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Employing buck controller in buck boost design

Posted: 13 Aug 2010     Print Version  Bookmark and Share

Keywords:buck boost  buck controller  buck boost topology 

Electronic circuits typically operate from regulated positive output voltages, often provided by buck regulators. If a negative output voltage is also required, the same buck controller often can be configured in a buck-boost topology. A negative output voltage buck-boost, sometimes called a negative flyback, operating at 50 per cent duty-cycle provides an output voltage equal to the input voltage, only opposite in polarity. It has the ability to "buck" or "boost" the output voltage to maintain regulation by adjusting the duty cycle as the input voltage fluctuates.

A controller must be selected that can power up witsh the minimum input voltage minus a diode drop, but must also withstand Vin plus Vout while operating. The FET and diode must also be rated for this voltage range. The output voltage is regulated by connecting the feedback resistor at the output ground, since the controller is referenced to the negative output voltage. By carefully selecting just a few components' values and some minor circuit changes, a buck controller can work double-duty in a negative output buck-boost topology.

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