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DRV8818: Mixed decay

NAOKI AOYAMA
Guru 11215 points
Part Number: DRV8818

Hello,

Our customer use the DRV8818, have a question.

Is mixed decay active at full step mode?

Best Regards,

Naoki Aoyama

  • 0
    TI__Genius 14487 points
    Aoyama-san,

    Yes, all the decay mode options should be available for all stepping configurations.
  • 0 in reply to James Lockridge19
    TI__Genius 14487 points
    Aoyama-san,

    One of my colleagues just corrected me on my previous statement. DRV8818 only has slow-decay mode for full stepping. Even if it is set to mixed-decay or fast-decay, it will still decay slowly.

    A way around this is to send a signal to the sRN pin when you want the current to decay quickly.
  • 0 in reply to James Lockridge19
    Guru 11215 points

    Hello James-san,

    Thank you for your reply.

    The customer have an additional question.

    The customer have an issue of current control.

    The configuration of the customer as follows,

    Full step

    mixed decay

    VCC=3.4V  and Vdecay=1.7V

    C=470pF and R=33kohm placed at RCx

    tOFF=15.51usec, tDF=2.83usec, tBLANK=0.658usec

    SRn : L   synchronous  rectification

    I attach the file their issue,

    DRV8818_mixed_decay.pdf

    At first, it looks mixed decay is active. But after several msec, only slow decay is active.

    When mixed decay active, a current is under control. 

    Do you have any idea?

    Best Regards,

    Naoki Aoyama

  • 0 in reply to NAOKI AOYAMA
    TI__Genius 14487 points

    Hello Aoyama-san,

    My guess is that because the current is increasing, mixed decay mode turns off. The datasheet says "Mixed decay mode is only used while the current though the winding is decreasing; slow decay is used while the current is increasing (p12)." Figure 7 in the application note "Current Recirculation and Decay Modes" shows that when current increases, slow decay is used, but when current decreases, mixed decay is used. In the customer's waveform, the DC current level seems to be rising to the set threshold.

    What is the "Hold Indexer Mode" in the customer document that you sent? Is there a reason the customer needs mixed decay in this mode?

  • 0 in reply to James Lockridge19
    Guru 11215 points
    Hello James-san,

    I already checked the datasheet P12. So, I'm wondering which decay use at Full step.
    You say only slow decay mode is applied, but the mixed decay is applied in the evaluation of the customer.

    "Hold indexer" means the customer don't input the STEP pulse. A motor is keep once position:(HOLD)
    The customer use Full step and 1/8 u stepping, don't change the configuration of the decay mode.

    By the way, I want to confirm Figure 7 in the application note, it seems to be changed from Fast mode or mixed mode
    to slow mode in the middle. Is it correct?

    Best Regards,
    Naoki Aoyama
  • 0 in reply to NAOKI AOYAMA
    TI__Genius 14487 points
    Aoyama-san,

    The DRV8818 can only operate in slow decay mode for full step setting. This will happen regardless of the decay-mode setting when full-step is chosen.

    Regarding Figure 7 in the application note, I think you understand correctly. When the average stepper current rises, the device always defaults to slow decay, but when the average current decreases while set to mixed-decay mode, the first part of each microstep will have fast decay. In the middle of the figure 7 waveform, this changes because the average current begins increasing in the opposite direction.

    Does your customer require mixed decay for full-stepping while in Hold-indexer mode? Using the SRn pin, are they able to see mixed/fast decay during motor rotation?
  • 0 in reply to James Lockridge19
    Guru 11215 points
    Hello James-san,

    Thank you for your reply.
    I will feedback the customer the DRV8818 can only operate in slow decay mode for full step setting.

    The customer doesn't require mixed decay for full step setting.
    But the current regulation doesn't operate well. The current increase.

    I will let you know when new information.

    Thanks,
    Naoki Aoyama