MAX1759 is a buck/boost regulating charge pump that generates a regulated output voltage from a single lithium-ion (Li+) cell, or two or three NiMH or alkaline cells for small hand-held portable equipment. The MAX1759 operates over a wide +1.6V to +5.5V input voltage range and generates a fixed 3.3V or adjustable (2.5V to 5.5V) output (Dual Mode™). Maxim’s unique charge-pump architecture allows the input voltage to be higher or lower than the regulated output voltage. Despite its high 1.5MHz operating frequency, the MAX1759 maintains a low 50µA quiescent supply current.
Designed to be an extremely compact buck/boost converter, this device requires only three small ceramic capacitors to build a complete DC-DC converter capable of generating a guaranteed 100mA (min) output current from a +2.5V input. For added flexibility, the MAX1759 also includes an open-drain power-OK (POK) output that signals when the output voltage is in regulation. The MAX1759 is available in a space-saving 10-pin µMAX package that is 1.09mm high and half the size of an 8-pin SO.
