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DRV8245-Q1: IPROP bidir current measure

Luca Meloni
Intellectual 470 points
Part Number: DRV8245-Q1
Other Parts Discussed in Thread: DRV8706-Q1

Tool/software:

Good morning, TI experts.
I'm using a DRV8245 in an application with a brushed DC motor. This motor operates at 13.7 volts, with a nominal current of 10 amps. I wanted to know if the chosen driver is suitable for this motor.

I'd also like to know if, using the IPROP system and external resistor, it's possible to read any current flowing from the motor to the H-bridge. This is because in my application, the motor has a high moment of inertia. Therefore, to decelerate the motor in a controlled manner, I need to read the current from both the bridge to the motor and the motor to the bridge. Reading the driver datasheet, I understand that current measurement is only possible from the driver to the motor.

Can you confirm?

Regards

  • 0
    TI__Mastermind 45445 points

    Hi Luca,

    Thank you for your post.

    Luca Meloni said:
    This motor operates at 13.7 volts, with a nominal current of 10 amps. I wanted to know if the chosen driver is suitable for this motor.

    I assume this means continuous current requirement for this motor would be 10 A. Brushed DC (BDC) motors have startup inrush current which would be several times the nominal current. Based on this I'd think the startup inrush requirements may be orders of magnitude higher than 10 A. For such BDC motor DRV8245-Q1 may not be suitable. See below from its datasheet. The continuous currents that can be sustained without a thermal shutdown is under the 'DC' columns. 

    A more suitable option for such high continuous nominal current requirements would be a TI Smart Gate driver from our portfolio with external MOSFETs for the H-bridge. The DRV8706-Q1 has integrated current sense in-line amplifier to sense current from a in-line current sense resistor in series with the motor to read motor current from and to the bridge outputs. 

    Luca Meloni said:
    I'd also like to know if, using the IPROP system and external resistor, it's possible to read any current flowing from the motor to the H-bridge.
    Luca Meloni said:
    Reading the driver datasheet, I understand that current measurement is only possible from the driver to the motor.

    Current sensing in the DRV8245-Q1 measures current via the high side (HS) FETs only. This would include both the driving current and the recirculation current - current from the motor while the PWM drive is off. See below. I marked the recirculation current in blue - the downward slope of the current during internal current regulation PWM off which is also output on IPROPI. This also applies to the current circulation during PWM off while INx is driven by a PWM input.

    You can also see section 7.7 Switching Waveforms for H-bridge high side recirculation current path. 

    However, when the bridge is disabled (Hi-Z) or not driving, the current via the FET body diode due to BEMF, flowing from the spinning motor to the H-bridge will not be measured by IPROPI. An in-line current sensing as in the DRV8706-Q1 can do either direction current measurement. 

    Regards, Murugavel 

  • 0 in reply to Murugavel4637
    Intellectual 470 points 
    Thanks for the reply. My problem is the following: Motor stopped, PWM command 50% and CW direction. 
    The driver applies voltage to the motor, which, being stopped, tends to draw a lot of current. 
    The Itrip mechanism intervenes, the current is limited to 10 Amp, the motor starts and reaches speed. 
    So far so good. Now I have to reduce the motor speed to zero. 
    If I reduce the PWM to 25%, due to a large moment of inertia of the motor, which becomes a generator, 
    the current reverses sign (now it flows from the motor to the driver),the Vcc from my 12V power supply increase, 
    and it seems not to be measurable by Iprop. In fact, in this case the Itrip does not activate. 
    If I switch from 50% PWM to 0 PWM, the current from motor to driver would invert with very high values, 
    and the motor would stop very quickly. 
    To avoid these problems, a current regulation  is usually performed, but it is necessary 
    to be able to measure the current both from the driver to the motor and vice versa. 
    It seems to me that via Iprop, it is not possible to read the current from the motor to the driver.
  • 0 in reply to Luca Meloni
    TI__Genius 12051 points

    Hello Luca,

    Murugavel is OOO and I am reassigning this to Joshua for support while Murugavel is out.

    Thanks,

    Ibinu

  • 0 in reply to Ibinu Alaudeen Nadeera
    TI__Mastermind 24860 points

    Hi Luca,

    How long is this motor expected to be run for? At 10A of nominal current this is likely not the best device for this application unless the motor is being run for <1 second. I do agree with Murugavel that moving to one of our external FET devices like the DRV8706-Q1 would be best for this application.

    Which control mode is being used? In the case that PWM mode is being used, the device will be place in Hi-Z when both inputs are low and during this Hi-Z period, the High FETs are not conducting which also means IPROPI will not output the current. If you are using PWM mode please attempt to use PH/EN mode since this mode does not cycle into Hi-Z mode.

    Can you provide a capture of OUT1, OUT2, IPROPI, and the load current, this will help me better understand what the device is doing? If you could also provide the control mode selected and the VM bulk capacitance on the board?

    Regards,

    Joshua

  • 0 in reply to Joshua Horlander Cruz
    Intellectual 470 points 
    Thanks for your reply. 
    I'll bring the requested measurements on Monday. 
    The working method is PH/En.
    I would like to clarify that in my first post I am referring to regenerative current 
    and not recirculating current.