DRV8889-Q1: question about DRV8889 current regulation

Hi Zhiyuan,

Thanks for reaching out. Below are my answers, listed in the same order as the questions.

1. Yes you are correct. One thing to keep in mind though when VM decreases below Ifs x R, R being the winding DC resistance of the motor phases the current may not reach its intended peak value. Once the VM is sufficiently > Ifs x R, it will remain regulated at the target Ifs setting.

2. I assume you're referring to Table 7-3 in the datasheet. It refers to the percentage of peak current set by Ifs which is set by the reference voltage input on the VREF pin and the TRQ_DAC setting in the CTRL1 register. The ON/OFF ripple due to regulation will be very small compared to the current setting. When zoomed in the current waveform ripple would look like Figure 7-13 in the datasheet showing Smart Tune Ripple Control STRC decay mode (default at power on). 

ITRIP is defined by VREF input voltage and TRQ_DAC setting. The relationship formula between IFS (full-scale current) and VREF is provided in section 8.2.2.2 Current Regulation, page-57 of the datasheet SLVSEE9D. IVALLEY is internally set with a fixed value. In the DRV8889-Q1 (ITRIP – IVALLEY) = ΔI = (11mA + 1% of ITRIP). The goal is to offer the lowest ripple with current regulation as well as maximize the use of slow decay. The stall detection feature requires the device to operate with STRC decay mode, hence it is the default decay mode in this device.  

3. Yes when the DRVOFF is enabled and there is no STEP pulses input the DRV8889-Q1 indexer will be held in the specific step where the pulses were stopped. The Ifs for that particular step or microstep will be maintained (until the device go to sleep and wake up - then it will reset to 45 degrees electrical angle the initial excitation state) with current regulation loop which will include the TRQ_DAC setting and the VREF input.

4. This specification is provided by the motor vendor. Usually the motor datasheet also has a speed vs torque output plot for the specific motor current(s). Ifs can be determined by the output torque required from the motor in the application. If the Ifs setting is too low the torque output will be too low that may result in motor stall or losing steps. When Ifs is set too high the motor will heat up unnecessarily but will also produce more torque output. The excess heat can increase motor wear and tear and impact its life expectancy.

Regards, Murugavel

Hello expert,

      I have some concern about your answer and pls check,thank you!

      1.when VM decreases below Ifs x R,will the H bridge always on?From the datasheet the minimum VM to work is 4.5V,but if  VM decreases below Ifs x R,the stepper will not work normally am I right?

      2.I can't find Figure 7-13 in datasheet and can you provide?

      4.can you show a example about how to determine the Ifs according to the motor datasheet?

Hi Zhiyuan,

1. You are correct the minimum recommended operating VM for the DRV8889-Q1 is 4.5V. What I meant is different, for example let's assume VM = 6V and motor R (DCR) is 8Ω per Ohms law the current through the winding based on DCR will be 6/8 = 0.75A. In this case if you set Ifs to 1A that current will never be achieved as well as there won't be any current regulation - meaning no tOFF in chopping although bridge drive is active, limitation due to physics. Ifs must be set well below 0.75 to achieve proper current regulation and waveform. In some low torque situations and slower step rate the stepper may still work under these conditions. But features such as stall detection may not work. 

2. Figure 7-13 in page-29 of the datasheet, section 7.3.6.6 Smart tune Ripple Control.  

3. Here's an example stepper datasheet Speed - Torque characteristics from a vendor along with winding current. https://catalog.orientalmotor.com/item/2-phase-bipolar-stepper-motors/shop-pkp-series-2-phase-bipolar-stepper-motors/pkp296d45aa. Based on the application need identify the desired pull-out torque and stepper speed. The current at this intersection should be the typical Ifs setting used for the application with this specific motor. This motor will generate up to 1.5 N-m with approximately 3A Ifs at 300 r/min speed of the motor. I hope this helps.

Regards, Murugavel

Hello expert,

    For question3 do you mean,in this application we need1.5 N-m at 300r/min speed.So from this figure we can find the related current is 3A?And the '1.5 N-m' is just the maximum torque the motor can provide,which should be no less than the actual torque needed in the detail application.Right?

    And what happened if we only provide 2A or less?From this figure we can't found the related Speed-Torque characteristics.

   I have a datasheet on hand but there is no Speed-Torque characteristics provided.Can we design the 8889 just base on the information below?

Hi Zhiyuan,

Yes that's correct. "And what happened if we only provide 2A or less? From this figure we can't found the related Speed-Torque characteristics.". The vendor did not provide that data, but we can say the maximum torque output will reduce as you reduce the full-scale current.  

Motor vendor should provide speed-torque characteristics. It also possible for customer to measure and do their own speed-torque characterization with suitable mechanical setup to measure torque. That said there is reasonable information in this table to design stepper motor driving for this motor using the DRV8889-Q1. The Vmin and Vmax voltage values are missing though. What are the application goals, operating parameters of the stepper for the customer application? What is the end application? Does the customer want to use stall detection feature? Does the customer have a DRV8889-Q1EVM? 

Regards, Murugavel

Hi Zhiyuan,

Do we have any updates? Thank you.

Regards, Murugavel