Smart battery current sensing
An increasingly relevant example of a low-side current sensing implementation is charge current monitoring for smart battery systems. In addition to the two battery terminals, these battery systems typically have two diagnostic signals: a single-wire data line for battery health, and a single-wire thermistor output for battery temperature monitoring. These diagnostics are referenced to the negative terminal of the battery.
When using a sense resistor in this application, a design engineer must take various error terms into account.
First, the resistor will dissipate energy from the battery as heat, making the system inefficient. Second, the voltage, VSENSE, developed across the sense resistor will be superpositioned on the thermistor voltage, VTHERM, such that the voltage seen by the charge controller is VSENSE + VTHERM, resulting in an error in the monitored battery temperature (figure 3). This could affect the charging control of the battery system and eventually hamper battery life. The sense resistor approach yields reduced accuracy when using lower sense resistor values, as discussed earlier in this article.
Table 1: Competitive comparison for 30A current sense applications.
In the case of the integrated Hall-effect sensor, the conductor resistance is lower (as low as 0.6 mΩ), which greatly reduces power dissipation. There is practically no voltage developed across the conductor loop terminals, which helps maintain the integrity of the thermistor’s diagnostic signal. Lastly, since the sensing is based on magnetic coupling, there is no dependence between conductor resistance and accuracy, simplifying the design.
Current sensing for motor control in white goods
Energy efficiency, safety, reliability, and low-cost are required in motor control circuits for appliances such as refrigerators, washing machines, and air conditioners (referred to as “white goods”). Integrated Hall-effect current sensors can provide compact current-sensing solutions with inherent high voltage isolation. Power consumption from current sensing can also be reduced significantly with integrated Hall ICs, since they have very low primary conductor resistance. This helps meet energy-efficiency requirements such as Energy Star.
These sensor ICs also provide a means of rapid detection of fault conditions that can prevent expensive system damage, improving reliability.
Alihusain Sirohiwala is a systems engineer at Allegro MicroSystems Inc.