Applications that require the control of a motor typically involve some type of current-sensing circuitry. Being able to sense the current through the motor allows adjustments, such as speed, to the motor’s current state if needed.
For example, in drones, each of the motors that control the propellers typically use a low-side current-sensing circuit to steer, stabilize and lift the drone through the air. In power tools like drills and reciprocating saws, low-side current sensing controls the speed of the tool based on how hard users pull the trigger. These products typically require a cost-sensitive design because they are sold in the consumer market space. In this blog post, I’ll discuss how to design a low-side current-sensing circuit for cost-sensitive applications.
One cost-effective option when designing a low-side current-sensing circuit is to use an operational amplifier (op amp) in a noninverting configuration. Figure 1 is a schematic of a typical low-side current-sensing circuit using an op amp.
Figure 1: Low-side current-sensing schematic
Equation 1 gives the transfer function of the circuit shown in Figure 1 as:
The design process for the low-side current-sensing circuit shown in Figure 1 breaks down into three simple steps:
Equation 4 calculates the size of the resistors, RF and RG, in the feedback network of the amplifier in order to set the gain calculated in Equation 3:
The TLV906x high-performance general-purpose amplifier family is designed for cost-sensitive low-side current-sensing applications due to its gain bandwidth (10MHz), slew rate (6.5V/µs), offset voltage (0.3mV) and input common-mode voltage range, which is specified at 100mV below the negative supply voltage. Table 1 highlights a few of the TLV906x family’s typical specifications.
The design in Figure 2 shows the final component values for a 0A to 0.5A low-side current-sensing circuit, with component values calculated by following steps 1 through 3.
Figure 2: 0A to 0.5A low-side current-sensing schematic
Popular applications such as drones and power tools require cost-sensitive low-side current-sensing solutions for motor control. In this post, I simplified circuit design into three simple steps: determine the maximum shunt resistor, calculate the gain of the amplifier that produces the largest output swing and choose your amplifier. In my next post, I discuss how to properly lay out a printed circuit board (PCB) for a low-side current-sensing circuit.
Thank You for the post.
One thought walks in my mind. What about short circuit in the load? So, until breaker, fuse or something else do not break extra current, voltage on shunt may be much greater than Vcc of the op amp. Are there ideas how to prevent op amp from smocking?
May be should to paste Schottky diode across the shunt or series resistor into “In+” pin of the op amp? Are there tipical solutions from your experience?
When the input voltage goes above the supply voltage the internal ESD diodes of the device will turn on. When this happens the current into the device must be limited to less than 10mA to avoid damaging the device. I recommend watching our TI Precision Lab videos on Electrical Overstress (EOS) for recommendations on how to protect the amplifier from being damaged.
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