• State Resolved
  • Locked Locked
  • Replies 8 replies
  • Subscribers 29 subscribers
  • Views 254 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

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.

Original question:

DRV8256: How to chose TOFF pin setting

DRV8256: Ivalley value with Smart tune Ripple Control

Yusuke Mukuno
Expert 7395 points
Part Number: DRV8256
Other Parts Discussed in Thread: DRV8426,

Hi Expert,

I cannot find the description about how to set Ivalley value in datasheet.

Could you please let me know it?

datasheet

Smart tune Ripple Control operates by setting an IVALLEY level alongside the ITRIP level. When the current level reaches ITRIP, instead of entering slow decay until the tOFF time expires, the driver enters slow decay until IVALLEY is reached. Slow decay operates similar to mode 1 in which both low-side MOSFETs are turned on allowing the current to recirculate. In this mode, tOFF varies depending on the current level and operating conditions.

Thanks

Muk

  • +1
    TI__Expert 4955 points

    Hello Mukuno san,

    I am happy to assist you today. I couldn't find specific information on the datasheet either. However, from looking at the DRV8889 datasheet and support, it seems to have a similar operation for dynamic ripple control.

    It seems that the Ivalley will follow Itrip, therefore you should only need to change Itrip to adjust Ivalley.

    In the following thread another expert confirmed that:

    "Ivalley is 1% lower than Itrip+12mA."

    Source:

    https://e2e.ti.com/support/motor-drivers-group/motor-drivers/f/motor-drivers-forum/823040/drv8889-q1-toff-and-ivalley-about-smart-tune-ripple-control-mode

    Please let me know if you have any other questions or if I can assist you with anything else.


    Best,

    Pedro Arango Ramirez

  • 0 in reply to Pedro Arango Ramirez
    TI__Expert 7395 points

    Hi Pedro-san,

    I'd like to confirm it again.

    For example, In case of customer set 1A for Itrip.

    1A+12mA x 1% = 10.12mA

    Ivalley = 1A - 10.12mA = around 990mA

    Is it correct?

    d-Itrip accuracy spec is below, (-12%min to 12%max at Itip<1A)

    I feel, 1% is too small. 

    Also, if the motor inductance is several mH, the chopping frequency will be several MHz. is this consideration correct?

    Thanks

    Muk

  • +1 in reply to Yusuke Mukuno
    TI__Expert 4955 points

    Mukuno-san,

    You bring up a valid and important point regarding the tolerance of Ivalley. Allow me to investigate this matter and I will respond within 24 hours.

    Regarding your second question about motor inductance, would you be able to expand more on your question. It is my understanding that a motor with higher inductance will have decreased current ripple allowing for the chopping frequency of the PWM to be lesser if so desired. I wouldn't think that for a motor inductance of mH you would need to go to MHz frequencies. 

    I believe that starts to become a concern when you are using extremely low inductance motors in the uH range:

    A good thread that talks about it can be found here:

    e2e.ti.com/.../low-inductance-motor-with-instaspin

    Let me know if I can assist with anything else in the meantime.

    Best,

    Pedro Arango Ramirez

  • 0 in reply to Pedro Arango Ramirez
    TI__Expert 7395 points

    Hi Pedro-san,

    Thanks, I wait your answer.

    Regarding second question,

    at VM=24, Motor inductance = 1mH, di = Itrip - Ivalley = 10mA (1A x 1%)

    The time of Ivalley to Itrip will be,

    dt = (L x di) / VM = (1mH x 10mA)/ 24V = 0.416us  (time of increasing current)

    Rough calculation for chopping frequency will be around 1MHz.

    Is this calculation correct?

    Thanks

    Muk

  • +1 in reply to Yusuke Mukuno
    TI__Expert 4955 points

    Hello Mukuno-san.,

    I have confirmed with another expert and since the 8256 belongs to the 84xxfamily, Ivalley is 1% below Itrip, minus 10mA. Therefore, if Itrip is set to 500mA, then Ivalley should be 481mA. This information is not currently in the datasheet and has been added to the list of changes for future revisions. 

    Regarding your second question. Your calculation for Ton (increasing current time) is correct, but it is incomplete for assessing chopping frequency.

    It is my understanding that to define chopping frequency you will need both the Ton and Toff of the device, for both current increasing and current decreasing. 

    Therefore chopping frequency should be

    1/(Ton + Toff)

    The equation for Toff should be dt= di * L/(I * 2Rdson * Rmotor)

    We must use I * R instead of Vm to account for the back emf from the motor. 

    I would be whatever Itrip is set to,

    dI would be the difference between Itrip and Ivalley. 

    It is 2Rdson at is in this mode slow decay current recirculation is used. 

    Without taking Toff into account the chopping frequency will be much much higher, as Toff will always be several times bigger than Ton in this setting.

    Let me know if there is anything else I can be of assistance on.

    Best,

    Pedro Arango Ramirez

  • 0 in reply to Pedro Arango Ramirez
    TI__Expert 7395 points

    HI Pedro-san,

    Thank you for your answer.

    I confirmed DRV8426 datasheet.

    Is it meaning, DRV8256 has same feature of DRV8426, below?

    Q1

    I don't understand the calculation method for Ivalley =481mA at Itrip 500mA setting.

    Maybe I think,  Ivalley = Itrip - (Itripx1%) - 10mA.  is it correct?

    (it will not become 481mA)

    Q2

    DRV8426 has 11mA offset from the datasheet.  is it 10mA in case of DRV8256? 

    Q3

    Is DRV8256 ripple setting also depend on Toff pin? (like above table, 1% at TOFF=0, 2% at TOFF=1, 4% at TOFF=High-Z, etc)

    Thanks

    Muk

  • +1 in reply to Yusuke Mukuno
    TI__Expert 4955 points

    Hi Mukuno-san,

    A1

    My apologies, I meant to type -19mA, not -10mA. I absolutely butchered the math there, 481 came from 500mA-19mA. I forgot to add the 1% difference. 

    The correct calculation for the 500mA case would be; Itrip - Itrip*0.01 - 19mA = 476mA

    A2

    The offsets are slightly different depending on the part, for DRV8256 it should be 19mA.

    A3

    Regarding Toff behavior, apparently there seems to be documentation in the DR8256 datasheet that Toff does control the Ripple:

    It is a safe assumption that Toff will behave in a similar way in the DRV8256. I will add it to the list of changes that need to be implemented in the datasheet. The only exception being that it will be 19mA instead of 11mA for the DRV8256.


    Best,

    Pedro Arango Ramirez