DRV8825: Holding Current

Kaile,

Would you help me check the following items:

1. Would you check xVREF voltage to replace "DIR" in your scope picture?

2. If you keep sending STEP signal, does the full scale current change?

3. "This issue only happen on mixed decay mode , fast decay mode is workable". How about slow decay? If we wait for one minute, is the current still kept at 773.8mA?

Hi Wang 

CH1 (Yellow): TB_HOLD

CH2 (Green): xVREF

CH3 (Blue): TB_STEP

CH4 (Red): output current

Fast decay : OK

Slow decay : NG

Mixed decay : NG

Kaile,

Thank you for the detail waveform comparison.  For the test waveform, we can see slow decay and mixed decay can not bring the output current back to reference voltage setting.

The slow decay just turns on both high side FETs or both low side FETs which gives a very low voltage across the motor winding. The output winding current may drop very slow with slow decay mode.

In tblank time, DRV8825 doesn't do the current regulation. On the slow decay zoom in waveform, the output current is driven up too much during the blanking time, the slow decay can not bring the current down quickly enough. The next switching cycle's tblank time, the output current keeps being driven up.  That is why you see the peak current is higher than the regulation point.

Mixed decay occurs only if the current through the winding is decreasing (per the indexer step table); if the current is increasing, then slow decay is used. So, if the current is stopped in the rising edge, the output current could also be higher than the regulation point.

Hi Wang 

I know TI release many new motor driver solution 

May I know the new one still have this kind of issue ?

DRV8424? Smart tune ?