Controller IC Eliminates Remote Sense Lines for Long Cable Runs

May 1, 2010 12:00 PM
Sam Davis, Editor in Chief

Linear Technology's LT4180 is a Virtual Remote Sense™ power supply controller that eliminates the remote sense connections used with power converters to compensate for the voltage drop in lines going to the output load. To understand use of the LT4180, we first have to look at a power supply's role in regulating voltage applied to a load, which means it must obtain an accurate load voltage in order to maintain tight regulation. The LT4180 addresses various applications where a high current load is located a long distance from the power supply, such as lighting systems, remote instrumentation, and surveillance equipment.

In a typical system, lines that connect the power supply's output to its load have some resistance, so load current causes an unwanted voltage drop between the power supply and its load. Therefore, this voltage drop produces an inaccurate output voltage to be fed back to the supply for regulating the load voltage. For the tightest regulation, the power supply should feed back an accurate load voltage. As shown in Fig. 1, many power supplies have a remote sense function to eliminate inaccurate feedback voltage. Connecting two remote sense lines (plus and minus) that draw virtually no current from the power supply to the load, the supply can get an accurate load voltage as feedback for load voltage regulation. For proper operation, steps should be taken to ensure the remote sense wires do not pick up noise by either twisting the two sense wires together and/or shielding them from noise. Also, you must observe the correct polarities with the +sense line connected to the +V load and the — sense line connected to the -V load.

Virtual Remote Sense (VRS) eliminates the need for the pair of remote sense wires. The basic VRS measures the incremental change in voltage that occurs with an incremental change in current in the output voltage wiring (Fig. 2). The VRS circuit uses this measurement to compute and compensate the voltage drop in the line from the power supply to the load.

In the application shown in Fig. 3, The LT4180 continuously interrogates the line impedance and corrects the regulator's output voltage. The IC maintains a corrected, regulated voltage at the load regardless of current or line impedance. Virtual Remote Sense takes over control of the power supply via its feedback pin (FB) that drives the opto-coupler. The VRS circuit maintains tight regulation of load voltage at RL.

Virtual remote sensing relies on sampling techniques to hold the different voltages during a correction cycle. Because switching power supplies are commonly used or loads may be pulsed, the LT4180 uses a variety of techniques to minimize potential interference in the form of beat notes that may occur. Besides several types of internal filtering, and the option for VRS/power supply synchronization, the LT4180 also provides spread spectrum operation on the dither. Spread spectrum techniques dither the interrogation frequency from cycle to cycle. With spread spectrum, low modulation index pseudo-random phasing is applied to Virtual Remote Sense timing. This lowers the average value of the peaks and prevents beat notes if the load is pulsed at a frequency close to the interrogation cycle.

SENSING LINE CURRENT

The voltage drop across R_SENSE is proportional to line current, IL. Select the value of R_SENSE so that it produces a 100mV voltage drop at maximum load current. For best accuracy, the V_IN and SENSE pins should be Kelvin-connected to R_SENSE.

Four track/hold circuits capture and hold input voltages obtained from R_SENSE (due to line current) at times determined by division ratio of the spread spectrum clock. The division ratio depends on the 3-bit code set by DIV2, DIV1 and DIV0. The track/hold circuits include a switch and a storage capacitor. In the track mode, the switch closes thereby coupling the input signal to the specific CHOLD(1, 2, 3, or 4) capacitor, allowing the storage capacitor to track the RSENSE input signal. In the hold mode, the switch opens, isolating the storage capacitor from the input signal and allowing it to hold the amplitude value of the input signal constant.

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