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Thermal and electromagnetic sensing can be used for monitoring and measuring electrical current but the simplest methods exploit Ohm's Law. When electrical current passes through a resistor, it develops a voltage that is directly proportional to the current. That simple current-sensing technique serves practical applications such as current monitoring, hot-swap controllers, fuel gaging and fault protection. It also raises some practical design issues, which will be discussed in this article.
High-Side Current-Sense Amplifiers
Resistor-based current sensing is simple, easy to use, low cost, extremely linear and requires no calibration. The statement that voltage across a resistor is directly proportional to the current through it is commonly known as Ohm's Law: V = I × R. As a caveat, note that all resistors dissipate power when current passes through them. Because the dissipation produces heat that, in turn, affects the resistance, power dissipation in a sensing resistor must be carefully assessed.
A larger sense-resistor value yields better accuracy, but dissipates more power: P = I2R, where I is the measured current and R the sensing resistance. The magnitude of the measured current is application-specific rather than a design parameter, so the sensing-resistor value should be as low as possible to minimize “joule heating.”
Choosing a small-valued sensing resistor yields a low-level sensing voltage across it, requiring an amplifier to boost that voltage to a level suitable for interface to a comparator, analog-to-digital converter or other external circuit. Low sensing voltages are vulnerable to the measurement error induced by the inherent bias current and input-offset voltage of amplifiers. For example, a practical full-scale sensing voltage might range from 50 mV to 200 mV. If the amplifier's maximum input-offset voltage is ±5 mV, the measurement error is ±10% at 50 mV (full scale) and even worse at lower currents.
The current-sensing amplifier must have low input-offset voltage and low input-bias current. A dedicated high-side current-sense amplifier (Fig. 1) places a current-sensing resistor between the voltage source (a battery, for instance) and the load. By avoiding extraneous resistance in the ground plane, that arrangement greatly simplifies the pc board layout and generally improves the overall circuit performance. Current passing through the sense resistor (RSENSE) develops a voltage drop, which is sensed by the op amp and drives the MOSFET transistor to sink current through R. The voltage drop across R equals the voltage across the sensing resistor:
KISENSERSENSE = IOR or
The sensor output current is proportional to the load current. Typically, a current mirror is included to increase the output current by a factor of K. If you require a voltage output, convert the current to voltage by placing an output resistor (RO) between the current output and ground. Resistors R and RO can easily be factory trimmed to achieve a current-sensing accuracy of 1% or better.
Current Monitor and Protection
High-rel power-supply circuits often incorporate short-circuit or overload protection (Fig. 2a). The IC shown (a MAX4373 current sensor) integrates a reference, comparator and latch. R1 and R2 set the trip current. The comparator compares the current-sensor output voltage with the reference voltage. When load current reaches the allowed maximum, the comparator output turns off the p-channel MOSFET switch by latching to logic high. No current flows to the load. The p-MOSFET remains off until a reset is applied or the power supply is toggled.
Battery chargers and other applications must guard against overcurrent due to short circuits and undercurrent due to open circuits. For this purpose, the current-window detector of Fig. 2b is similar to the Fig. 2a circuit but includes a second comparator for monitoring undercurrent. The two comparator outputs are open-drain, and therefore can be “wire-OR'd” together or can remain separate outputs. When the monitored current falls outside of the window, the IC alerts the system by asserting a fault condition.
The hot-swap controller is a specialized and more sophisticated current sensor, intended for use with system cards such as the I/O cards on a server. It allows you to insert or remove a card while the system is running, without interrupting the rest of the system. Without a hot-swap controller, inserting or removing a card can short circuit the system power supply and disrupt system operation. Also (without a hot-swap controller), the on-board capacitors are charged quickly when a card is inserted. The resulting in-rush current may temporarily pull the system supply voltage below the critical level.
The hot-swap controller (Fig. 3) is designed to overcome these problems. It incorporates a soft-start capability to reduce the in-rush current to a safe level, and when a fault occurs (overload or short circuit), the controller isolates the card from the rest of the system.
As an example, hot-swap controllers of the MAX5933A family allow you to safely insert and remove circuit cards from a live backplane without causing glitches on the backplane power-supply rail. During startup, the controller acts as a current regulator, using an external sense resistor and MOSFET to limit the amount of current drawn by the load. Internal circuitry slowly increases the monitored current, thereby avoiding a large in-rush.
The sense resistor also sets the current limit. If the FB input senses a short circuit, the IC reduces (folds back) the current limit by a factor of 3.9. Using a 25-mΩ sense resistor, for example, sets the normal operating current limit at 1.88 A, but a short circuit suddenly reduces that limit to 480 mA. Hot-swap controllers often include a timer that shuts off the MOSFET and protects the power bus when the current limit is not reduced within a given time period. Other hot-swap features include protections against undervoltage, overvoltage and excessive temperature.
