Current sensing, in general, is widely used in a variety of systems and applications; it is often beneficial to know the amount of current being delivered to a load. For example, in low-power consumer products (e.g., portable computing devices, cell phones, etc.), the supply current can be monitored for controlling system operation to optimize battery life. Knowing the load current can also be used to make safety-critical decisions in over-current protection circuits.
When sensing load current, a designer typically chooses to place a sense impedance (e.g., resistor or transistor device) either between the supply rail and load, or between the load and ground; the former is called high-side sensing whereas the latter is called low-side sensing. Ideally, the sensing circuitry added to a given circuit for detecting the load current has no impact on circuit performance. In practice however, there is likely some observable effect on circuit performance resulting from, among other factors, parasitic contributions of the sensing circuitry.
Traditional sensing circuits sometimes rely on replicating the load current (scaled or unscaled), either through the use of a current mirror and/or operational amplifier, to generate a sense output signal that is proportional to the sensed load current. However, because most current replication circuits are closed loop, speed must be limited in order to maintain stability, which is undesirable, particularly in high-speed switching applications.