This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

DRV8434S: Reflection time to current when changing Vref and SPI register

Part Number: DRV8434S

Hi experts,

I have questions about the time from when the current value is set until the actual set current is output? Could you please answer them?

[Question]

(1) Response time or reflection speed [A/sec] when current output changes by switching VREF with SPI register constant
(2) Response time or reflection speed [A/sec] when current output changes by setting SPI register with VREF constant
If there is a corresponding part in the data sheet, it would be greatly appreciated if you could tell me where to look.

[Background]

My customer wants to vary both the VREF and SPI register settings to achieve a variety of current outputs. For convenience of microcomputer control, they set the current in order as follows: VREF switching → SPI setting (or vice versa order)
If you switch in order like this, a maximum time lag of 3 ms will occur between these two operations due to the specifications of the microcomputer.

Depending on the VREF and SPI settings, it over/under shoot the target current when switching as  the figure below shows. In the case of actual over/under shoot, they are investigating how much it will exceed in 3 ms. To clarify this point, they need your answer of above question.

Thank you for your help.

Best Regards,

Taito Takemrua

  • Hey Taito, 

    I will have to put in a design ticket to get the exact value, but I am fairly certain the response time is much faster than 3ms.  I would guess the response time is in the nanosecond range, so your over/undershoot current time would be close to 3msec.  

    I put in a design ticket for this question, it typically takes them 3-5 business days to respond.  

    Regards,

    Jacob

  • Hi Jacob,

    Thank you for your comment. It is helpful for me. I will ask again if my customer need more information.

    Best Regards,

    Taito Takemura

  • Hey Takemura-san,

    • VREF pin has opamp buffers at 1MHz bandwidth before they change the reference for coil current
    • TRQDAC(SPI) has 10MHz digital CLK before sending the updated bits to DAC to change the reference for coil current

    So changing the reference for coil current takes < 10us across all conditions. Its almost instantaneous.

    But coil current catching up with this reference depends on the coil inductance (which normally takes few tens/hundreds of usec to few msec)

    T = L * (New_I_reference) / VM

    If increasing current, VM  & L decide. If decreasing current, VM, L & chosen decay mode decide.

    Regards,

    Jacob