DRV8955: DRV8955 'fault' indicated when powered on.

Hi Tom,

Tom Tsao said:

Unfortunately, the DRV8955 indicated a 'fault' when powered on. The impedance of both VREF pins to ground was only about 300 ohms, although the outputs were not short-circuited.

Could you please help check again if there is any issue in the circuit?

The fault indication suggests the device may have experienced thermal shutdown. With current chopping DCR 0.3 Ω shouldn't be an issue as long as there is sufficient inductance in the stepper coil. We do not have a 0.3 Ω DCR unipolar to try with an EVM but we have a 0.4 Ω DCR, 3.5 mH bipolar stepper. I connected one coil between an output and VM (high side load) similar to your schematic. See below waveform captures. VM = 24 V, MODE = GND and TOFF = 330 kΩ to GND to replicate the settings in your schematic. Keep in mind while the unipolar common is connected to VM +24 V the IN1 to IN4 must be HIGH for output off and LOW for output on. This means the default inputs from the MCU must be 1 on IN1 to IN4.

VREF1,2 = 1.9 V, INx = 0, blue is coil current, yellow is OUTx voltage showing tON and tOFF:

VREF1,2 = 3.3 V, INx = 0, blue is coil current, yellow is OUTx voltage showing tON and tOFF:

These captures prove current regulation is working with 0.4 Ω DCR stepper coil at the output.

Also note only slow decay is supported and there is a small tON blanking time before tOFF kicks in for every tON and tOFF current regulation cycle. This means there will always be a minimum tON and minimum ouput current even when VREF1,2 = 0V up to the minimum tON time which may be a couple of μs for this device. 

What is the desired current setting for the coil maximum current? Based on this VM may have to be reduced from 24 V to 12 V because you are using a very low DCR stepper motor. What is the target current required for the stepper and what is coil inductance? Based on this information we can tell whether the DRV8955 would be suitable to drive this motor or not. You can also follow the section 8.2.2.2 Power Dissipation and Thermal Calculation formulae to see if the desired out currents in your application can be supported without a thermal shutdown. Thank you.

Regards, Murugavel 

Hi Tom,

The DRV8955 cannot support 3 A per phase. Besides 0.94 mH is very low inductance, and with 0.3 Ω DCR you'd need very low Rdson output FETs much lower than 300 mΩ. Will you drive the stepper in full-step or half-step mode? Are you considering microstepping (this would require MCU to support variable VREF voltage inputs and synchronized INx control) ? 

Please review the DRV8952, DRV8962 datasheets. This device features 4 half-bridges with each half-bridge capable of driving up to 5 A full-scale current. However, we still need to consider the total power dissipation - please refer to power dissipation calculations in the Application and Implementation section of the product datasheet. The DRV8962 is also available in DDV package that requires external heatsink and can support up to 10 A per half-bridge. EVMs are available for evaluation. Thank you.

Regards, Murugavel