If you want a 9V to 3V Step-Down Converter here is a simple circuit using LT1073 from Linear technology.The LT®1073 is a versatile micro-power DC/DC converter.The device requires only three external components to deliver a fixed output of 5V or 12V. The very low minimum supply voltage of 1V allows the use of the LT1073 in applications where the primary power source is a single cell. An on-chip auxiliary gain block can function as a low battery detector or linear post-regulator.The LT1073 is good idea,it used for dc to dc converter.Average current drain of the LT1073-5 used as shown in the Typical Application circuit below is just 135mA unloaded,making it ideal for applications where long battery life is important. The circuit shown can deliver 5V at 40mA from an input as low as 1.25V and 5V at 10mA from a 1V input.source

Here is DC-DC converter using LM2576 circuits.It performs same function as the commonly known voltage regulator LM7805 from National Semiconductor. The 7805 voltage regulator dissipates a lot heat. The higher input voltage, the more heat is generated. The extra input energy is converted to heat, keeping the output voltage regulated at 5V.source

Sometimes you need a simple negative power supply. The best example is the contrast PSU for common small LCD device. Building -5V from a battery or a wallmart supply isn’t really easy.Here is a simple circuit that is able to provide -5V, -12V, -15V. The circuit used the MAX 764/765/766 series. You can simply change the chip to provide the good output.

The MAX764/MAX765/MAX766 inverting switching regulators are highly efficient over a wide range of load currents, delivering up to 1.5W. A unique, current-limited, pulse-frequency-modulated (PFM) control scheme combines the benefits of traditional PFM converters with the benefits of pulse-width-modulated (PWM) converters.

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As we know,the white LED is hard to drive to reach maximum efficiency.These requirements can be easily met using the LT1618 DC/DC converter from Linear Technology.With the circuit above you can drives white LEDs with an input voltage range from 1.6V to 18V,the LT1618 can provide LED drive from a variety of input sources, including two or more alkaline cells, or one or more Li-Ion cells.source

A SEPIC (single ended primary inductor converter) is a DC-DC converter which allows the output voltage to be greater than, less than, or equal to the input voltage. The output voltage of the SEPIC is controlled by the duty cycle of the control transistor. Here is a USB to 5V SEPIC DC/DC Converter with Short-Circuit Protection.This circuit is ideal for applications where the output must reduce to zero during shutdown. The input current limit not only helps soft-start the output but also provides short-circuit protection, ensuring USB device compliance even under output fault conditions.source