DRV8889-Q1: Unexpected output inrush when nsleep pulled up

Hello Zhiyuan,

Thank you for the post. You're right the DIS_OUT default value for DRV8889-Q1 is 0 (that's what you meant) which will enable the H-bridge as soon as nSLEEP is 1 and DRVOFF = 0. If externally DRVOFF was made 1 using MCU GPIO then H-bridge won't be enabled as soon as nSLEEP = 1.

That said there is a 'A' variant of this device which is DRV8889A-Q1 which had default DIS_OUT = 1. There are few other differences (added benefits) in the 'A' version that is mentioned in the datasheet.

I hope this helps.

Regards, Murugavel

Hello expert，

    Thank you for your reply, do you mean base on DRV8889-Q1 and the fix pull down resistor on the DRVOFF without MCU control, this issue can not be removed?

    If the answer is yes, how to evaluate the risk? Does it cause the device damage, or the unexpected movement of the stepper motor?

    Is DRV8889A-Q1 and DRV8889-Q1 PIN-PIN and without any external schematic change?

Hello Zhiyuan,

Yes if the DRVOFF is permanently enabled by a fixed connection, the bridge will be enabled and both windings will be energized with winding currents as per the home position described in page-17 of the datasheet. See snippet below.

This is typical for stepper drivers whenever the driver is enabled. The stepper will be energized and generate hold torque with this current and keep the load from moving due to gravity or other types of forces in the mechanical system. The current through the windings would be 71 % of the IFS set by VREF input. The default value for TRQ_DAC is 100%. Depending on the supply voltage used and the VREF voltage input this may result with over temperature or over current protection kicking in and protecting the device. We do not expect the device to get damaged if left indefinitely in this condition. However, this could over heat the stepper motor and permanently damage it by overheating similar to high hold current continuously on, scenario. To avoid this, the customer can immediately write 1111b to the TRQ_DAC register to set the current to 6.25% of IFS. This will reduce the hold current to a low value and avoid over heating of the device and the motor even if left in this condition for a long duration. 

Regards, Murugavel 

Thank you.

    We write the DIS_OUT=1 immediately instead of write 1111b to TRQ_DAC register as you mentioned. I think the effect is similar, am I right?

    Any why A=B=71% IFS can provide the hold torque? 

Hi Zhiyuan,

Yes, writing DIS_OUT =1 will actually disable the H-bridge which means no holding torque is generated by the stepper except its built-in detent torque. This may be good for your application. Some customers need a holding torque in their application. Typically they use a value set by the TRQ_DAC. Usually a stepper generates higher holding torque with a lower current than its running torque with higher current.

Stepper motor generates holding torque whenever at least one of the phases A or B has a current flow through it. The highest holding torque is generated while both A and B are at their highest value which happen to be 71% default or 45 degrees electrical angle, IFS * Sin(45) and IFS * Cos (45), assuming IFS = 100% of the full-scale current. In microsteps mode one of the phases can achieve 100% current while the other phase is at 0 at specific angle positions. The indexer table shows it in the datasheet. Again this is based on Sin and Cosine calculations.  

I hope this helps. If all your questions were answered could you please close this thread at your end? You are welcome to create a new thread for future questions. Thank you.

Regards, Murugavel