Voltage Drop on Power Lines
Output voltages of DC/DC converters or LDOs typically are precisely regulated at the location the feedback divider is connected. In case of longer connections to the load (for example, when it is not on the same PCB), the precision of the regulation suffers from the way the connection is established. Remote sense is not always an option because an additional feedback line is needed which adds cost and is potentially susceptible to noise. A practical example demonstrating the need for voltage drop compensation is a car center console USB-Port with charging capability. The Infotainment Head-Unit contains the electronics and is located in the dashboard. The USB-Port is a passive implementation and located in the center console connected via three meters of cable. To charge mobile digital devices, the USB current capability has to be 2A and more. Stringent USB port supply voltage limits increase the need for compensation as the voltage drop is not acceptable.
Figure 1. Block-diagram Car Center Console USB Charging Port
The weight also restricts the cross section of the wires that can be used. This means a voltage drop which depends on the load current must be expected. Since this voltage drop cannot be fed back to the converter, compensation can only be done by adjusting the output voltage of the converter to match the voltage drop along the cables. In the case of the car center console USB charger, a USB charging port controller and TPS2546 power switch is in series with the power line. This could potentially open up the connection from Vbus to Vout which would prohibit further remote sense on Vbus for this application.
Figure 2. Equivalent Circuit Car Center Console USB-Port
Implementing Voltage Drop Compensation
The voltage drop compensation is implemented by measuring the current and feeding back the current information to the feedback resistor network of the converter. In this implementation, the characteristics of the power supply is changed to compensate a voltage drop on a power line.
Figure 3. INA213 Droop Compensation
Carsten Thiele’s article How to extend a power supply for droop compensation published on EETimes offers a detailed description including design help in a cookbook manner.
USB Charger Reference Design
Based on the article is the Automotive USB Charger with Linear Droop Compensation Reference Design. This design demonstrates an automotive USB charging port that offers short-to-battery protection. With the help of the shunt amplifier, the design aims to compensate the effects on Vdroop when the smartphone/tablet is connected to the USB charger using a cable that is roughly 2-3 meters in length.
Step-Down Converter Reference Design
An additional reference design build and tested by Juergen Neuhaeusler is the Step-Down Converter with Cable Voltage Drop Compensation. This design details a power management circuit which is capable to accurately regulate the output voltage at the end of a cable without using sense connections. The implementation is based on a completely integrated step down converter circuit enabling a small total solution while still capable of handling up to 3 A at the output. The related Application Report offers as well a design guide to calculate the components.
Above reference designs found under TI Designs offer detailed description, schematic/block diagram, test data and design files including gerber and bom.
INA213-Q1 Voltage Output, High/Low-Side Measurement, Bi-Directional Zero-Drift Series Current Shunt Monitor.
TPS62130 3-17V 3A Step-Down Converter with DCS-Control in 3x3 QFN Package
TPS2546-Q1 USB-Charging Port Controller and Power Switch With Load Detection