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DRV8825 supply current question

Matthew Reaves
Prodigy 40 points
Other Parts Discussed in Thread: DRV8825EVM, DRV8825

Hi,

I have gone through the power dissipation calculation for my 8825 application. 

Application specs:

Stepper Motor:  5.7 ohms per coil, 1A max per coil.

multistep set to 1/4

Very low frequency needed for step function (1/4 turn within 1 second, then stopped until needed again at random intervals).

Input Va and Vb power supply: 15Vdc

For the power dissipation calculation, I assumed slow decay, and the Switching frequency very high compared with true to provide some buffer.  My current limiter (Vref) is set to limit at approximately at 0.95A, though I used 1A for ease of calculation.

The total I came up with is 1.4W.  At this power, I am very safe for temp issues even at our highest expected ambient, so I have no issues there.  What I need to know, however, is what is the current draw on my 15Vdc source at each Voltage (a & b) input?  I need this so I can request the proper trace width(s) to the PCB designer. 

Is it as simple as I(input) = P(total) / V(input)  ---->  I = 1.4W / 15Vdc = 0.1A ?   Or are there other factors to consider before we can get from total power dissipation to input current draw at the source voltage pins?

Thank you,

Matt

  • 0
    TI__Guru** 114905 points

    Hi Matt,

    It appears you have overlooked the power used by the motor. That is 2.85W per coil @ 1A when running.

    For current, each coil can use up to the full 1A if stopped during microstepping. When holding at a full step consumes approximately 71% of the current limit per coil. This will total 1.4A. Also not included is the internal regulators, charge pump, and switching losses.

    For your application, designing for a current draw at 1A per coil (total of 2A) should work. If you have room, making the traces wider reduces the trace resistance and inductance for better system performance.

    Once estimated, it is also a good idea to measure the current for confirmation.

    For reference, the DRV8825EVM layout and schematic are good resources to use when developing your board. The files can be found in the Tools and software section of the DRV8825 product folder. Scroll down to the Software section once there.

  • 0 in reply to Rick Duncan
    Prodigy 40 points

    So to better conserve current, I could do the following?:

    1) if the 1.8 degree resolution of the (non-multistep) step gives me the desired positioning needed, then i can save 29% of the current used to hold the motor at a specific point.

    2) the 1A is just the motor max current limit, so if I find setting my current limit (lets say 1/4 A) still allows my motor to move and hold the parts within my system with no noticeable ill effect, I can do so.  This is where I assume motor force, torque, and other mechanical properties come into play.

    (Sorry if it sounds like I am lost, I am traditionally an Instrumentation Engineer and assigned this task to stretch my design abilities a bit).

  • 0 in reply to Matthew Reaves
    TI__Guru** 114905 points

    Hi Matt,

    Matthew Reaves said:
    1) if the 1.8 degree resolution of the (non-multistep) step gives me the desired positioning needed, then i can save 29% of the current used to hold the motor at a specific point.

    You are not saving any current with microstep vs full step at the same chopping current. The RMS current using microstep and full step is the same, at .707 * the full scale current.

    Matthew Reaves said:
    2) the 1A is just the motor max current limit, so if I find setting my current limit (lets say 1/4 A) still allows my motor to move and hold the parts within my system with no noticeable ill effect, I can do so.  This is where I assume motor force, torque, and other mechanical properties come into play.

    You are correct. If you can reduce the chopping current with no ill effects, this is where you can save current. You must be careful doing so, because the motor can lose torque as it speeds up. Your motor supplier should be able to help you pick the optimal motor for your application.