High-side current sensing, how to choose the right device

In many applications such as motor control, solenoid control, and power management (such as DC-DC converters and battery monitoring), high-precision high-side current sensing is required. In such applications, monitoring the high-side current rather than the return current can improve diagnostic capabilities, such as determining short-circuit current to ground and continuously monitoring the return diode current, avoiding the use of sampling resistors, and maintaining ground integrity. Figures 1, 2, and 3 show typical high-side current sampling configurations for solenoid control and motor control, respectively.

Figure 1. High-side sensing in a typical solenoid valve control

Figure 2. High-side sensing in a typical H-bridge motor control.

Figure 3. High-side sensing in typical three-phase motor control.

In all of the above configurations, the pulse width modulation (PWM) common-mode voltage on the sampling resistor that monitors the load current swings from ground to the power supply. The period, frequency, and rise/fall time of this PWM input signal can be determined by the control signal from the power stage to the FET. Therefore, the differential measurement circuit that monitors the voltage on the sampling resistor requires extremely high common-mode voltage rejection and high voltage handling capabilities, as well as high gain, high accuracy, and low offset - in order to reflect the true load current value.

In solenoid control using a single control FET (Figure 1), the current always flows in the same direction, so a unidirectional current sensor is sufficient. In motor control configurations (Figures 2 and 3), the motor phases are split, which means the current in the sense resistor flows in both directions, so a bidirectional current sensor is required.

Many semiconductor vendors offer a variety of solutions for high-side current sensing, and design engineers studying this type of application have discovered that these solutions can be classified according to two distinct high-voltage structures: current sense amplifiers and difference amplifiers.