AN3669| Application Note
Maxim > App Notes > AMPLIFIER AND COMPARATOR CIRCUITS Keywords: power-sensing, circuit breaker, current sense, power monitor, high side, current mirror, hall effect, current switch
Oct 10, 2005
APPLICATION NOTE 3669
Power-Sensing Circuit Breaker Breaks New Ground
Abstract: Most circuit breakers sense current. Fuses, for example, protect a circuit by blocking the current when it exceeds a limit. While current-sensing circuit breakers work well for constant DC or RMS supply voltages, power sensing is safer for a variable supply voltage such as the battery in a notebook computer. This article shows the method and benefits of using a power-sensing approach rather than a current-sensing approach to implement a circuit breaker.
Introduction
The main supply voltage in a notebook computer drops when the wall cube is unplugged. This happens because battery voltage is generally lower than that from a wall cube. Even the voltage of a Li+ battery changes from 4.1V when fully charged to less than 3V when nearly discharged. Thus, a power-sensing circuit breaker is preferable for systems like notebook computers, in which power is limited and the voltage source is not constant.
Theory of Operation for a Power-Sensing Circuit
Power delivered to a load is defined as the load voltage multiplied by the load current. A power-monitoring integrated circuit, therefore, must include a current-sensing circuit with voltage output and an analog multiplier. The high-side current sensor provides an output voltage proportional to load current, which is multiplied by the load voltage to obtain an output voltage proportional to load power (Figure 1).
Figure 1a. A basic power sensor includes a current sensor that measures current and produces a proportional voltage, which is multiplied by the load voltage to obtain a voltage output proportional to load power. The lowcost MAX4210 all-silicon power sensor.
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