Advantages of Integrated Current Sensing

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Advantages of Integrated Current Sensing [Copy link]

 

By Ryan Kehr

Many brushed and stepper motor applications must monitor and regulate current. For brushed motors, current information can be used to determine changes in load conditions or to limit starting and stall currents. For stepper motors, high levels of micro-stepping require regulation of current for each step.

Figure 1 is a graph of current versus time showing the start-up curve for a brushed DC motor. In this example, the current is limited to approximately 2 amps before the motor reaches a steady-state condition of less than 1 amp. Without current regulation, the same motor could peak at over 14 amps. Not only does this require an over-designed power supply to support this transient, but the motor driver also needs to be rated to reliably handle the peak current.

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Figure 1. DC motor current vs. time

Traditionally, this current limiting can be achieved using external shunt resistors in the ground path. The voltage drop across these shunts is monitored and compared to a reference voltage that is either internally or externally provided, depending on the device. Since the full load current passes through these shunts, the resistors must be power resistors and are up to 1206 in size at a size that is almost as large as the integrated circuit itself, which measures 3.2 mm x 1.6 mm. Figure 2 compares the size of the shunts and the DRV8870 DC motor driver. Figure 3 shows the DRV8871 with integrated current sensing . In the DRV8871 , a single 0402 resistor is used to set the current limit threshold. As shown in Figure 3, integrated current sensing can save board and component costs.

Compared to power resistors, low-power signal path resistors require less board space and reduce bill of materials (BOM). In addition, no power is lost in the resistor and another source of heat is eliminated in a design that is likely already under thermal budget pressure.

Figure 2. DRV8870 with external shunt

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Figure 3. DRV8871 with integrated current sensing

Now that the benefits of current limiting and integrated current sensing are clearly understood, we can also add monitoring capabilities to the latest family of brushed DC motor drivers through the IPROPI pin. The DRV8873 is the latest addition to the brushed motor driver family and provides an analog current output that is proportional to the current flowing through the bridge. This proportional current passes through a small resistor, and an external microcontroller can be used to monitor the voltage proportional to the current to determine changes in load conditions and act accordingly. With this internal current monitoring function, there is no need for external power resistors and regulation circuits such as operational amplifiers.

Finally, since stepper motors traditionally require two external sense resistors to feed current back to the driver, we can improve stepper motors to better benefit from the PCB size and BOM advantages of integrated current sensing. Figure 4 compares the traditional driver PCB footprint and the DRV8886AT that benefits from internal current sensing.

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Figure 4. Board layout reduction

A less obvious benefit that can only be realized during the PCB layout process is that the layout can be simplified when choosing a device with integrated sensing capabilities. Therefore, there is no longer a need to route around large external shunt components, nor is there a need to break out the power, output, and GND routing that would require larger traces to carry the device current.

In summary, integrated current sensing in TI’s latest family of motor drivers removes expensive power resistors from the BOM, reduces board size, component count, and simplifies PCB routing.

Table 1. Alternative device list

Device Name	Device Description
DRV8871	3.6A Brushed DC Motor Driver with Integrated Current Sensing (PWM Control)
DRV8871-Q1	3.6A Automotive Brushed DC Motor Driver with Integrated Current Sensing (PWM Control)
DRV8873	10A H-Bridge Motor Driver with Integrated Current Sense and Current Sense Output
DRV8873-Q1	Automotive H-Bridge Motor Drivers
DRV8876	37V, 3.5A Brushed DC Motor Driver with Integrated Current Sense and Current Sense Output
DRV8876-Q1	Automotive H-Bridge Motor Driver with Integrated Current Sensing and Regulation
DRV8884	1.0A Stepper Motor Driver with 1/16 Microstepping and Integrated Current Sensing (STEP/DIR Control)
DRV8885	1.5A Stepper Motor Driver with 1/16 Microstepping and Integrated Current Sensing (STEP/DIR Control)
DRV888AT	2A Stepper Motor Driver with Integrated Current Sensing and Adaptive Decay Mode