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DRV8462EVM: Some confusion about amperage values in 8462 registers and Full scale current setting in EVM GUI

Edward Brakus
Prodigy 40 points
Part Number: DRV8462EVM

Hi

The datasheet for the 8462 shows most amperage settings as a value from 0-255.  This is a scaling of some value X, but I'm not sure what X is.   Is it relative to the Full Scale Current or something else?

Regarding Full Scale Current, there is a setting for this in the EVM GUI, and its an absolute rather than relative value.  But I dont see any corresponding register in the datasheet.  Capturing the registers from the EVM GUI before and after changing this value shows no change to the register values.

Could you help me understand whats going on here a bit?

Also, the GUI doesn't show Control register 14.

Thanks in advance

-E

  • 0
    TI__Mastermind 44595 points

    Hi Edward,

    Yes these are relative to the full-scale current. Please see below snippet from the datasheet. We will include these formulae in the mouse over help tip text for these widgets in the next release of the GUI.  

    The full-scale current setting in the EVM GUI defines the VREF input to the device. The required VREF voltage for the set full-scale current is calculated and output by a 12-bit DAC in the MSP430 MCU used in the EVM. It is calculated using this formula, IFS (A) = VREF (V) / KV (V/A). We will include this information for the help txt for this widget in the next release. This setting does not change any registers like you observed. You can measure the VREF voltage on header J6. This will change when the full-scale current widget value is changed.

    Did you rework the DRV8462EVM with a production sample silicon or did you get a replacement DRV8462EVM. If it was a replacement EVM it would carry a pre-production silicon marked "PDRV...." vs, the production silicon would be marked "DRV.....". If you're using with production silicon use GUI ver 1.0.1 else use GUI ver 1.0.0. When you use GUI ver 1.0.1 you'll see a popup window informing about an updated firmware. You should do a firmware update using the 'File > Program Device' menu.  In both versions of the GUI CTRL14 Register displays at the end because of its address.

    Below snippet is from the GUI v1.0.1, HELP menu.

    Regards, Murugavel

  • 0 in reply to Murugavel4637
    Prodigy 40 points

    Thanks Murugavel.

    Sorry to require some handholding here. I am an embedded sw guy operating maybe a step too far into ee land.

    Regarding the formula.

    IFS (A) = 3.3 V x TRQ_DAC / KV (V/A)

    Perhaps its the KV that's throwing me off.

    Listed as 0.66 nominal, that would be IFS(A) = 3.3V * 256 / 0.66.  = 1280.  I must be misinterpreting this.

    Could you give me a real example for a  IFS of 2A?

    Thanks again.

    -E

  • 0 in reply to Edward Brakus
    TI__Mastermind 44595 points

    Hi Edwards,

    No worries, I'm happy to help. It's great you're not afraid to venture into the EE land! TRQ_DAC in the formula is referring to the values in the associated table which is a percentage value of the current scalar. The 'KV' is the nominal Volts to Amps scaling factor for converting VREF to Ifs. For 100% TRQ_DAC, VREF = 3.3V will translate to 3.3/0.66 = 5A Ifs. So for 2A, which is 66.67% of 5A, the TRQ_DAC setting would be 170 or 0xAA. I hope this helps.

    Regards, Murugavel     

  • 0 in reply to Murugavel4637
    Prodigy 40 points

    Bingo.  That was the missing piece of the puzzle for me.  I understand now.  Thanks again for your help!