# Calculate Current Regulation - DRV8825

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

About current regulation the datasheet says:

9.2.2.2 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.

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?

• Study up on how chopper-drives work, then measure the current through either winding, not the power-supply. Single-step to find the maximum. – Tut Feb 29 '16 at 20:53
• 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? – Torsten Römer Mar 4 '16 at 16:06