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TMS320F28027F: Current measurement

Carlos Silva85
Expert 4915 points
Part Number: TMS320F28027F
Other Parts Discussed in Thread: CONTROLSUITE,

Hi Colleagues,

I'm designing an EVM for motor control. Currently we are basing our design on the TIDA-00285 software.

My question: The motor control stuff expects the measurements to be bidirectional, with the zero current at 1.65V (1/2 Supply). It so happens we are not measuring or planning to implement regenerative functions and would like to change the code so that the reference is zero and any current measured is always positive.

Is it possible to change the software to accommodate for this? If so, what changes are required?

  • 0
    TI__Guru** 116490 points
    The phase/line current of 3-phase motor has positive and negative always in a rotation regardless of run as motor or generator, so the reference for zero must be half voltage range of ADC (1.65V).
  • 0 in reply to Yanming Luo
    TI__Expert 4915 points
    Hi Yanming,

    We are measuring low side, not inline.
  • 0 in reply to Carlos Silva85
    TI__Guru** 116490 points
    Using shunt resistor on low side is the same as phase inline, it has bi-directional (P/N) current also.
  • 0 in reply to Yanming Luo
    TI__Expert 4915 points
    Yanming, can you tell me how the reference voltage is internally set up on the launchpad?
    On my design the external reference pin is already in use, therefore I'm trying to decide if I set the opamps with a 1.65V fixed reference or a buffered 1/2 voltage divider from VCC.
  • 0 in reply to Carlos Silva85
    TI__Guru** 116490 points

    Not need to set reference voltage on current C2000 launchPad, the reference voltage is set on current sampling amplify circuit. You can refer to the Motor Control EVM board, for example, High Voltage Kit, DRV8301Kit or DRV8312kit, the hardware files for these EVM Boards can be found in TI controlSUITE (\ti\controlSUITE\development_kits) if you installed it on your PC. Below is from High Voltage kit, the voltage reference is from a regulator and is added on positive input of a amplifier.

  • 0 in reply to Yanming Luo
    TI__Expert 4915 points

    Hi Yanming,

    I'm designing one such board.

    I need to set the reference voltage for the ADC to compare the input with. Section 6.9.1.1.1 mentions 3.3V.  

    I'm unsure what happens, if say, the supply varies... Say it drops to 2.9V, for example...

    • Is the ADC output limited to 3600 rather than 4096 (which is the absolute value of 2.9V?)
    • Does the output now consider 2.9V as 4096 because it is comparing the absolute input voltage to its own supply?

    Similarly, I would like to use the internal on chip comparator (section 6.9.1.3.1) to trip the PWM module using one of the trip zone inputs and the internal DAC.

    • The value should be a voltage generated by the internal DAC and compared to the ADC's for the motor current.

    Would you be able to assist me with the above?

  • 0 in reply to Carlos Silva85
    TI__Guru** 116490 points
    This is based on which c2000 MCU is used, you could share the schematic with me, I would like to check it for you.
  • 0 in reply to Yanming Luo
    TI__Expert 4915 points
    This thread and the datasheet link I provided are for the TMS320F28027F. Do you need anything specific I might be missing...?

    I'm looking for the source code to configure the peripherals as above, as in demo source code. The schematic should be irrelevant at this point, Ill first test it on a launchpad and if I'm happy with we can then discuss how to port it.
  • 0 in reply to Carlos Silva85
    TI__Expert 7215 points

    Hi Carlos,

    The ADC uses Vref as full output (4095). You can either use internal (3.3v via internal bandgap reference) or external reference voltage. If you use the internal reference, the reference voltage is 3.3v. However, since voltage reference is a bandgap circuit followed by a buffer, it cannot exceed the input voltage of the chip (Vdda). So if the input power drops, the full range will drop accordingly. Say if Vdda = 3.1V, then 3.1V will become 4095.

    Same thing for the DAC.

    You can easily test this by feeding any launchpad with external power supply below 3.3 V and connect an ADC pin to the VCC. If you read around 4095, then it will prove the internal reference drops with Vssa.

    Best regards,

    Han