Invert your biased voltage


During system development, it’s not often that we think about the power supplies needed for the system. Typically, lab power supplies are used for the very first tests of a new system, such as an industrial temperature sensor. These are just turned on and off manually in order to simplify the testing of the system, write code, and do basic debugging. But as the system comes together and takes on its final form factor, the power supplies must be integrated into the system. It’s often at this point that we realize just how many rails we have in our systems and how much easier our life would be if we had a bit fewer. Thus begins the process of optimizing the system’s power architecture.

One rail that is particularly difficult to optimize is a negative voltage rail. Even more difficult is when this rail is just used as a bias supply for op amps or other basic circuits and draws very little power. To achieve the system concerns of small size, high efficiency, and good output voltage regulation puts a strain on the already few options for creating negative voltage rails. A negative voltage charge pump, for example, may not be as efficient as required for a battery-powered sensor at very light loads.  A dedicated inverting buck-boost converter, however, adds another IC to the Bill of Materials (BOM) as well as its inductor and other passive components required for its operation. This grows the BOM disproportionately to the amount of output power on the negative rail.

Luckily, many Texas Instruments wide-Vin buck converters can be used in an inverting buck-boost configuration. Since the same IC can be used as a buck converter and an inverting buck-boost converter and with the same passive components, the BOM is greatly simplified. The TPS621xx wide-Vin, low power buck converters especially are optimized for high efficiency at light load currents. For example, the TPS62120’s 11 µA of Iq and the TPS62175’s 4.8 µA of Iq enable over 70% efficiency in the inverting topology even below 1-mA load currents.  Furthermore, the DCS-Control architecture used in the latest TPS621xx devices offers superior output voltage regulation compared to other topologies. At 2mm x 2mm and 2mm x 3mm, respectively, these devices will not take up excessive board area and their high integration and supporting application notes, "Using the TPS62120 in an inverting buck-boost topology" and "Using the TPS62175 in an inverting topology" (SLVA478 and SLVA542) are sure to ease system integration.

                                                                        TPS62175 in an inverting topology

In which applications do you have a need for a low power negative voltage rail?

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