A4988 Current Limiting Not Matching Datasheet Formula - Much higher...

[DigitalDesigner]

Hi,
I’ve implemented the attached schematic, which is fairly standard, using the very common A4988 stepper motor driver (datasheet attached).

I configured the ADC so that Vref = 50 mV, and I’ve verified that this is indeed the voltage present at the VREF pin. The sense resistor value is Rsense = 0.05 Ω.

If relevant, I am driving it in full step mode.

According to the datasheet formula, the maximum motor current should therefore be:

ItripMax = Vref / 8 / Rs = 0.05 / 8 / 0.05 = 125mA

However, I am measuring approximately 1.05 A RMS, which is obviously far higher than expected.

Due to mechanical constraints, I cannot directly access the motor leads to measure the coil current. The value above is inferred from a steady and repeatable increase in current drawn from the bench power supply. If the RMS current is already 1.05 A, the peak current must be even higher.

Has anyone encountered a similar issue or can help identify what might be causing this discrepancy?

Thank you

[sarahMCML]

I suspect that your ref input  of 50mV is not what it seems, since to get that voltage would mean programming the DAC, which gives 32mV per step, only the binary value of 3 or 4 (neglecting the 2 least significant zeros here!) at the most, allowing for the voltage divider.
I calculate the DAC would need to be loaded with about hex 34 to output 1.05 Amps, whereas it would be hex 0C (3) or 10 (4) for ~50mA.