When using buck or linear regulators, the voltage is typically regulated to a set value to power the load. In some applications (e.g., laboratory power supplies or electronic systems requiring long cables to connect various components), the presence of voltage drops on interconnects makes it impossible to guarantee an accurate regulated voltage at the desired location. Control accuracy depends on many parameters. One is the DC voltage accuracy when the load requires a continuous, constant current. Another is the AC accuracy of the generated voltage, which depends on how the generated voltage changes with load transients. Factors affecting DC voltage accuracy include the required reference voltage (which may be a resistive divider), the behavior of the error amplifier, and other power supply influencing factors. Key factors affecting AC voltage accuracy include the selected power rating, backup capacitor, and the architecture and design of the control loop.
However, in addition to all these factors that affect the accuracy of the generated power supply voltage, other effects must be considered. If the power supply is located separately from the load that needs power, there will be a voltage drop between the regulated voltage and the location where the power is required. This voltage drop depends on the resistance between the regulator and the load. It could be a cable with plug contacts or a long trace on a circuit board.
Figure 1. Physical distance between the voltage regulator and the relevant load
Figure 1 shows the resistance between the power supply and the load. The voltage drop across this resistance can be compensated by slightly increasing the voltage generated by the power supply. Unfortunately, the voltage drop across the line resistance depends on the load current, i.e., the current flowing through the line. Higher current results in a higher voltage drop compared to lower current. Therefore, the load is powered by a regulated voltage with rather low accuracy, and this regulated voltage depends on the line resistance and the corresponding current.
A solution to this problem already exists. An additional pair of leads can be added in parallel with the actual wiring. Kelvin sensing leads are used to measure the voltage on the electronic load side. In Figure 1, these additional leads are shown in red. These measurements are then integrated into the power supply voltage control on the power supply side. This method is effective, but the drawback is the need for additional sensing leads. Since high current is not required, the diameter of these leads is typically very small. However, incorporating sensing leads into the connecting cable to achieve higher current introduces additional workload and cost.
Voltage drop compensation on the connection line between the power supply and load can be achieved without the need for an additional pair of test leads. This is particularly useful for applications with complex and costly cabling where EMC interference can easily couple to the voltage test leads. A second approach is to use a dedicated line voltage drop compensation IC like the LT6110. This IC is inserted into the voltage generation side, and the current before entering the connection line is measured. The power supply's output voltage is then adjusted based on the measured current, allowing for very precise regulation of the load-side voltage regardless of the load current.
Figure 2. Using LT6110 to adjust the power supply output voltage to compensate for voltage drop on the connection line.
Using components like the LT6110 allows for adjustment of the power supply voltage based on the load current; however, this adjustment requires knowledge of the line resistance. Most applications provide this information. If, during the device's lifespan, the wiring is replaced with longer or shorter wires, the voltage compensation implemented with the LT6110 must also be adjusted accordingly.
If the line resistance may change during device operation, components like the LT4180 can be used to virtually predict the line resistance using an AC signal when there is an input capacitor on the load side, thereby providing a high-precision voltage to the load.
Figure 3. Virtual remote measurement of the line using LT4180
Figure 3 shows an application using the LT4180 where the resistance of the transmission line is unknown. The line input voltage is adjusted based on the corresponding line resistance. Using the LT4180, no Kelvin sensing of the line is required; voltage regulation is achieved simply by gradually changing the line current and measuring the corresponding voltage changes. The measurement results are used to determine the voltage drop in the unknown line. Optimal adjustment of the DC/DC converter output voltage is then achieved based on the voltage drop information.
This measurement method is effective as long as the nodes on the load side have low AC impedance. It is effective in many applications because the load following a long connection line requires a certain amount of energy storage. Due to the low impedance, the output current of the DC/DC converter can be regulated, and the line resistance can be determined by measuring the voltage across the connection line.
Whether a stable power supply voltage can be obtained depends not only on the voltage converter itself, but also on the power supply line of the load.
in conclusion
The required DC accuracy can be improved by adding an additional Kelvin sense line. Alternatively, integrated circuits can be used to compensate for voltage drops on the line, eliminating the need for a Kelvin sense line. This approach is useful if Kelvin sense lines are too expensive, or if existing wiring must be used and no additional sense lines are available. Using these design techniques, higher voltage accuracy can be easily achieved.
About the Author
Frederik Dostal studied microelectronics at the University of Erlangen in Germany. His career began in 2001 in power management, where he has been active in various application roles, including a four-year stint in Phoenix, Arizona, focusing on the development of switch-mode power supplies. He joined Analog Devices in 2009 and currently works as a field application engineer in the Power Management division, based in Munich, Germany.
Disclaimer: This article is a reprint. If it involves copyright issues, please contact us promptly for deletion (QQ: 2737591964). We apologize for any inconvenience.
