“Generally speaking, DC-DC switching regulators are fixed voltage outputs. Is there a way to dynamically adjust the output voltage of the power supply using digital control signals? This article describes how to use ADI’s dedicated digital-to-analog converter (DAC), the LTC7106, to dynamically adjust the output voltage of a power supply.
Generally speaking, DC-DC switching regulators are fixed voltage outputs. Is there a way to dynamically adjust the output voltage of the power supply using digital control signals? This article describes how to use ADI’s dedicated digital-to-analog converter (DAC), the LTC7106, to dynamically adjust the output voltage of a power supply.
How to realize the dynamic adjustment of the output voltage of the power supply?
Resistor divider circuit + potentiometer
For dynamic adjustment of the output voltage of a DC-DC switching regulator, the most common practice is to set up a resistor divider circuit. As shown in the figure below, we use a potentiometer to replace one of the voltage divider resistors to dynamically adjust the voltage output of the DC-DC switching regulator.
Figure 1: Using a potentiometer to adjust the DC-DC switching regulator output (Image credit: ADI)
This method is very effective, but requires us to do it manually. If you need to use a digital control signal to control the output voltage, a good way is to input a positive or negative current to the feedback node above. Therefore, we need a small DAC specially developed for dynamically adjusting the output voltage.
Resistor divider circuit + DAC
Let’s take ADI’s DAC LTC7106 as an example: as shown in the figure below, the LTC7106 outputs a current to the resistor divider circuit, so that the voltage on the feedback pin of the DCDC switching regulator changes, so that the output voltage of the DC-DC switching regulator can be digitally controlled signal changes.
Figure 2: LTC7106 DAC connected to DC-DC power supply feedback path (Image source: ADI)
This approach imposes a requirement on the DAC. When the DAC has no digital control signal, it cannot have current output, otherwise it may cause unnecessary voltage to appear when the DC-DC switching regulator is started.
Specialized DACs can overcome this phenomenon, such as ADI’s LTC7106 is designed specifically for this application. As long as there is no valid digital control signal, there is no current (ie high impedance) on the LTC7106 output pin IDAC, thus avoiding unwanted voltages during circuit startup.
The LTC7106 is a 7-bit DAC capable of up to 1 LSB (least significant bit), 1µA resolution. We can design a suitable resistor divider circuit according to the characteristics of the DAC to match the accuracy of the DAC. Below we illustrate through a design example.
Design example based on LTC7106
As shown in the figure below, the LTC7150S is used as a DC-DC switching regulator to provide a 1.5V voltage output. Suppose we now need the power supply to dynamically adjust the power supply output voltage between 1.5V and 1.0V.
Figure 3: LTC7106 design example (Image source: ADI)
According to the LTC7106 data sheet,
The greater the magnitude of the LTC7106 output current IDAC, the better the accuracy of the LTC7106. The IDAC output range of the LTC7106 can be -63µA to +63µA in normal mode.
Assuming to choose =10kΩ, =6.65kΩ, according to the formula:
We can derive the IDAC range: IDAC = (1.5VC1.0V)/10kΩ = +50µA.
Assuming IDAC least significant bit LSB=1μA, the output voltage range of Vout is 1.5V to 1.0V.
Simplify Development with the LTC7106 Development Board
We can also use the LTC7106 development board DC2620A-A to shorten the development process.
Figure 4: LTC7106 development board DC2620A-A
DC2620A-A can be connected to a computer through the interface module DC1613A
Figure 5: DC2620A-A connected to a computer (Image source: ADI)
The LTpowerPlay graphical user interface can be displayed on the computer. Control signals on the computer via PMBus or I2C controls the LTC7106.
Figure 6: LTpowerPlay graphical user interface (Image credit: ADI)
LTpowerPlay also provides diagnostic and debug capabilities for programming or tuning power management schemes in a system, or diagnosing power problems when powering up a power rail.
Summary of this article
Using the LTC7106 can easily achieve dynamic adjustment of the output voltage of the power supply with reliable operation. It should be noted that the DAC output accuracy has errors. For example, the LTC7106 has a positive output of ±0.8% and a negative output of ±1.5% over the entire temperature range. Care must be taken to ensure that the control loop stability and output voltage ripple are within a reasonable range for the desired output voltage.