Motor drive technology that is most easily misunderstood: drive current rating

If you've ever looked for a Texas Instruments device, you've probably seen the tool shown in Figure 1. This product selection tool is powerful enough to help you quickly review hundreds of devices.

Figure 1: Product Selection Tool

However, as the saying goes, useless input will bring useless output. So understanding the different specifications of the device sought is the key to finding the right device. In our issue of the Motor Drive Forum question and answer, I will mainly talk about the most easily misunderstood motor drive specifications: drive current rating. I will also check the two drive current parameters, peak output current and root mean square (RMS) output current.

The peak output current is the maximum current that the motor driver can deliver without regard to the thermal limits of the device and its package. This limit is usually set by the overcurrent protection (OCP) of the motor drive (Figure 2).

Figure 2: OCP limit (peak output current)

Example of protection limits in the DRV8840 datasheet

When the current drawn is greater than the OCP limit, the driver stage will be turned off. An exception to the peak output current limit is that the motor driver will allow a higher current for a short period of time during the OCP anti-spike timer. The anti-spike timer ensures that the noise of the switch or output capacitor does not trigger the OCP and causes the shutdown.

The waveform in Figure 3 shows a typical OCP event of the motor driver output during a short circuit. The yellow trace is the input signal, the blue trace is the fault output signal, and the pink trace is the current flowing through the output stage. At the beginning, the current rises rapidly. After a brief overshoot (this overshoot does not affect the output stage), the current is limited to approximately 9A by an analog current limit. After approximately 2.5 μs, the OCP anti-spike timer expires and the current temporarily stays at the 9 A analog current limit, which exceeds the OCP limit. On this device, the OCP level is approximately 3A. At this point, the output stage is disabled, the current drops to a value of zero, and a fault occurs.

Figure 3: OCP event

The RMS output current is the maximum RMS (or DC) current that the motor driver can support under fixed conditions. Typically, these conditions include operating at a nominal voltage, an ambient temperature of 25 ° C, and using a standard JEDEC-compliant printed circuit board (PCB). This limit is usually set by the overtemperature protection of the motor drive. Once the device exceeds the overtemperature limit, the driver stage will shut down. This ensures that the device does not operate outside of its safe working area.

Figure 4: Over temperature limits for the DRV8840 data sheet

Other resources:

Learn more about hot topics on the TI.com/thermal website.

Read this "Understanding Motor Driver Current Ratings" application report to learn more about the output current rating and the protection mode used by TI motor drivers.

Search for questions and answers in the TI E2ETM motor-driven community or ask the motor application team.