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LMD18245 BASIC: How to controll the DAC like it is at typical Application

Matthias Wenczel
Prodigy 20 points
Other Parts Discussed in Thread: LMD18245

Hi,

 

I have built up the LMD18245 Full bridge motor driver as described in the document on page 15.

The only different is, that I'm going to drive a motor with 0,2mH and 2,8 Ohm (instade of 12mH and 12Ohm).

My problem is, that I could not control the DAC on the LMD18245!

I do control the direction and the brake.

What am I doing wrong?

 

Any answer is welcome!

Matthias

 

  • 0
    TI__Guru** 114905 points
    Hi Matthias,

    Our experts have been contacted.

    FYI -- Your post is being moved to the motor driver forum.
  • 0
    TI__Expert 3920 points
    Hi Matthias,
    Do you mean you are not getting any output from the OUT1 and OUT2 pins? Lowering your winding inductance and resistance will increase your current as compared to the example schematic, and therefore, you will need to make sure your Rs value (which sets current threshold) allows for this increase in current. Can you double check this value in your schematic and make sure it's appropriate for the current you are trying to drive?

    For details on the effect of Rs on maximum current and DAC output, take a look at "The Comparator, Monostable and Winding Current Threshold for Chopping" section and Table 1.

    Tim
  • 0 in reply to Tim Reyes
    Prodigy 20 points

    Hi Tim,

    thank you for your answer!

    I allways get out the max current from Out1 and Out2! Only if I put M1, M2, M3 and M4 to 0V it stops. The break and the direction pin works. As soon as Break is 0V and any Pin of M1 to M4 is 5V (Uref) I get the full current.

    I have had a closer look on page 12 Table 1. But I could not find how to choose the capacitor placed next to Rs.

    Regards

    Matthias

  • 0 in reply to Matthias Wenczel
    TI__Expert 3920 points

    Matthias,
    Check you max current, and see if you are getting a chopping waveform like shown in Figure 9 of the datasheet.

    Also double check your VDAC REF voltage and RS values, input them into equation 2 so that you know what currents to expect based on your digital input. It is possible that you are hitting your current limit even when your digital input is as low as D=1 (This means M1-4 = 0001)