Typically, power supplies are set to a fixed output voltage to supply energy to the load. However, depending on the application, variable voltage may be required. For example, microcontrollers can be operated more efficiently by adjusting the microcontroller core voltage according to the operating state. In this article, we explore how to regulate the power supply output voltage during operation using a digital-to-analog converter (DAC) developed for this specific purpose.


Typically, the output voltage of a voltage converter can be set using a resistor voltage divider. This method works well as is for a fixed voltage. However, when the output voltage needs to be variable, one of the voltage divider resistor values must be adjusted, and this adjustment can be done dynamically using a potentiometer. Figure 1 shows a simple circuit using a buck, that is, a step-down topology switching regulator IC, as an example of such a circuit.


However, for many applications, using potentiometers in the circuit is not practical. In many cases, the voltage must be set using digital signals. One solution is to provide a low level of positive or negative current to the FB node. To achieve this, a small DAC designed to dynamically regulate the output voltage can be used.

Figure 2 shows an example circuit with a DC-DC converter and an LTC7106 DAC added to the feedback path. In principle, any voltage converter equipped with an external, accessible feedback pin can operate in this manner.

The LTC7106 has a current output that supplies current to a resistive voltage divider circuit, so the reference voltage of this switching regulator IC appears on the switching regulator's FB pin for various output voltages. Therefore, the output voltage is set, and the FB pin receives the required regulation voltage.

Unlike many other DACs with current output, the LTC7106 is designed so that no current flows to the IDAC pin unless there is a valid digital command. As a result, no unwanted voltage is formed during circuit startup.

The LTC7106 is a 7-bit DAC that can operate at 1 A per LSB or 4 A per LSB depending on the application. The highest resolution can be achieved when operating at 1 A per LSB. Therefore, it is recommended to set the resistor voltage divider of the switching regulator to 1 A per LSB of the LTC7106.

The output of this current DAC has an accuracy of ±0.8% in the positive range and ±1.5% in the negative range over the entire allowable temperature range.

Figure 3 shows that the LTC7106 can be easily programmed using the LTpowerPlay graphical user interface.

Of course, even circuits using the LTC7106 have limitations in controlling the output voltage. Switching regulators or linear regulators can only generate the intended voltage. Linear regulators or step-down switching regulators can only generate an output voltage lower than the input voltage. It is also necessary to test the voltage conversion circuit to verify that control loop stability and output voltage ripple are within an appropriate range for the desired output voltage.

Using a compact current DAC like the LTC7106 allows for easy dynamic adjustment of the output voltage. The LTC7106 is designed to operate reliably with minimal wiring.

※ Author Introduction

Frederik Dostal is a power management expert who has worked in this field for over 20 years. 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 on customer projects as an FAE. He then worked in Phoenix, Arizona for four years as an application engineer, responsible for switch-mode power supplies. He joined Analog Devices in 2009 and has since held various positions related to product lines and technical support for European customers. Currently, he works as a power management expert at the ADI Munich office, leveraging his extensive knowledge of design and applications.