I am driving a bipolar hybrid stepper motor rated 1A with a DRV8825 at 12 V.

About current regulation the datasheet says: Current Regulation

In a stepper motor, the set full-scale current (IFS) is the maximum current driven through either winding. This quantity depends on the xVREF analog voltage and the sense resistor value (RSENSE). During stepping, IFS defines the current chopping threshold (ITRIP) for the maximum current step. The gain of DRV8825 is set for 5 V/V.

Current Regulation

To achieve IFS = 1.25 A with RSENSE of 0.2 Ω, xVREF should be 1.25 V.

In my circuit, RSENSE is 0.1 Ω. Following, in order to get IFS of 1.0 A, xVREF should be 0.5 V.

However, when I set the reference voltage to 0.5 V, I can measure only 0.5 A in both windings in full-step mode. Through experimentation I found that I get 1.0 A with a reference voltage of 1 V - which should give 2 A following the equation above.

On page 4, the datasheet says the VREF input voltage should be 1 - 3.5 V with the following note:

(2) Operational at VREF between 0 to 1 V, but accuracy is degraded.

Can it really be so inaccurate just slightly below 1 V, or am I getting something wrong here?

  • 3
    \$\begingroup\$ Study up on how chopper-drives work, then measure the current through either winding, not the power-supply. Single-step to find the maximum. \$\endgroup\$ – Tut Feb 29 '16 at 20:53
  • \$\begingroup\$ Following your advice, I can measure 0.5 A in both windings at 0.5 V ref voltage in full-stepping mode. I suppose "either winding" here means "one of both windings", not "both" as what I thought. So, at least in full-stepping mode, the current the motor is rated for relates to the sum of current in each winding? \$\endgroup\$ – Torsten Römer Mar 4 '16 at 16:06

The answer is yes, accuracy can be degraded. The test engineers at TI left you that note to let you know that if you operate the VREF below 1V then the circuit will not preform like the equation that they provided. This is probably because they didn't put a super accurate opamp\comparator in silicon to measure the current (in figure 6) and it has some kind of common mode range or offset.

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  • \$\begingroup\$ Now measuring correctly, there either is an error with factor 2 or I just don't get it. In order to have 1 A in each winding single stepping in full-step mode, I have to set the ref voltage to 1 V, not the expected 0.5 V. \$\endgroup\$ – Torsten Römer Mar 4 '16 at 16:35

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