DRV8889-Q1: Motor Damage without config

Lyto

Part Number: DRV8889-Q1

Tool/software:

Dear Support Team,

Under which condition is it possible that the driver, without any configuration (thus should be running with the default config), passes the VM supply voltage directly to the motors?

Every time I do not send a config to the motors and only enable the driver, I see a current draw of more than 1A from my supply, although the motor is rated at only 180mA. The motor heats up very quickly, as if the driver were practically connecting VM directly to the motor.

My Vref is 1.1V, so the maximum current should actually be:

$1.1*100%/2.2 A= 0.5A$

When I send my normal configuration with 50% TRQ_DAC, everything works fine again.

Vm is 28V. Motor phase resistance = 16Ohm.

Thank you very much for your support.

Best regards,

Lyto

1 month ago

0 Murugavel4637 1 month ago

TI__Mastermind 44615 points

Hi Lyto,

Thanks for your question.

Lyto said:
Under which condition is it possible that the driver, without any configuration (thus should be running with the default config), passes the VM supply voltage directly to the motors?

When the DRV8889-Q1 is powered up, awake from sleep by nSLEEP = 1 and the outputs are enabled with DRVOFF = 0, the outputs will drive the coils with 71 % of the set current by VREF input voltage because the TRQ_DAC register would be 100 % at power on or at every wake up from sleep event, with no STEP pulses input or STEP = 0. This would be a regulated current.

This is when the coils would be powered by VM voltage, however this would be a switched voltage. See below waveform capture. VA is the differential voltage across coil A, VB for coil B and IA, IB are coil A and B currents. If the motor is in this condition aka HOLD current mode it is possible for the motor to overheat and potentially damage the coils. However if braking force is required this can be avoided. Designers mitigate the coil heating by setting a lower HOLD current by configuring the TRQ_DAC register to a lower value. If no HOLD current is needed then DRVOFF can be 1, which will disable the outputs to 0 current.

Lyto said:
My Vref is 1.1V, so the maximum current should actually be:

$1.1*100%/2.2 A= 0.5A$

Correct. With this setting the coil HOLD mode currents in both coils would be 0.5 x 0.71 = 355 mA.

Lyto said:
When I send my normal configuration with 50% TRQ_DAC, everything works fine again.

With TRQ_DAC at 50 % the HOLD mode current would decrease to 178 mA.

Lyto said:
Vm is 28V. Motor phase resistance = 16Ohm.

The coil currents would be regulated to the set value regardless of the VM voltage and stepper DCR ohms. Even though VM/R = 1.75 A, because of chopping constant current regulation the coil current will remain at the set value. The voltage across the could would be switched but while conducting it would be at VM level. The driver will never pass the un-chopped (unswitched) VM supply voltage directly to the coils unless the driver itself was damaged, for example its HS-FET shorted and failed. Thank you.

Regards, Murugavel

0 Lyto 1 month ago in reply to Murugavel4637

Prodigy 120 points

Hello Murugavel,

thank you for your answer. This was also my understanding. Now i measured the chopper current without config and i get the following.

The mean current ist 478mA. The ripple is noticeable large. But I should get a mean of 71%*(1.1V/2.2V)*100%= 355mA right?

Thanks

Best regards,

Lyto

+1 Murugavel4637 30 days ago in reply to Lyto

TI__Mastermind 44615 points

Hi Lyto,

Lyto said:
The mean current ist 478mA. The ripple is noticeable large. But I should get a mean of 71%*(1.1V/2.2V)*100%= 355mA right?

Sorry that table I referenced was for microstep mode. This would not match the default step mode for this device.

At power up or wake up default the step mode would be in Full-step 100 %. For this step mode the 45 ° HOME position current for both coils would be 100 % of the set value. See below. The peak of the ripple would be close to 500 mA, which seems to be what the capture you shared is showing. Thank you.