Applications requiring control of motors usually include some type of current sensing circuit. The ability to sense current through a motor can help designers make adjustments such as speed based on the current state of the motor.

For example, in UAV applications, each motor controlling the propeller usually uses a low side current sensing circuit to control the UAV to travel, stay or rise in the air. In electric tools such as drilling rigs and reciprocating saws, the low side current induction controls the speed of the tools according to the force of the user to press the trigger. These products usually require cost sensitive design because they face the consumer market. In this blog post, I’ll show you how to design low side current sensing circuits for cost sensitive applications.

In the design of low side current sensing circuits, one of the most cost-effective methods is to use non inverting configuration operational amplifier (OP AMP).

1. Calculate the maximum shunt resistance. When the current from the load (Iload) flows through the shunt resistor (rshunt), a voltage potential (vshunt) is formed on the shunt resistor. Vshunt is considered as the “ground terminal” of the system load. Therefore, it is recommended to keep the maximum load current of vshun below 100mV to avoid problems when connecting with other systems with real 0V grounding.

2. Calculate the gain of the amplifier. The operational amplifier amplifies vshunt to produce Vout_ Min to Vout_ Max output voltage swing, where Vout_ Min and Vout_ Max is the minimum and maximum output swing limits of the amplifier respectively. In this formula, the gain of the amplifier is calculated by calculating the gain of the amplifier generated by the feedback formula 3.

3. Select operational amplifier. In low side current sensing applications, if the current is bidirectional, the common mode voltage can be at or below ground potential; therefore, the input common mode voltage range of the amplifier must be at or below ground potential. The tlv9062 is a high performance general purpose amplifier designed for cost sensitive applications with an input common mode voltage range as low as ground potential.

Tlv906x series high performance general purpose amplifiers can be used in cost sensitive low side current sensing systems because of their gain bandwidth (10MHz), voltage swing rate (6.5V / μ s), offset voltage (0.3mV) and input common mode voltage range of 100mV below negative supply voltage. Table 1 lists some technical parameters of tlv906x series.

Applications such as UAVs and power tools require cost sensitive low side current sensing solutions to control motors. In this article, I simplify the circuit design to three simple steps: determine the maximum shunt resistance, calculate the amplifier gain that produces the maximum output swing, and select the amplifier. In the next article, I will discuss how to design a printed circuit board (PCB) for low side current sensing circuits.