No, it is not the limit but if you read the very first page of the spec sheet it clear says for the TQFN package (which is what I see on the sticks I bought from Digikey) can handle 1.4A RMS. Yet on the same page it says you can use the stick with motors whose coils draw up to 2 amps. The two are not mutually exclusive. But it also means you don't want to drive them at 2 amps and you shouldn't need to.

And a bit of conservatism is always appropriate, so I wouldn't even get close to the 2 amp figure (or even 1.4 if I could get away with it).

The fact that JoAnn asked the question because the drivers were 'smoking hot' as she said means the Vref needed to come down a lot.

On the question of power - motors generate their torque from current - the voltage (DC or chopped) is just a way of delivering the current. So calculations about power are not really the issue.

Yes, you are correct: The CFG3 jumper is not connected. That's the problem with looking at too many spec sheets under too many tabs in Adobe.

I was happy to hear from JoAnn that she turned the Vref down and the motors still seemed to operate nicely and the drivers were cooler. Good-that is what should have happened. Short of putting a representative load on the motor (either a scope or some kind of friction disk with a dynometer to measure rotational torque) there is not much to be done. I wouldn't worry about it, frankly, since these motors have a lot of torque anyway and telescopes are heavily geared down.

Cheers,

Mark
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Also, in addition to the 1.4Amp (RMS) figure quoted on the front page (for the TQFN package) see page 26, which gives recommended current ranges.
For external sense resistors they recommend no more than 1.4 amps. So while it may not be a hard limit the implications are pretty clear: Keep to 1.4Amps or less, or accept the risk.

Cheers,

Mark

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On Wed, Jan 17, 2018 at 10:28 pm, Mark Christensen wrote:

Also, in addition to the 1.4Amp (RMS) figure quoted on the front page (for the TQFN package) see page 26, which gives recommended current ranges.

I believe that you're correct and my take on this was wrong.  My misunderstanding was with which current is referred to.  The current into the driver is well below 1.4A RMS x 2 (per coil) at 12V.

On the question of power - motors generate their torque from current - the voltage (DC or chopped) is just a way of delivering the current. So calculations about power are not really the issue.

Sure voltage is just a way of delivering the current, but how fast do you want it delivered (power?)  A current of 2A at 1mV, for example, isn't going to operate a stepper motor with the same torque as 2A at 1V, and so on.  Somewhere both current and voltage enter into it so the driver must limit power and we know it does (that trim pot, sense resistors, current chopping.)  So the driver (current) spec. isn't for the 12V supply voltage into this stepper motor's 1.1 ohm coils which I assume it tries to do (but cuts off the current before it rises the 10.9A level?)  And it isn't for the overall limited current (per coil) at 12V which is 0.37A  It's perhaps based on the stepper motor coil resistance (which the driver can "know") and the current level we set on the driver (trim-pot) since those two can represent all that needs to be known about the stepper motor to drive it properly?

Mark, as far as I can understand the formulas in the wiki for TMC2100 are wrong?
http://reprap.org/wiki/TMC2100

I've checked the datasheet and your formula is correct. Why then reprap and all other websites are calculating with wrong formulas?

Antonio,

Probably because they just cut and pasted from data from other stepSticks. That kind of thing is why I always go back to the manufacturer data sheets.
Speaking in general terms, there is a lot of useful information out there and there is a lot that is wrong as well.

Mark
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I'm still not understanding why the RepRap formula is wrong. 

I admit to not understanding the TMC2100 datasheet content on this subject entirely, I don't see how the exact values in the table on Page 23 (Section 7) relate to the "RMS RUN CURRENT CALCULATION" in Section 8 on the following page but the values are close to what I'd expect and for the SSS's 0.11 Ohm sense resistors (all RMS):

Table (interpolation:)  1.78A
Formula (RRCC): 1.74A
RepRap Wiki: 1.77A

Now those values are just the basis.  The SSS TMC2100 is designed with current scaling so that trim pot changes VAIN from 0V to 2.5V (20k fixed and 20k pot as a voltage divider) which then scales the current from 0A to 1.77A (about.)  This additional calculation is covered by the formula in "CURRENT REDUCTION BY ANALOG SCALING" in Section 8.

The RepRap Wiki formulas look right to me:

Their example shows 1V = 0.71A... so lets see (1V/2.5V=0.4, 0.4*1.77A=0.708A)

[I just spent some more time looking over the RepRap Wiki and even though the formula they show works their discussion just above in that section appears to be in error.  They claim Imax is 1.77A and Irms is 1.25A which goes against what I understand and also the Watterott FAQ which puts Imax at 2.5A and Irms at 1.77A]

It seems the SSS TMC2100 uses the QFN package which is rated for 1.2A (RMS) as opposed to the TQFP which can handle 1.4A.  So I guess if you have excellent cooling (fan/big heat sink) the TMC2100 SSS maxes out at 1.2A RMS and can fully power a 1.7A rated stepper motor using a Vref of 2.4V 1.7V.  [Note: the Watterott GitHub shows the TMC2100, TMC2130, and TMC2208 all as 1.2A RMS "motor phase current".  The TMC2208 datasheet says it's 1.4A RMS (and a lower RDSon makes me think they might run a bit cooler.)]