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DRV8818: bulk capacitance and charge pump question

Dean G.
Prodigy 60 points
Part Number: DRV8818
Other Parts Discussed in Thread: DRV8886AT, DRV8886ATEVM

Hello. I am working on a module design using DRV8818. During applications testing, a prototype failed with short circuit across VM/GND, and short circuit across VDD/GND. I repeated the experiment twice with identical result. I am trying to understand the mode of failure.

My best guess is inadequate bulk capacitance. The PCB is very small, so I could only fit 20uf. Section 9.1 of datasheet briefly describes how the "system responds to excessive current demands with change in voltage".

I am trying to understand how & why the device was damaged. Is there any information available about this mode of failure, internal functional schematic, or case study of DRV8818?

  • 0
    TI__Guru** 114905 points
    Hi Dean,

    These types of failures are usually due to electrical overstress. The cause can vary, but based on your prior post it could be during the power down sequence.

    As the device powers down, there will be a point when the outputs are disabled and the current is flowing through the body diodes of the output FETs. This can cause the VM voltage to increase. If the bulk capacitance is too small, the VM voltage could exceed the absolute maximum voltage.

    What is your VM and what is the full scale current used? What step rate are you using?
  • 0 in reply to Rick Duncan
    Prodigy 60 points
    VM = 12 volts
    full scale current = 1.25 amps
    step rate = 10 RPM = 267 Hz with 1/8 microstepping
  • 0 in reply to Dean G.
    TI__Guru** 114905 points
    Hi Dean,

    It is recommended to check for voltage spikes when powering down. Also you should check to ensure the current is being regulated properly in 1/8 modes.

    If space is a constraint, there are other devices that are smaller with similar capabilities and fewer components needed.

    The DRV8886AT would be a good choice because you no longer need the two external current sense resistors, or all the associated Rs and Cs. This would free up room for larger bulk capacitors.

    The DRV8886ATEVM is available on line if you are interested.
  • 0 in reply to Rick Duncan
    Prodigy 60 points

    I increased the bulk capacitance from 20uf to 66 uf in my latest prototype. So far no problems.

    Could it help to add rectifier diodes very close to VM pins? The datasheet says VMx minimum is -0.3 V.  The idea is to add diodes to prevent negative voltage spikes greater than 300mV. (diode Vf less than 200mV.)


    I am looking at DRV8886AT for future projets. Amazing chip with many features. Definately more room for bulk capacitance, and cost savings too.

    The only shortcoming is peak current. My goal is to drive NEMA 34 motors at 3.4 amps full scale. An older revision of my PCB has been performing well. The PCB uses PowerPeg thermal connectors to keep the DRV8818 cool. I ran into problems when trying to make the PCB smaller, by removing capacitors.