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Queries about the nice current amplifier circuit in the DRV8312 and other reference designs/development kits

Other Parts Discussed in Thread: DRV8312, MOTORWARE, DRV8301

Hello,

Can you provide some  background on the thinking behind the current sensor circuits in the DRV8312 and related development kits? Specifically, the rail connection and the frequency-response-setting components. We have heard high praise for these little amplifiers and are trying to understand them better:

1. The cap that I believe is there to bring in the frequency response of the op amp to give it a pole in the neighborhood of 700 Hz is in a place whose purpose is not obvious, in the input arm with the input resistor that sets the current into the feedback loop, rather than parallel with the sensing resistor or the feedback resistor. It creates a near-in zero in addition to a far-out pole...could you give some insight into this? It is marked as optional, whereas the much larger cap that is in parallel with the sense resistor is not marked as optional, but with that really tiny sense resistor, the pole created by the larger input cap is still quite far out. What was the thinking behind the frequency response design for this little amplifier?

2. Can you provide some background on the 33 Ohm resistor on the power input, between the opamp rail input and the board rail?

Thanks!

Regards,

JS

  • Janet,
    Only portion I can comment on is the voltage filter pole. This is discussed in the SPRUHJ1 user's guide, but it needs a slight update. Originally we encouraged a filter pole slightly larger than the maximum frequency of the motor. Once we started testing these very high speed motors (we have run up to 4 KHz but 2 KHz is more practical) we realized that having a voltage filter pole this large would be an issue. Also, the larger the pole the more it is influenced negatively by any component drift. So we were seeing pretty poor performance of the filter components at higher frequency.

    we adjusted our recommendation and now have in our MotorWare "setting user.h" spreadsheet a recommendation of a minimum of 200 Hz with a maximum of about 500 Hz, and you only need to insure that it is > IQ_FREQUENCY / 4.

    This pole must be updated as a software pole in the user.h as we cancel this HW pole out in the library as part of our voltage filter. It provides a very clean signal which helps FAST work the best.

    I've assigned this thread to someone from the DRV8301 team to see if they can comment further.
  • Hi JS,

    I did not design this kit but will take a stab at the intended functions.

    1. I am unsure as to the function of the RC on the -IN opamp. The cap in parallel with the sense resistor is intended to snuff out any high frequency noise that might be cause by the switching half-bridge such as ringing or flyback.

    2. This is possibly intended to create a small low-pass filter for the power rail removing noise from the system supply to the opamp.