Electronic Design

Circuit Design Innovations

Xphase relies on lossless average inductor current sensing to provide high accuracy at low cost. This technique requires an external inductor with a series-connected resistor-capacitor (R1, C1) pair shunted across the inductor (see the figure). The time constant of the resistor-capacitor combination is chosen to match that of the inductor (inductance/equivalent series resistance, or ESR). If the time constants match, the voltage across C1 will be identical to the voltage across the inductor's parasitic resistance, ESR.

A measurement of this voltage allows calculation of the current output of a particular phase. The accuracy of this measurement depends on the tolerances of four parameters (LOUT, ESR, R1, and C1) across a range of operating conditions. Any mismatch in LOUT, R1, and C1 affects only the ac portion of the current-sense signal and can be compensated in the control loop. A high-speed differential current-sense amplifier in the Phase IC is used to sense the current signal across C1. To compensate for the increase in the inductor's ESR with temperature, the current-sense amplifier's gain is made inversely proportional to temperature.

To achieve accurate current sharing among the different phases, the output of each current-sense amplifier is compared against an average of all current-sense amplifiers. This average value is transmitted over the current share bus to each Phase IC. If the current in a phase is smaller than the average current, a share adjust amplifier will activate a current source. That current source reduces the slope of its pulse-width-modulation (PWM) ramp to increase its duty cycle and increase the output current of that phase.

The achievement of an overall system setpoint accuracy of 0.5% for output voltage represents an advance over existing controllers that typically offer tolerances of 1% or greater. To achieve this precision, the company trims the controllers' internal voltage reference after the chip is packaged. Another contributing factor is the relatively small size of the controller die versus other controllers. The small die size lowers the mechanical stresses that degrade the accuracy of the voltage reference.

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