Positive regulator makes negative dc/dc converter

摘要

Power-supply designers can choose from a plethora of available positive buck regulators that can also serve as negative boost dc/dc converters. Some buck regulators have a negative-feedback reference voltage expressly for this purpose, but ICs that have positive-reference feedback voltages far outnumber these negative-feedback regulators. You can take advantage of this greater variety of devices by using a positive buck switch-mode regulator to create an excellent negative boost converter. All you need are a few small modifications to the typical buck-converter configuration. Figure la shows a ―5V-input to ―9-output, 1.4A negative boost converter using the LT1765EFE positive-buck-converter, switch-mode regulator. This IC accepts 3 to 25V input, uses a 1.2V feedback voltage, and has an internal 3A power switch. The 1.25-MHz switching frequency of the LT1765EFE helps reduce the size of the inductor and input and output capacitors. Figure 1b shows a typical positive-buck-converter application for the LT1765EFE: a 12V-input to 3.3V-output, 2.2A dc/dc converter. Figure 2 shows an efficiency plot for the regulator in Figure 1a.