Linear Tech cuts two wires from PSU remote sensing | Electronics Weekly

Linear Technology has announced an add-on chip that compensates for output wire losses in PSUs without resorting to remote sensing.

Dubbed LT4180, it is claimed to work with any regulator topology in both isolated and non-isolated power supplies including bricks, modules and adjustable linear regulators.

The output conductor impedance is continuously interrogated by forcing the output current up and down by 5%, and from voltage measurements the device compensates for both load current and line impedance with line voltages from 3.1 to 50V.

“Output correction is obtained by modulating the output and measuring the incremental change in voltage that occurs with a change in current,” said the firm. “Any modulation noise is eliminated by the supply bypass capacitor located at the load.”

A potential draw-back is that this bypass capacitor may have to be added to the system output terminals to implement this form of compensation.

Optional dithering spreads the spectrum of any noise created by the modulation.

Although Linear’s automatic compensation technique appears to be new, compensation that is set at the design stage to allow for output wire losses is not uncommon, for example CamSemi has a family of off-line regulators with it built-in.

CamSemi provides it to allow wall adapter manufacturers to cut copper cost by using thinner output wires.

Linear’s chip regulates the main PSU by breaking into the original feedback loop between its potentiometer and the PSU’s feedback connection, or by driving an opto-isolator.

As yet, the firm is not publically discussing system output voltage precision, although the chip is known to have an internal voltage reference accurate to +/-1% over temperature.

Applications for automatic compensation are sought in remote instrumentation, battery charging, wall adaptors, notebook power, surveillance equipment and halogen lighting.

“The provides a new function for power supply designers. Excellent regulation is obtained without sense wires, eliminating the need for point-of-load regulators,” said v-p engineering and company CTO Robert Dobkin. “Difficult regulation problems such as long wire runs or system retrofits can be accommodated. With halogen lights, the light output drops more quickly than power, so keeping the correct voltage more than compensates for any additional cost of the power supply.”

Linear points out that a 10% error in voltage results in a 30% change in light output of halogen bulbs.

“With a 12V system it’s easy to get a 1V drop in the wiring, decreasing the light intensity and changing colour temperature,” said Linear.

The chip comes in a SSOP-24 package and three temperature grades are available: -40 to 85°C, -40 to 125°C and -55 to 125°C.