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Power tip: Compensating for cable drop

Posted: 21 Oct 2013 ?? ?Print Version ?Bookmark and Share

Keywords:power supply? remote sensing? USB charger? differential amplifier? transconductance?

Sometimes your power supply design calls for better regulation than you can achieve without the complexity of remote sensing. A prime example of this is an offline USB charger where the power supply must compensate for 0.5V of cable drop without the cost and bulk of two extra wires. The voltage to the output needs to be in range of 4.75-5.25V. Without remote sensing, this cannot be achieved with typical component tolerances and the 0.5V drop on the output cable.

The standard approach to this problem measures the output current through a sense resistor with a low offset voltage differential amplifier. Then the output voltage from the amplifier is turned into a current source and subtracts current in the voltage sensing circuit, thereby raising the output voltage. A simpler approach that eliminates the transconductance amplifier is shown in figure 1.

The output of the amplifier U1B is the output voltage minus the amplified current sense voltage. If the amplifier output is held constant by a closed loop, the output voltage (Vo) rises as the load current is increased. This can be used to compensate for cable drop from Vo to the actual load by suitable choices of R1, R3, and R4.

Figure 1 also shows how the circuit can be unstable. The EAout equation shows a simplified expression for the amplifier output voltage in terms of Vo. In this simplification, the cable resistance and the load resistance are lumped as RLOAD, and there is no capacitance assumed at the load.

Figure 1: A single op-amp can compensate for cable drop.

Note that the amplifier voltage has two terms, one positive and one negative. If at some frequency, the magnitude of the second term is larger than the first term, the phase of the amplifier output changes 180 degrees, which can create an oscillator. This typically is not a problem when the current sense resistor is connected between the output capacitor and the load. It can be a serious problem if the current sense resistor is connected between the output inductor and output capacitor.

Figure 2 shows how simple cable drop compensation can be.

Figure 2: This method of compensation has one less amplifier than a traditional approach. (Click on image to enlarge)

This is a 12V to five-volt buck regulator that can be used in an automotive USB charger. The power supply would be in an assembly that plugs into a power port, and the load would be powered through a cable. The heart of this circuit is the control IC, U1, which closes the feedback voltage loop as well as provides the power switches of the buck regulator. Internally to the IC, the voltage at the feedback pin is compared to a one-volt reference. This information is used to set the duty cycle of the power switches. The voltage at the feedback pin is set by the R5/R7 divider, so that the circuit regulates TP9 to five volts. The amplifier U4A subtracts an amplified current sense voltage from the voltage at TP4.

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