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DRV8434: DRV8434 decay mode, bulk capacitor calculation and time propagation

Rajat Jhunjhunwala
Prodigy 110 points
Part Number: DRV8434

Hello TI,

I am working with DRV8434 and have a query on my bulk capacitor calculation. In app circuit it is stated 100uF but with out any justification.

My calculation is from a app note of TI for bulk capacitor calculation: SLTA055 app note.

C=1.21*Itr^2*L/V^2.

Iload = 1

Lmotor = 1.4mH

Vripple = 3.2 , as battery input is max 32V. C value got is 165uF

Kindly let me know if the formula for the application is right.

Question 2) The decay mode that must be chosen for the application. Bipolar stepper motor 1Amp and 150 deg rotation. Oil level controllng application.

Question 3) How do i calculate the complete time propagation delay from controller to motor input? Will it depend on decay mode?

  • 0
    TI__Guru 55605 points

    Rajat,

    1)  This is the app note I recommend and calculation looks good to me.  

    2)  Please use one of the smart tune modes.  Either dynamic decay or ripple control.  

    https://www.youtube.com/watch?v=8i_HDJmxais

    https://www.ti.com/lit/an/slvaes8/slvaes8.pdf

    3)  Not really sure what you are asking here.  If you are asking for the time from rising edge of STEP input to increasing/decreasing current in the motor coil, that is ~1us.  It will not depend on decay mode.

    Regards,

    Ryan

  • 0
    TI__Guru 56650 points

    Rajat,

    1. The equation is based on a buck converter and may be good for a DC/DC converter. I am not sure where 1.21 comes from.

    In motor drive application, the bulk capacitor can reduce the input voltage jump due to the back drive current from the motor. The input bulk capacitor needs to absorb the motor sending back current. 1/2 * C * (Vin_max^2-Vin^2) = 1/2 *L*I^2

    C is the input bulk capacitor value; Vin is the input voltage before the current sending back from motor; Vin_max is input maximum allowing voltage; L is the motor inductance; I is the motor maximum current.

    2. Decay mode is important for the winding current control. DRV8434 can support smart tune dynamic decay which can automatically optimize the fast decay percentage. So, if you don't know what decay mode should be used, you can select smart tune dynamic decay.  

    3. DRV8434's input signal is STEP and DIR pin to control the motor speed and spin direction. There is not propagation delay here.

    Regards,

    Wang Li

  • 0 in reply to Ryan Kehr
    Prodigy 110 points

    Hi Ryan,

    Thankyou for your reply.

    1) I wanted to understand how to calculate the current flowing through the winding from AOUT1 to AOUT2 and BOUT1 to BOUT2.

    2) We are using 1/32 ustepping and need to calculate the electrical angle and AOUT/BOUT current through windings.

    As in datasheet it's only for 1/8 ustepping. 

    I have the bipolar stepper motor winding resistance. Kindly help in this.

    Regards,

  • 0 in reply to Wang5577
    Prodigy 110 points

    Hi Wang and Ryan,

    Thankyou for your reply.

    1) I wanted to understand how to calculate the current flowing through the winding from AOUT1 to AOUT2 and BOUT1 to BOUT2.

    2) We are using 1/32 ustepping and need to calculate the electrical angle and AOUT/BOUT current through windings.

    As in datasheet it's only for 1/8 ustepping. 

    I have the bipolar stepper motor winding resistance. Kindly help in this.

    Regards,

  • 0 in reply to Rajat Jhunjhunwala
    TI__Guru 56650 points

    Rajat,

    1. The output current d equation can be found in datasheet. The full scale current IFS(A) = VREF/1.32. On each micro-step, the current can be found in Table 7-3. Relative Current and Step Directions. 

    2. The output current is a sinusoidal waveform. AOUT = IFS x sin (electrical angle); BOUT=IFS x cos (electrical angle).

    electrical angle = (n-1) * (90 degree)/m;    n is total step number; m is micro-stepping setting. For example: the 2nd step of 1/32 micro-stepping, the electrical angle is (2-1)x 90 / 32= 2.8125 degree

    Regards,

    Wang Li

  • 0
    Prodigy 110 points

    Hello Ryan and Wang,

    The driver input are suggested to be kept Hi-Z for particular stage or working. Shall I consider Hi-Z as floating the pin or add a 1Mega Ohms pull up resistor?

  • 0 in reply to Rajat Jhunjhunwala
    TI__Guru 56650 points

    Hello Rajat,

    It seems you are asking the Hi-Z input.

    Hi-Z input means the input pin is floating; or the input source doesn't inject any signal to the Hi-Z pin; or the input signal can set the input voltage in Hi-Z mode voltage range. In Hi-Z mode, we don't need the one Mega ohm pull-up resistor. 

    BTW, Hi-Z output means that power FETs are off in that output circuit.

    Regards,

    Wang Li