DRV8899-Q1: different current draw between two drv8899 at same motor

Hi Fabian,

Thanks for posting your question in this forum.

Fabian Zeulner said:

I am using DRV8899 without any SPI configuration, so its in its default configuration when powered.

Without any SPI configuration the registers will have assigned default values as per the datasheet. The device will operate in full-step mode with 100 % current and with TRQ_DAC at 100 %. The coil current IFS will be based on VREF voltage. With VREF = 1 V, the IFS would be 1/2.2 = ~ 455 mA. The IFS current will be regulated to +/- 455 mA only if (VM - VBEMF) > IFS x DCR of the motor, VBEMF is the BEMF voltage which will be 0 when the motor is stopped and a finite value depending on the BEMF constant of the motor. , DCR is the DC resistance of the stepper motor coils.

Fabian Zeulner said:

but with one DRV8899 i get around 300mA current draw.  with a different DRV8899 (but same schematic design, and also 1V VREF) i get around 180mA current draw.
I am using the exact same Stepper motor for that test.

Would this be while stepper motor is stopped or while the motor is stepped and spinning? In full-step mode both A and B current would be 100% (+/-) depending on the step position. The device to device variation could be within the current regulation accuracy specification - see highlighted data in the below table from the datasheet.

See below table. Based on this, the current draw on VM will be 2x the IFS at all times with the stepper motor connected to the driver.  

Please confirm the following:

1) (VM - VBEMF) > IFS x DCR of the motor. What is the VM voltage and the stepper motor specifications R and L values? 

2) When the motor is still (STEP pin = 0) you can measure 100 % IFS coil currents for coil A and coil B.

3) When the motor is spinning, the current waveform for coil A and coil B would represent a square wave with + / - 100 % IFS amplitude. Please share the current waveforms of coil A and coil B for analysis.

Thank you.

Regards, Murugavel 

sorry for the longer delay in response, and thats for the support.

spec of the motor:

- resistance / phase  6,4 ohm

- impedance 2.6mH at 1khz

we are running it with 12V

the current difference is after i enable the IC, without any motion happening,  so its in the initial Step index.

when i increase the voltage, the current drops. so it seems its getting power limited, but the current does not raise to the 455mA you mentioned.

Hi,

Due to US national holiday, please expect some delay for our response.

we actually found 1 problem, which is stupid ... but had a solder mask problem which generated connectivity issues.
this did solve the problem of only 1/2 of the current,  so i now get consistant current of around 300mA

I just dont understand why i dont see your original mentioned 450mA

Hi Fabian,

Glad you found the problem with connectivity.

After power on, the default microstep setting would be full-step 100 %. Assuming the customer did not change this setting and VREF was 1 V with TRQ_DAC = 100 %, IFS should be 455 mA. If full-step 71 % was set then it would be 323 mA. Please verify if this was changed or TRQ_DAC was changed. Thank you.

Regards, Murugavel   

hello,

I can verify that the ~300mA are measured at the 12V supply to the DRV8899 after power on of the chip.

Power on here means:  
- added 3.3V 
- no SPI interaction
- Vref to 1V

so it should boot in the default settings, which is 100%.

so right now its also not a real problem for us, as now all systems behave the same and run at the ~300mA
Its more that i would like to understand the difference to the calculation.

if there is something else i should check or measure, let me know.

Fabian