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MCF8329A: Current sensing

Ludovic Martin
Prodigy 20 points
Part Number: MCF8329A

Hello,

I've a problem regarding how to calculate the resistor Rsense with the given calculation in $7.3.5 (MCF8329A datasheet). Rsense = (Vso - Vref /2)/(CSA_GAIN * I) with Vso between [0.25V to 3V]

What is Vso? Is it the voltage between Rsense terminals?

What's happen if Vso is equal to 1.5V? Using calculation is produce Rsense = 0ohm???

Can you help me?

Regards,

Ludovic

  • 0
    TI__Expert 7450 points

    Hi Ludovic,

    I will clarify VSO node details.. There is a current sense amplified which measures voltage drop across RSENSE and amplified signal goes to comparator, ADC etc internally. 

    RSENSE should be selected such that not much I2R drop, get better resolution with lower current. 

    If we take RSHUNT of 10mOhm 

    1. CSA has four level of Gain selection 5V/V, 10V/V, 20V/V, 40V/V
    2. BASE CURRENT is 15-bit value which is calculated based on gain settings It is given by {1.5/(RSHUNT*CSA_GAIN)}
      1. Scaling: 32768 / 1200 Example: for 15A, enter 15 * 32768 / 1200
    • Base Current will be 7.5A for CSA Gain of 40V / V with R Sense Shunt of 10mOhm or 3.75A for CSA Gain of 20V/V . For 7.5A write 0xCC to GD_CONFIG2-> BASE_CURRENT

    Thanks and Best Regards,

    Venkatadri S

  • 0 in reply to Venkatadri Shantaram84961
    Prodigy 20 points

    Thank you for the response.

    I'm not sure to be agree with you: for au 10mOhm resistor and CSA_GAIN of 40V/V I calculate 1.5/(0.01*40) = 3.75A but not 7.5A as written, can you confirm?

    I'm using motor studio (downloaded version is 1.12 but after installing in help menu I can see it is  version 0.1.9, can you confirm?). In the advance tuning menu--> System Level Configuration, when I change CSA_GAIN or RSHUNT value, it doesn't update GD_CONFIG2 register. It may be a tool bug.

    Thank you for your help,

    Regards,

    Ludovic

  • 0 in reply to Ludovic Martin
    TI__Expert 7450 points

    Hi Ludovic,

    Thanks for noticing error, I meant 40V/V for 3.75A and 7.5A with 20V/V for 10mOhm shunt.

    The GUI version is not updated in that release. We are updating new version soon with all the issues fixed.

    Thanks and Best Regards,

    Venkatadri S

  • 0 in reply to Venkatadri Shantaram84961
    Prodigy 20 points

    Hi Venkatadri,

    When do you think a new release will be available?

    I've to evaluate the component (asap) but actually due to a lot of strange GUI behaviours I'm not able to validate my design.

    Regards,

    Ludovic

  • 0 in reply to Ludovic Martin
    TI__Expert 7450 points

    Hi Ludovic,

    We will provide you update by 2nd week of April.

    We can still use the Register map view int he same GUI to tune. 

    In GUI 1.12 version there are errors in JSON loading. I suggest you to write word by word from the reg map view and then write into EEPROM.

    After EEPROM is written power cycle the board and read back the data and verify if all registers are proper.

    Please make sure to follow this before you start running the motor.

    Thanks and Best Regards,

    Venkatadri S