• State Resolved
  • Locked Locked
  • Replies 7 replies
  • Answers 3 answers
  • Subscribers 48 subscribers
  • Views 502 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

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.

DRV595: DRV595 in conjunction with a 2.7V DAC

Chris Rawle
Prodigy 240 points
Part Number: DRV595
Other Parts Discussed in Thread: DAC7571

Hello all,

I have a DRV595 which I intend to use to drive a peltier to heat and cool. The plan is to use a 2.7V 12bit DAC  (DAC7571) to provide a variable voltage input (0-2.7) to the IN+ pin and use a voltage divider to set the IN- pin to 1.35 V.  This way the voltage range from the DAC about the IN- pin value of 1.35V is symmetric . We do not need the to use the full range of the DAC output however as this will result in an over 4A operation current for the peltier which is why the lower operating voltage for the DAC is selected to make better use of the 12 bits resolution.

The differential input pins to DRV595 are 'biased to 3V' however.  If possible I would like to know why this is done and how this then works with the input voltage to IN+/IN- which is other than 3V.  Presumably the bias voltages are 'weak' and over ridden by whatever voltage value I provide to the IN+/IN- pins?

To my mind it is not clear from the DRV595 data sheet why then an input bias voltage should necessarily be suggested as 3V or the requirement?

http://www.ti.com/lit/ds/symlink/drv595.pdf

http://www.ti.com/lit/ds/symlink/dac7571.pdf

  • 0
    TI__Mastermind 19390 points

    Hi Chris,

    Are you worried about the input signal on IN+/- too high to destroy the device?

    The DAC output is AC signal, what's the common voltage of the DAC output?

    I want to confirm the 2 questions. Since the DRV595 is old device, I can confirm with designer.

    Thanks!

    Derek,

    Regards

  • 0 in reply to Yanming Gong
    Prodigy 240 points

    Hi Derek,

    I am not worried about destroying the DRV595.  The Peltier is only 1.17 Ohm so we can not use very much input range (difference between IN+ and IN-) without also exceeding 4A for the DRV595. Hence I use a low voltage DAC and low reference voltage to IN- as otherwise we can not sure the full 12bit resolution from the DAC even with the DRV595 set to its lowest gain.  Does that make sense?

    The DAC7571 consists of a string DAC followed by an output buffer amplifier - I would think this is a DC voltage? Common voltage of the DAC output is not in the data sheet.

    Regards,

    Chris.

  • 0 in reply to Chris Rawle
    TI__Mastermind 19390 points

    Hi Chris,

    Got you.

    Ideally, the IN+(2.7V) and IN-(1.35V) are same phase. The amplifier output will match your design.

    But in fact, the IN+ and IN- are not same phase completely. Once IN+ and IN- have some delay, they have some phase difference. Then the amplifier will have big output instantaneously. That may destroy the Peltier. That's big risk.

    Derek,

    Regards

  • 0 in reply to Yanming Gong
    Prodigy 240 points

    Hi Derek,

    thanks for that explanation.  The Peltier is large physically and capable of many more amps than DRV595 can supply so I think that the Peltier will be fine.  

    But could you please explain a bit more regarding the phase difference between IN+ and IN-?  IN- is fixed at 1.35V with a voltage divider sourced from the same 2.7v source which powers the DAC.  Where does the phase difference originate from?  In my mind the output from the DAC simply varies as we desire from 0-2.7V.  Or are you saying the output from the DAC may not smoothly and monotonically change between voltages commanded and instead have spikes or noise in the output causing large current fluctuations from the DRV595?  In that case do I need a low noise DAC or maybe LC filter on the output of the DAC?

    Regards,

    Chris.

  • 0 in reply to Chris Rawle
    TI__Mastermind 19390 points

    Hi Chris,

    The phase delay is related to the divided resistors and the layout trace.

    The unexpected delay may be too big to generate spike noise on output.

    I am afraid this risk. That needs your evaluation.

    Derek,

    Regards

  • 0 in reply to Yanming Gong
    Prodigy 240 points

    Hi Derek,

    I had three test boards made of my proposed circuit.  It works as expected.  I don't understand the phase delay you mention as the 1.45 V on IN- is constant and only the voltage to IN+ varies as the DRV595 requires.  How is the DAC different to any other variable voltage source to IN+? 

    At any rate - the DRV595 has over current and over temperature protection so any spikes should not damage the DRV595, only potentially the Pelter - is that the concern regarding the spikes, damage to the TEC rather than the DRV595?

    Thanks again.

    Chris.

  • 0 in reply to Chris Rawle
    TI__Mastermind 19390 points

    Hi Chris,

    I only afraid about the mismatch between the IN+ and IN-. The mismatch may lead the pop or peak on output.

    If you can control the input trace matched, I think the use case is no problem.

    Derek,

    Regards