• State TI Thinks Resolved
  • Locked Locked
  • Replies 5 replies
  • Answers 3 answers
  • Subscribers 64 subscribers
  • Views 1191 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

CCS/UCD3138A: question about DAC and RAMP

liwei qi
Expert 1160 points
Part Number: UCD3138A
Other Parts Discussed in Thread: UCD3138

Tool/software: Code Composer Studio

In voltage loop, I can get the A point voltage and set the DAC value though the register of RAMP.

FeCtrl0Regs.RAMPDACEND.bit.RAMP_DAC_VALUE = XX

FeCtrl0Regs.DACSTEP.bit.DAC_STEP = YY

What value i can set? in the picture, the DAC is 10bits. And the register of DAC_STEP is 18bits. is the 4 bits dither ignore?

And the error signed 9bits result is XN. is it right?

  • 0
    TI__Mastermind 18370 points
    The DAC_STEP has a 10 bit fractional part, allowing very precise and also very slow ramps. There are 10
    fractional bits, below the dither portion of the DAC. There are 8 bits matching the DAC value, including
    dither.

    For more details please refer to chapter 3.3.3 DAC Ramp Steps in Technical Reference Manual (TRM):

    www.ti.com/.../sniu028a.pdf

    And yes XN is a signed 9 bit value.
    Hope this helps.

    Regards,
  • 0 in reply to Yitzhak Bolurian
    Expert 1160 points
    which register I can use to read the signed 9 bit value? FILTERXNREAD.XN?
    How to calculate the XN form (EAP0-EAN0)? XN = RAMP_DAC_VALUE - EADC? i don't know the register if have the same scale.
  • 0 in reply to liwei qi
    TI__Genius 17090 points

    The best place to read the EADC output is the EADCRAWVALUE in the Front End Regs.  Looking at the XN register you mention is a good way to make sure that the error value is making its way into the Filter. 

    You can also look at the EADCVALUE register.  The ERROR_VALUE bitfield in that register will also give you the EADC value.

    The ABS_VALUE bitfield in the same register will give you the combination of the EADC value and the DAC value scaled at a nominal 1.5625 mV/bit like the EADCDAC.

    Your equation is correct, and they do have different scaling.  The EADC is at a nominal 1 mV/step, and the EADCDAC is at a nominal 1.5625 mV/bit.

    For the full register descriptions, see Section 3.7 in the UCD3138 Technical Reference Manual.

  • 0 in reply to Ian Bower
    Expert 1160 points
    In voltage loop control, if XN overflow, what will be happen?
    I have an issue. The output voltage change to 0. For the output volotage change to normal. it need to give a big duty ,but it don have pwm output from UCD3138 PWM pin. And the RAMP_DAC_VALUE is about 0.7V which is set by user. The max value of XN is 2^9*1 = 0.512V. The XN maybe overflow. So i doubt that is the reason. Right?
    Thanks!
  • 0 in reply to liwei qi
    TI__Genius 17090 points

    If the signal goes outside the dynamic range of the EADC, which is what you are describing, the XN will just be at the minimum or maximum level, whichever is appropriate. 

    I suspect that you have something happening somewhere else in the loop.  I suggest that you try on an open loop board, or on a board with the FETS disconnected, and use the CPU SAMPLE function to make sure the rest of the loop is working.