The fundamentals to translating the analog world into the digital domain reduces to a handful of basic parameters. Voltage, current, and frequency are electrical parameters that describe most of the analog world. Current measurements are used to monitor many different parameters, with one of them being power to a load.
There are many choices of sensing elements to measure current to a load. The choices of current sensing elements can be sorted by applications as well as the magnitude of the current measured.
This write up is part two of a three part series (Part one can be seen here) that discusses different types of current sensing elements. The focus of this paper is to evaluate a direct current resistance (DCR) sensing architecture that allows for lossless current sense in some power applications. This paper explains how to design a DCR circuit for power applications. This paper analyzes the drawbacks of the architecture as well as providing a way of improving the current measurement technique through calibration.
DCR circuits are commonly used in low supply voltage applications where the voltage drop of a sense resistor is a significant percentage of the supply voltage being sourced to the load. A low supply voltage is often defined as any regulated voltage lower than 1.5V.
A DCR sense circuit is an alternative to a sense resistor. The DCR circuit utilizes the parasitic resistance of an inductor to measure the current to the load. A DCR circuit remotely measures the current through an energy storing inductor of a switching regulator circuit. The lack of components in series with the regulator to the load makes the circuit lossless.
A properly matched DCR circuit has an effective impedance with respect to the ADC that equates to the resistance within the inductor. Figure 1 is a simple schematic of a DCR circuit. Before deriving the transfer function between the inductor current and voltage at the input of the ADC, let’s review the definition of an inductor and capacitor in the Laplacian domain.
Xc is the impedance of a capacitor related to the frequency and XL is the impedance of an inductor related to frequency. ω equals to 2πf. f is the switching frequency dictated by the regulator. Using Ohms law, the voltage across the DCR circuit in terms of the current flowing through the inductor is define by Equation 2.
In Equation 2, Rdcr is the parasitic resistance of the inductor. The voltage drop across the inductor (Lo) and resistor (Rdcr) is the same as the voltage drop across the resistor (Rsen) and capacitor (Csen). Equation 3 defines the voltage across the capacitor (Vcsen) in terms of the inductor current (IL).
The relationship between the inductor load current (IL) and the voltage across capacitor simplifies if the component selection in Equation 4 is true.
Comments are closed.
