Applications that require control of the motor typically include some type of current sensing circuit. The ability to sense current through the motor can help the designer make adjustments such as speed based on the motor current state.
For example, in drone applications, each motor that controls the propeller typically uses a low-side current sensing circuit to control the drone to travel, stay, or rise in the air. In power tools such as rigs and reciprocating saws, low-side current sensing controls the speed of the tool based on the force of the user pushing the trigger. These products often require cost-sensitive designs because they face the consumer market. In this blog post, I will show you how to design low-side current sensing circuits for cost-sensitive applications.
One of the most cost-effective ways to design low-side current sensing circuits is to use a non-inverting configuration op amp (op amp). Figure 1 is a schematic diagram of a typical low-side current sensing circuit using an operational amplifier. 
Figure 1: Low-side current sensing schematic
Equation 1 is used to calculate the current transfer function in Figure 1:
among them
. The low-side current sensing circuit design process shown in Figure 1 is divided into three simple steps:
1. Calculate the maximum shunt resistance. When a current from the load (ILOAD) flows through the shunt resistor (RSHUNT), a voltage potential (VSHUNT) is formed across the shunt resistor. VSHUNT is considered the "ground" of the system load. Therefore, it is recommended to keep the maximum load current of VSHUN below 100mV to avoid problems when connecting to other systems with true 0V ground. Equation 2 is used to calculate the RSHUNT value:
2. Calculate the gain of the amplifier. The op amp amplifies VSHUNT to produce an output voltage swing from VOUT_MIN to VOUT_MAX, where VOUT_MIN and VOUT_MAX are the minimum and maximum output swing limits of the amplifier, respectively. Equation 3 is used to calculate the gain of the output swing required by the amplifier:
Equation 4 is used to calculate the magnitude of the resistors RF and RG in the amplifier feedback network to set the gain calculated in Equation 3:
3. Select an op amp. In low-side current sensing applications, if the current is bidirectional, the common-mode voltage can be at or below ground; therefore, the amplifier's input common-mode voltage range must be at or below ground. The TLV9062 is a high performance general purpose amplifier designed for cost sensitive applications with an input common mode voltage range down to ground.
The TLV906x family of high performance general purpose amplifiers can be used in cost sensitive low side current sensing systems due to their gain bandwidth (10MHz), slew rate (6.5V/μs), offset voltage (0.3mV), and input common-mode voltage range. 100mV below the negative supply voltage. Table 1 lists the technical parameters of some TLV906x series.
Parameter Specifications Supply Voltage Range ((V+)-(V-)) 1.8V to 5.5V Quiescent Current 538μA Gain Bandwidth Product (GBP) 10MHz Input Voltage Noise 10nV/√Hz Slew Rate 6.5V/μs Offset Voltage 0.3mV Input Bias current 0.5pA input common mode voltage (V-)-100mV to (V+)+100mV
Figure 2 shows the final component values ​​of the 0A to 0.5A low-side current sensing circuit. For the calculation, please refer to steps 1 to 3 above. 
Applications such as drones and power tools require cost-sensitive low-side current sensing solutions to control the motor. In this article, I simplified the circuit design into three simple steps: determining the maximum shunt resistance, calculating the amplifier gain that produces the maximum output swing, and selecting the amplifier. In the next article, I will discuss how to design a printed circuit board (PCB) for a low-side current sensing circuit.
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