Fuel Gages and Battery Management
The current-sensing amplifier shown in Fig. 1 is a relatively simple and general-purpose device. Specific applications such as fuel gaging and battery management, on the other hand, require additional functions and features to be integrated on-chip (Fig. 4). Fuel gaging is important for battery applications, in which the battery capacity is monitored with precision to optimize the system performance and prolong battery life.
For example, the battery pack in a laptop computer often integrates a smart fuel gage for monitoring and supervising the charging and discharging. Such gaging devices usually have a digital coulomb counter that tracks the accumulated charge and discharge actions. Thus, a given battery is fully charged when it accepts a certain amount of charge (in coulombs, C). Similarly, a battery is empty (discharged) when a given amount of charge is taken from it. Remember that 1 A of current equals 1 C/s. Thus, the time integral of current equals the total charge. A current-sensing amplifier measures battery current, and the coulomb counter acts as a time integrator that accounts for the total charge flow during a charge or discharge cycle.
The current sensor for fuel-gaging applications requires a bidirectional current-measurement capability. When charging a given battery pack, the maximum charge is set by the user. When the coulomb counter reaches the set value, it alerts the microcontroller to stop charging because the battery is fully charged. Similarly, during discharge due to normal battery use, the gage functions as a fuel gage that informs the user how much battery capacity remains. When the coulomb counter reaches a set minimum limit, it prevents over-discharge by alerting the microcontroller that the battery is empty. Thus, the coulomb counter prolongs battery life by preventing excessive charging or discharging.
The current sensor also provides overload and short-circuit protection by continuously monitoring current flow. By shutting off the MOSFETs in response to a short circuit, the current-sense amplifier disconnects the battery to protect it against short-circuit faults.
Dynamic Power-Supply Controller
For the power amplifiers used in handset applications, an accurate control of the power amplifier supply current maximizes battery usage and talk time. When the handset is near a base station, and therefore does not require high transmitting power, you can reduce the power amplifier supply current and still maintain a good transmitting signal. On the other hand, when the handset is far away from the base station or too much interference is present, the transmitter requires high output power and high supply current. Thus, dynamic adjustment of the power amplifier supply current minimizes the power consumption while maximizing talk time.
Though similar to the current sensor in Fig. 1, the power amplifier current controller (Fig. 5) incorporates an error amplifier and operates in a closed loop. Its operation is similar to that of a current source. The error amplifier (A3) compares and integrates any voltage difference between the IR drops across RSENSE and RG1, and feeds that output to the gain control (GC) input of the power amplifier. A3 increases the gain and the power amplifier output power, and consequently the supply current, until the two IR drops are equal. The voltage-to-current converter comprising A2, Q1 and RG3 controls the voltage drop across RG1, and users control the power amplifier supply current externally, via the PC input:
Advanced Power Sensing
For the battery in a notebook computer, whose terminal voltage varies as the battery discharges, power monitoring is safer than current monitoring and preferable to it. Power delivered to a load is defined as the load voltage multiplied by the load current. A power-monitoring integrated circuit must therefore include a current-sensing circuit with voltage output and an analog multiplier. The high-side current sensor provides an output voltage proportional to the load current, and that voltage is multiplied by a fraction of the load voltage to obtain an output voltage proportional to load power.
The MAX4210 power-monitor IC, for example, is designed to monitor the battery in a notebook computer. Its common-mode voltage range (4 V to 28 V) accommodates a variety of battery voltages. To measure current, you insert a sense resistor in the path between power source (battery) and load. The current-sense amplifier then feeds a voltage proportional to load current to one input of the analog multiplier. The other multiplier input connects to a voltage divider connected to the load. (Load voltage must be reduced by a divider, because the multiplier's maximum input voltage is only 1.1 V.) Multiplying these two voltages produces an output voltage proportional to load power. Like the current sensor, this analog multiplier is factory trimmed to achieve good accuracy.
A solid-state power-sensing circuit breaker (Fig. 6), useful for protecting batteries from short circuits and overpower faults, blocks current to the load when it detects an overpower fault. When it detects a fault, the p-channel MOSFET (M1) turns off and stays off until you press the manual reset button or apply a logic high to the CIN2 input. You also can reset the circuit breaker by cycling the input power, which causes the LE pin to go low and unlatch the comparator output COUT1. The RC network connected to the comparator (R3-R4-C1) prevents false transitions during the power-up voltage transient.
Thus, current sensors based on Ohm's Law are simple yet effective circuits for a variety of applications: power-supply protection, battery fuel gaging and dynamic power-supply control. Modern ICs integrate most of the components needed to implement a low-cost, high-performance, all-silicon current-sensing system. Factory trimmed to better than 1% accuracy, such ICs improve system performance, reliability and safety.