Read REFv on A4883

[i]"The A4983 supports such active current limiting, and the trimmer potentiometer on the board can be used to set the current limit.
One way to set the current limit is to put the driver into full-step mode and to measure the coil current without clocking the STEP input.
The measured current will be 0.7 times the current limit (since both coils are always on and limited to 70% in full-step mode).
Please note that the current limit is dependent on the Vdd voltage.

Another way to set the current limit is to measure the voltage on the REF pin and to calculate the resulting current
limit (the current sense resistors are 0.05Ω). See the A4983 datasheet for more information."[/i]

If you read .51 volts on the REF pin. How do you convert that to amp limit? The actual reading to a motor lead was 350mA at around mid Pot setting. tia


From the datasheet:

ITripMAX = VREF / ( 8 x Rs)

Where ITripMAX is the maximum current limit of each of the driver’s full H-bridges (i.e. the limit per stepper coil). Rs is the coil’s current sense resistor, and both such resistors have a value of 0.05 Ohms on our carrier board.

A VREF of 0.5V could produce a maximum current of approximately 1.3 A per coil. Note that if you are full-stepping, you will only ever see approximately 71% of this current flowing through each coil. If you are microstepping, the percentage of this max current passing through the coil will depend on the step you are on as shown by the %ITripMAX column of Table 2 in the datasheet (and %ITripMAX can be as high as 100% for some steps).

- Ben

Thanks much Ben.

So if my motor is rated at 0.33A and I want to figure out what VRef to set, then I would:
vRef = MotorAmps * (8 * .05)
vRef = .33 * (8 * .05) = 0.132

So make sure I dial down the pot to not exceed 0.132 volts on the vRef pin? tia

That sounds right. If you do this and measure the current through one of the stepper coils while in full-step mode (and without clocking the STEP input), do you see approximately 70% of 330 mA, or 0.23 A?

- Ben

Yep. About exactly. Thanks Ben. Another question if I may.
I have read the datasheet. But I am still not understanding practical difference between Sleep and Enable. What senerio would you use one over the other? tia


Enable just turns off the output FETs but leaves everything else running. It uses more power than sleeping. Also, when you sleep the microstep translator loses its position, which means the next time it turns on, it will not return to the microstep position it was at.

I haven’t tried it myself but you might be able to disable it to save power when there is no load on the stepper but you need to return to the same microstep position you were at. Also, since the step pin is still enabled when you disable the output FETS, you could also use enable to cycle through the microstep translator table without sending power to the motor.

- Ryan