When a step-down regulator or linear regulator power supply is used, the voltage is generally regulated to a set value to supply power to the load. In some applications (for example, laboratory power supplies or electronic systems that require long cables to connect various components), it is not possible to ensure that accurate regulation is always provided at the desired point due to various voltage drops on the interconnects Voltage. 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 varies with load transients. Factors that affect the DC voltage accuracy include the required reference voltage (perhaps a resistive divider), the behavior of the error amplifier, and some other influence of the power supply. Key factors affecting AC voltage accuracy include the power level selected, backup capacitors, and the architecture and design of the control loop.

However, in addition to all these factors that affect the accuracy of the generated supply voltage, other effects must be considered. If the power supply is spaced apart from the load that needs to be powered, there will be a voltage drop between the regulated voltage and where the power is needed. This voltage drop depends on the resistance between the regulator and the load. It could be a cable with plug contacts or a longer trace on a circuit board.

Figure 1. Physical Distance Between Voltage Regulator and Associated Load

Figure 1 shows that there is resistance between the source and the load. The voltage loss across this resistor can be compensated by slightly increasing the voltage generated by the power supply. Unfortunately, the voltage drop developed across the line resistance depends on the load current, which is the current flowing through the line. High current results in a higher voltage drop than low current. Therefore, the load is powered by a rather inaccurate regulated voltage, which depends on the line resistance and corresponding current.

There is already a solution to this problem. Can be connected in parallel with actual wiring to add an additional pair of connections. Measure the voltage on the electronic load side using Kelvin sense wires. In Figure 1, these extra lines are shown in red. These measurements are then integrated into the supply voltage control on the supply side. This works well, but has the disadvantage of requiring an extra sense lead. These leads are usually very small in diameter as they do not need to carry high currents. However, setting up the measurement lines in the connecting cables to obtain higher currents entails additional work and higher costs.

It also compensates for the voltage drop on the connection line between the power supply and the load without the need for an additional pair of sense leads. This is of particular interest in some applications where cabling is complex and expensive and the resulting EMC disturbances can easily couple into the voltage test leads. The second option is to use a dedicated line drop compensation IC such as the LT6110. Plug this IC into the voltage generating side and measure the current before entering the connecting wire. The output voltage of the power supply is then adjusted based on the measured current, enabling very precise regulation of the load-side voltage regardless of the load current.

Figure 2. Using the LT6110 to regulate the output voltage of a power supply to compensate for the voltage drop on the connecting line

With components such as the LT6110, the supply voltage can be adjusted according to the corresponding load current; however, this adjustment requires knowledge of the line resistance. Most apps provide this information. If the connecting wires are replaced with longer or shorter connecting wires during the lifetime of the device, the voltage compensation implemented with the LT6110 must also be adjusted accordingly.

If the line resistance may change during device operation, a component such as the LT4180 can be used to provide a high-precision voltage to the load by virtually predicting the line resistance from an AC signal with input capacitance on the load side.

Figure 3. Virtual Remote Measurement of Lines Using the LT4180

Figure 3 shows an application using the LT4180 where the resistance of the transmission line is unknown. The line input voltage is adjusted according to the corresponding line resistance. With the LT4180, no Kelvin sense line is required, and voltage regulation can be achieved by simply changing the line current in steps and measuring the corresponding voltage change. Use measurements to determine voltage losses in unknown lines. Optimal regulation of the output voltage of the DC/DC converter is achieved based on the voltage loss information.

This measurement works well as long as the nodes on the load side have low AC impedance. Effective in many applications because loads behind long connecting lines require a certain amount of energy storage. Due to the low impedance, the output current of the DC/DC converter can be adjusted and the line resistance can be determined by measuring the voltage on the front side of the connecting line.

Whether a stable supply voltage can be obtained is not only related to the voltage converter itself, but also to the power line of the load.