August 1, 1997

Autotransformer regulator inverts 12V to produce ­12V

Dana Davis, Maxim Integrated Products, Sunnyvale, CA

You can create a dc/dc regulator with an internal switching MOSFET that inverts 12V to produce an output of 200 mA at ­12V (Figure 1). The IC is a high-efficiency device whose low quiescent current (120 µA maximum) is the product of a CMOS process that limits the absolute-maximum input-to-output voltage to 21V. Thus, to avoid having 24V across its terminals, the IC must isolate itself from the inductor-flyback voltage. It can do so by driving either an external switch in a nonbootstrapped configuration or an internal switch in a flyback-transformer configuration.

Autotransformer T₁, a center-tapped inductor with a 1-to-1 turns ratio, offers a design alternative. Terminal LX flies back to 1/2V_OUT plus a diode drop, or approximately ­6V. The V+ terminal remains at 12V, yielding 18V maximum between V+ and LX--well within the 21V limit. Because IC₁ drives the gate of its internal MOSFET between the V+ and OUT voltages, you normally connect OUT to V_OUT to ensure sufficient gate drive. (In a typical application, the chip inverts 5V to produce ­5V.) In this circuit, the 12V input provides adequate gate drive, so OUT connects to ground. (DI #2067)

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