Single-inductor circuit generates bipolar supply voltages

Figure 1: This single-inductor, 200mA power supply derives +15V/−14V from +5V
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The circuit is capable of meeting the demands of most analog circuitry, including audio, instrumentation, and industrial applications. It provides output currents up to 200 mA with low output ripple (100 mV_pp), and drives several amplifiers drawing 5 mA to 10 mA each.

A voltage-mode, step-up switching converter requiring no expensive current-sense resistors (U1) generates the +15 V output using a fixed on-time and minimum off-time. The negative rail is generated by an external charge pump consisting of D2, D3, C5, and C6. Because the negative rail is not regulated, it is not as stable as the positive rail, and varies with current drawn from the positive supply, Figure 2 Note also, that U1's latched-fault mode shuts down the controller in response to a short circuit.

Figure 2: Load regulation for the −14 V output in Figure 1 depends on load current drawn from the +15 V output (curves shown are for 20 mA and 200 mA drawn from +15V).
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The +15 V output delivers as much as 200 mA, with efficiencies up to 85% and about 44 mV of output ripple, Figure 3.

Figure 3: Output efficiency (a-upper) and ripple (b-lower) are as shown for the +15 V output in Figure 1, loaded with 200mA.
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Load regulation for both outputs is shown in Figure 4.

Figure 4: These curves from Figure 1 show the output regulation for +15 V, with 200 mA load on −14 V (a-upper), and the output regulation for −14 V, with 200 mA load on +15 V (b-lower).
(Click on image to enlarge)

As for all switching converters, a good printed circuit board (PCB) layout is essential for low-noise operation.

About the author
Perwez Nawabi has been an applications engineer at Maxim Integrated Products, Sunnyvale, CA, for three years, following his graduation from San Jose State University with a BSEE.