DRV8825 thermal issues and holding current

Timothy Starr,

1. If the voltage value is high (>12V), it'll not affect the Rdson. At Ta = 25 C, Rdson = 0.2 each. At Ta = 85C, Rdson = 0.25 each. Your requirement is @ 35C + temp rise due to power dissipation within the part at 1A RMS.

Power Dissipation for 8825 = 4*I(rms)*Rdson = ~0.8W

Theta (JA) = 31.6 C/W implies that DRV8825 will heat up by = (31.6*0.8) = 25.3 C.

Therefore, Rdson value that you require is at 35 + 25.3 = ~ 60C. You can assume a linear increase in Rdson for temperature rise from 25C to 85 C in most cases. If you need more accurate estimates, please feel free to ask me.

2. Holding current: You can reduce holding current by reducing the Vref voltage. In many applications, there is a resistor divider on Vref Pin and FETs parallel to the resistors. You can turn off/ On the FETs using your MCU and thus changing Vref voltage. If you need more information on this, please feel free to ask me.

3. The temperature will rise by 31.6 C per Watt dissipated within the part. So, the maximum power that you dissipate is (150-35)/31.6 = ~3.6 W Please note that since Rdson increases with increase in temperature, you might have to use higher values for Rdson than at 85C. Also, other than the power disspated in the FETs, there's also switching loses from the FETs turning ON and OFF.

Thanks
Vivek

Vivek Shankar

Applications Engineer

Motor Applications Team

Texas Instruments Inc.

Thanks Vivek; that was helpful. Another question, though:

According to the DRV8825 datasheet, the nHOME signal goes low (active) whenever you go through the home position at 45 degrees.

So my interpretation is that for a industry standard two phase stepper motor with the full step angle of 1.8 degrees (200 steps per revolution) the nHome signal goes low 200 per revolution when the electric angle between the phases, according to Table 3, is 45 degrees. So TI call “home position” not one spot per revolution but rather a spot in micro stepping within a step. Please confirm or explain. Thanks,

-Tim Starr on behalf of JK@NX

Timothy,

That is correct. The nHOME signal goes low when the electrical angle is 45 deg. It might occur multiple times per mechanical revolution. However, for the typ motor that you have mentioned, nHOME will go low 200/4 times (assuming you are running in Full step) per physical revolution.

Thanks

Vivek

Hi Tim,

The PDF for all of our EVM's schematics can be found with the EVM CD Image which can be downloaded on the respective device's datasheet. I am copying the link for this particular one here:

http://www.ti.com/litv/zip/slvc342a

The nHome signal can be used for many things although I am not certain what other people use it for. What it gives you is a point in reference into the internal lookup table. For example, if you are using 32 degrees of microstepping, the HOME will be asserted on step 17. There are some intricate algorithms in which you want to keep track which step is being generated. Although you know when steps are generated, if there was no home you could not know which one was being generated. One you synchronize you can go on counting steps internally. You will need to resynchronize whenever you change resolution or possibly if you change direction.

Hope the info helps,

Jose Quinones