The backplanes of popular multi-supply buses (such as VME, VXI, and PCI) each provide power-limited outputs of 3.3 V, 5 V, and ±12 V (or ±24 V). If adding line cards to these systems increases the requirements for 3.3- or 5-V power, the power budget thus may dictate that a circuit tap its power from the lightly loaded −12-V source.

The figure shows a circuit that derives 3.3 V from −12 V by boosting the −12-V supply to 15.3 V (with respect to the −12-V rail) and provides up to 300 mA. Transistor Q2 level-shifts the 3.3-V output to the feedback level of −10.75 V.

The circuit occupies approximately 6.25 cm² of pc-board space and employs a PWM boost controller that delivers 1 W at 83% efficiency levels.

Because the sense resistor (R_SENSE) limits peak currents to about 120 mA, the n-channel switching MOSFET (Q1) can be an inexpensive logic-level device. Resistors R1 and R2 permit selection of V_OUT to either 3.3 V or 5 V.

Note that the feedback voltage at equilibrium (pin 5, as determined by the trip point of a comparator internal to IC1) is 1.25 V above the PGND terminal (pin 7). To calculate the appropriate R1/R2 values:

V_FB = −12 V + 1.25 V = −10.75 V

Choosing an arbitrary value for R1:

I2 = 1.25 V/R1 = 1.25 V/12.1k = 103 µA

Therefore,

R2 = (V_OUT − V_BE) / I2 = (3.3 V − 0.7 V) / 103 µA = 25.2k

IC1 has another capability that's useful in RF applications, DAQ modules, and other noise-sensitive designs: It lets you program the switching frequency between 100 kHz and 500 kHz using an external resistor. The circuit's high frequency (100 kHz) maintains high efficiency while allowing the use of small inductor and capacitor values.

The circuit is optimized for a 1-W output in applications associated with desktop-PC power supplies. In these applications, efficiency must exceed 83% since the −12-V bus is power-limited to 1.2 W. If necessary, the output power can be increased to 3 W (3 V at 1 A or more) by scaling L1, D1, and R_SENSE. A diode can be added in series with R1 to block unwanted reverse currents.

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