Virtual remote sensing for accurate power delivery

EDN Europe, 15 Apr 2010

Linear Technology’s LT4180 is an IC that you can use to improve the performance of a power supply; you can achieve improved voltage regulation at the point-of-load without adding an extra pair of sense connections in a four-wire configuration.

The diagram shows how the 4180 connects: it monitors the supply output voltage, using a sample-and-hold circuit, and drives the supply’s voltage-trim, or voltage-adjust, input. Between the power supply and the load is an indeterminate resistance; as Linear’s Bob Dobkin, whose creation the 4180 is, points out, this is not only unknown but also variable – wiring resistance varies with temperature, and connector losses vary with age, wear and contamination. The 4180 imposes an ac modulation at, typically 8 – 16 kHz, on top of the dc output of the supply, by driving the trim input. The ac level is typically +/- 10% of the nominal supply voltage. The chip then measures (using the sense resistor shown in the diagram) the ac current. The ac component encounters a short-circuit prior to the load itself, in the form of the capacitor across the load terminals, and therefore the loop resistance seen by the ac signal is identical to the wiring resistance. The 4180 therefore takes the voltage drop seen by the small ac signal, scales it up to the level of the nominal supply voltage, and applies it as a dc correction signal to the trim input. The PSU steps up its output in response to the trim input, compensating for the losses in the connection to the load. Effectively, wherever you place that load-side capacitor, becomes the control point at which the PSU – augmented by the LT4180 – maintains regulation.
You might use the device with any power supply that delivers power over a long connection, or in difficult situations such as system retrofits; Dobkin mentions notebook computer PSUs as a typical example. He points out that for user convenience, their makers like to provide low-voltage wiring that is thin and light – and therefore, high resistance. He says that if you equip such a PSU with the 4180’s “virtual remote sensing” you can eliminate a second voltage regulator in the notebook – or other load – along with its heat sink and other components, and the overall outcome can be a lower bill-of-materials. Another application is low-voltage lighting circuits; the output of halogen lamps drops rapidly if their voltage falls below nominal, and the 4180 allows you to regulate their supply to the correct level at the lamp terminals.
You can use the 410 with almost any controller chip, existing PSU, or module, Dobkin says; he cites a test configuration he created with a Vicor module and a very large load-side capacitor, that worked well. It will handle voltages in the range 3 to 50V and works with boost and buck converters of any topology in isolated or non-isolated configurations, with the capability to drive an optocoupler. The chip delivers as much as a 50:1 improvement in regulation at the load. Dobkin calls the added ac modulation the “dither” frequency; you can adjust it to avoid other frequencies present in your design and, if your supply is already handling a load that has a regular variation in its current demands, you can add a spread-spectrum component to the dither frequency to prevent it from interfering with the frequencies in the load current. Your load almost certainly has a large capacitor across its power terminals already, Dobkin says, but if you need to upgrade it, it should not require any substantial changes – “100 to 500 F is typical.” The dither frequency is a square wave but the capacitances at power supply and load ends of the connection “soften” the waveform edges so that the extra ac component does not create any EMC issues.
The device comes in three temperature grades, extended, industrial and military, and costs from $2.95 (1000) in a 24-pin SSOP package.