This is the circuit diagram of 5VDC single polarity to 12VDC dual polarity converter based on IC LM2595-12 from National Semiconductor. This circuit will convert 5V DC single voltage (+) become 12V DC dual polarity output (+), GND, (-).

Many electronic devices need a ?12V power supply. Typical examples include analog circuits or RS-232 driver power supplies. The ?12V typically needs to be generated from a 5V system bus. The solutions normally used involve a multiple secondary transformer or multiple switching regulators. These solutions can be complicated, may require custom transformer design and may have poor efficiency and poor regulation. The 5VDC single polarity to 12VDC dual polarity converter circuit shown in above image is simple, uses only one switching regulator IC, uses a small number of components, and provides good regulation at a high efficiency. Additionally, all the components used in this circuit are off the shelf components.

The converter circuit in above image uses an LM2595-12 (buck SIMPLE SWITCHER?) based switching regulator to generate both the +12V and the -12V outputs from 5V input. The LM2595 is configured as an inverting buck-boost converter to obtain the negative output. The positive output is generated using an additional winding in the off-the-self inductor (CTX250-4 from Coiltronics) used in this circuit. Only one additional diode (D3) and a capacitor (C3) are needed to generate the positive output.

This circuit come from Application Note #1118 by National Semiconductor about Simple Regulator Provides ?12V from 5V Source using LM2595-12.
Download the application note AN-1118:

AN-1118 - 5V to 12V DC Converter

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This is a 5VDC to 12VDC boost converter circuit built based LT1070. The LT^?1070/LT1071 are monolithic high power switching regulators. They can be operated in all standard switching configurations including buck, boost, flyback, forward,inverting and “Cuk”. A high current, high efficiency switch is included on the die along with all oscillator, control and protection circuitry.

5VDC to 12VDC LT1070 Boost Converter circuit Reference: LT1070 datasheet