This article aims to provide tips on power management by introducing circuits that convert positive (+) voltage to negative (-) voltage and negative (-) voltage to positive (+) voltage, respectively.

In addition, it explains how to convert positive voltage to negative voltage by slightly modifying a step-down (buck) regulator circuit, and how to convert negative voltage to positive voltage by modifying a boost converter.

■ Preface

Power converters are generally used to generate the stable supply voltage required by a circuit from existing voltage rails.

In most applications, a buck converting switching regulator is used to lower the voltage.

There are times when the voltage needs to be increased, in which case a step-up boost converter is primarily used. These two topologies, buck and boost, are widely used in various applications.

However, there are times when the voltage needs to be inverted depending on the situation. In most cases, it is necessary to convert positive (+) power supply voltage to negative (-) voltage; this is done to supply power to sensors in the signal path that can process signals with positive and negative voltage deviations.

For example, this applies to cases where an operational amplifier (op amp) uses +5V and -5V power supplies to process signal voltages.

In addition, some applications require a negative voltage to safely shut off the gate of a MOSFET.

To generate a negative voltage from an existing positive voltage, an inverting topology as shown in Figure 1 is suitable.

In this method, positive voltage is converted into negative voltage.

This circuit has a simple structure, does not require a transformer, and can be implemented with a standard buck switching regulator IC.

The output voltage of the buck regulator is connected to the system ground (GND), and the GND pin voltage becomes a negative voltage, which can be adjusted through resistor dividers Rfb1 and Rfb2.


In addition to these applications, there are sometimes cases where negative voltage needs to be converted to positive voltage.

A typical example is the case in the telecommunications field where -48V power needs to be converted to +48V.

There are also such applications in the industrial sector. There is an excellent solution for this that does not even require a transformer.

Figure 2 shows an inverting boost converter that converts negative voltage into positive voltage.

This circuit uses a boost regulator IC instead of a buck regulator. The power levels must be designed so that the body diodes of the switching elements each conduct in the correct direction.


Several additional components are required to efficiently convert negative voltage into positive voltage and implement various functions.

For example, level conversion is required for various setting pins, such as soft start or clock synchronization functions.

The allowable voltage range for these pins is determined by the GND pin voltage of the switching regulator IC, which is based on the negative input voltage.


For stable switching even under higher power conditions, it is advisable to use a dedicated switching regulator IC that converts negative voltage to positive voltage.

One such IC is the LTC7899, a switch-mode power supply (SMPS) boost converter controller specifically designed to convert negative voltage to positive voltage.

The LTC7899 uses an external switching element, operates synchronously, and supports a very wide voltage range of up to 135V between the input and output.

Figure 3 shows the LTC7899 used in the inverting circuit in LTspice®, a simulation tool that enables circuit simulation.

■ Conclusion

Negative and positive voltages can be easily inverted without a transformer.

Many applications require such voltage inversion, and simple solutions and various power conversion ICs exist for this purpose.

However, to achieve excellent performance and simple design simultaneously, it is desirable to use a dedicated switching controller such as the LTC7899.

※ Author Introduction
Frederik Dostal has been working in this industry for over 20 years as a power management expert. He majored in Microelectronics at the University of Erlangen in Germany and joined National Semiconductor in 2001, where he gained extensive experience implementing power management solutions for customer projects as an FAE. During his four years at NS, he worked in Phoenix, Arizona, as an Application Engineer responsible for Switch Mode Power Supplies (SMPS). He joined Analog Devices in 2009 and subsequently held various positions related to product lines and European technical support. Currently, leveraging his extensive knowledge of design and applications, he works as a power management expert at the ADI Munich office.