October 23, 1997

Simple regulator provides ±12V

Musiri Shrivathsan, National Semiconductor Corp, Santa Clara, CA

A simple circuit can provide ±12V from a 5V input using only one switching-regulator IC and a few off-the-shelf components. This circuit provides good regulation at a high efficiency. The circuit configures the switching regulator, IC₁, as an inverting buck-boost converter to obtain the negative output. The circuit generates the positive output using an additional winding of an off-the-shelf inductor, L₁. Only one additional diode, D₃, and capacitor, C₁, are necessary to generate the positive output.

During the on-time of the switching cycle, the circuit applies the supply voltage across the inductor to charge it. Meanwhile, C₂ and C₃ supply the load. During the off-time of the switching cycle, the energy stored in the inductor transfers to the output capacitors. IC₁ regulates the negative output. The coupling between the windings of the inductor enables regulation of the positive output.

D₄ prevents the ­12V output from going positive above a diode drop during the turn-on of the circuit. D₁ provides isolation between the output and the input and helps increase closed-loop stability. IC₁ operates at a switching frequency of 150 kHz, which allows for small inductors and capacitors.

The ­12V output varies between ­11.99 and ­11.97V under a load variation of 50 to 100 mA and a line variation of 4.5 to 5.5V. The 12V output varies between 11.15 and 11.45V under the same conditions. The 12V output is lower than the ­12V output because of the voltage-divider effect of the leakage inductance and a mismatch in the forward voltage drops of D₂ and D₃. However, the leakage inductance in this circuit does not lead to losses, such as those in a flyback converter.

This circuit's efficiency varies between 76 and 83% over the same line and load variations. The ripple voltage on both outputs is about 95 mV. Most of this ripple voltage results from the ESR of output capacitors C₁ and C₂, which have an ESR of about 0.65 ohms. You can obtain a smaller ripple voltage by using output capacitors with higher capacitances and smaller ESRs. (DI #2095)

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