FEATURE POWER ELECTRONICS
Improving regulation for remote power and instrumentation
Greg Zimmer, Senior Product Marketing Engineer, Signal Conditioning Products at Linear Technology Corporation discusses the route to improving voltage regulation without remote sense
S
imply connecting a power supply to a load is not always so simple. Voltage drop due
to wires, cables, connectors and circuit board traces can cause considerable regulation errors at the load. It is not uncommon for some applications to have long wires between the power supply and the load. In these applications, as the load current increases, the voltage delivered to the load drops, with the result appearing as if there is a load regulation issue with the power supply. The traditional approach to solving this problem is to sense the output voltage with a dedicated pair of wires extending from the regulator’s feedback network to the load. While this approach works well, an additional pair of wires may not be desirable due to its impact on cost, weight, reliability, etc. Another possible solution is to increase the gauge of the cable/wire to reduce resistance; however, this leads to a higher cost and weight for the wire and can be impractical for retrofitting an existing system. Finally, one can place a regulator at the point-of-load. This may require additional regulators with added cost/complexity, rather than a single, central regulator. It can also reduce power efficiency for linear regulators (with less power delivered to the load) and it may not be practical due to physical constraints (such as size) and environmental constraints (such as thermal load, operating temperature). Linear Technology offers a novel approach
to this problem with its LT6110 wire drop compensator. This device is designed to improve regulation for remote power and instrumentation, such as high power USB, Power-over-Ethernet (PoE), battery charging, surveillance equipment, high power lighting, portable entertainment, remote cameras, well logging and down-hole drilling. The LT6110 works by measuring the load current and providing a small current into the regulator’s feedback to raise the regulator’s output voltage. It is designed to work with standard feedback circuits as well as “current source” reference devices such as Linear Technology’s LT3080. At the heart of the LT6110 is a precision
high side current-sense amplifier with a current-mode output, designed for controlling the output voltage of an adjustable power supply or voltage regulator. As increasing load current is sensed, this device raises the output of the
18 DECEMBER/JANUARY 2015 | ELECTRONICS
by 1V, a correction current needs to flow through the regulator feedback resistor. This correction current is generated by the LT6110. Since the regulator feedback resistor is 20K and the desired correction voltage is 1V, the current (IIOUT value of IIOUT
) needs to be 50uA. The is determined by the VSENSE
voltage, which is forced across the input resistor of the LT6110. Thus, the input resistor should be set to 400Ohms. The improvement in load regulation for this
example can be easily demonstrated with a simple LTSpice circuit simulation . Using an LTSpice model with an LT1965 linear regulator driving a 1A/1Hz load, the load regulation is dramatically improved with an LT6110. The LT6110 operates with supply voltages
regulator to compensate for voltage drop at the load. In most cases, setting up wire drop
compensation is simple. Consider a camera mounted atop an observation pole with a power unit located 50 feet away. A 5V supply voltage is dropping to 4V, as a result of 1Amp of current draw, implying 1Ohm of line resistance.
Figure 1:
The LT6110 wire drop compensator from Linear Technology is designed to improve regulation for remote power and instrumentation
that range from 2V to 50V and is fully specified for operation from -40°C to 125°C. The LT6110 can compensate for small or large voltage drops to a high degree of precision, and it can compensate for load regulation issues within the regulator itself. Generally, a reasonable estimate of wire resistance will provide good compensation. For example, a 50 percent uncertainty in wire drop compensation will still improve load regulation by a factor of 2; a 10 percent uncertainty would improve load regulation by a factor of 10. The LT6110 provides two current-mode outputs that are proportional to the load current, one sinking and one sourcing. This allows the LT6110 to adjust the output voltage of a wide variety of regulators. Either output may also be used to monitor the load current. Low DC offset allows for the use of a small sense resistor, as well as precise control of small variations in wire voltage drop. The LT6110 can sense load current up to
With the LT6110 wire drop compensation, a correction current is generated to adjust the regulator feedback. This current is placed at the regulator’s feedback node and will be conducted through the feedback resistor. Thus, this correction current is chosen such that it boosts the regulator’s output by the amount of voltage drop: ICORRECTION
= VDROP
Where VDROP load and RF
/RF is the voltage drop to the is the feedback resistor on the
regulator. The LT6110 is configured such that it
generates this correction current, ICORRECTION (See Figure 2).
Considering the remote camera
application, an LT6110 wire drop compensator can be inserted between the regulator and load. With 1Amp flowing through the 20mOhms built-in sense resistor, a 20mV sense voltage is generated. To adjust the regulator output
Figure 2:
The LT6110 is configured such that it generates this correction current, ICORRECTION
3Amps with its internal sense resistor, and an external sense resistor can be used for higher current. Two current-mode outputs are provided; either is available for wire drop control, or simple current monitoring. Low DC offset allows for the use of a small sense resistor and precise control of small wire drop, and the 180kHz signal bandwidth enables fast response to load changes. With this combination of features, the LT6110 can be used with all types of regulators and across a broad range of applications. This small, simple, and low cost solution does not require adding point-of-load regulators or sense wires, and enables the use of thinner wires.
Linear Technology Corporation
www.linear.com 01628 477066
Enter 205 / ELECTRONICS
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