Controller IC Eliminates Remote Sense Lines for Long Cable Runs

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

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Fig. 4 shows the timing diagram for virtual remote sensing (VRS). A new cycle begins when the power supply and VRS close the loop around VOUT (regulate VOUT = H). Both VOUT and IOUT slew and settle to a new value, and these values are stored in the track-and-hold capacitor (track VOUT high = L and track IOUT = L). The VOUT feedback loop is opened and a new feedback loop is set up commanding the power supply to deliver 90% of the previously measured current (0.9IOUT). VOUT drops to a new value as the power supply reaches a new steady state, and this information is also stored in the track/hold capacitor. At this point, the change in output voltage (ΔVOUT) for a -10% change in output current has been measured and is stored in the track/hold capacitor. It is amplified by a gain of 10 to obtain the total correction to the output. The four track/hold circuits store the amplitude values in sequence to capacitors at CHOLD1, CHOLD2, CHOLD3, and CHOLD4. During the next VRS cycle, the total voltage is fed back to the power supply, which adjusts the voltage at the load to compensate for voltage drops due to line resistance.

To insure orderly start-up, the LT4180 has a soft-correct function. When the RUN pin rising threshold is first exceeded (indicating VIN has crossed its undervoltage lockout threshold), power supply output voltage is set to a value corresponding to zero wiring voltage drop (no correction for wiring). Over a period of time (determined by CHOLD4), the power supply output voltage ramps up to account for wiring voltage drops, optimizing load-end voltage regulation. A new soft-correct cycle is also initiated whenever an overvoltage condition occurs.

The LT4180 will work with either isolated (with an opto) or non-isolated power supplies. A variety of power supplies and regulators having either an external feedback or control pin can be used with the LT4180. Tying the supply's existing inverting input to ground disables its error amplifier (Fig. 5). This converts the error amplifier into a simple constant-current source with the output voltage then controlled by the LT4180's drain pin. This method eliminates the regulator error amplifier from the control loop so the control loop with VRS in the LT4180 can control the output.

Isolated power supplies and regulators may also be used by adding an opto-coupler (Fig. 6). LT4180 internal regulator (INTVCC) supplies power to the opto-coupler LED. In situations where the control pin VC of the regulator may exceed 5V, a cascode may be added to keep the DRAIN pin of the LT4180 below 5V (Fig. 7).

Three temperature grades are available. They include an extended grade version from -40 to 85°C, an industrial grade version from -40°C to 125°C and a military grade option from -55°C to 125°C. The LT4180 comes in an SSOP-24 package.