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DRV8412: Protecting pins from unusual conditions

Part Number: DRV8412

Hi

We plan to use DRV8412 in electronic kit for kids. Kids will have access to out pins of DRV8412. So we need to protect it against:

  1. ESD
  2. Short circuit. This will happen often.
  3. Connecting of an external power supply (9V battery). So even if all pins are pulled to ground where will be a constant current ~1A

PVDD is 12V. Common load is >3 Om. Will it be enough if we put such protection:

  • Denis,

    All DRV8412 pins have an internal ESD protection following the datasheet ESD spec. DRV8412 also provides the output short protection. An ESD diode can be added to the output to protect the output pin if the ESD event is out of the datasheet spec.

    If the PWM frequency is 100kHz (10us cycle time), the C1 would be charged and discharged every switching cycle. The power loss would be 0.5C(V^2)fs=0.5*0.5uF*9V*9V*100kHz=2.025W due to this 500nF output capacitor. I would recommend selecting a lower capacitor value according to the maximum PWM operating frequency.

    DRV8412 also provides the output short to power and ground protection. But, it should be noted that a properly functioning overcurrent detector assumes the presence of a proper inductor or power ferrite bead at the power-stage output. Short-circuit protection is not guaranteed with direct short at the output pins of the power stage.

    For normal operation, inductance in motor (assume larger than 10 μH) is sufficient to provide low di/dt output (for example, for EMI) and proper protection during overload condition (CBC current limiting feature). So no additional output inductors are needed during normal operation.

    However during a short condition, the motor (or other load) is shorted, so the load inductance is not present in the system anymore; the current in the device can reach such a high level that may exceed the abs max current rating due to extremely low impendence in the short circuit path and high di/dt before oc detection circuit kicks in. So a ferrite bead or inductor is recommended to use the short-circuit protection feature in DRV841x2. With an external inductance or ferrite bead, the current will rise at a much slower rate and reach a lower current level before oc protection starts. The device will then either operate CBC current limit or OC shut down automatically (when current is well above the current limit threshold) to protect the system.

    For a system that has limited space, a power ferrite bead can be used instead of an inductor. The current rating of ferrite bead has to be higher than the RMS current of the system at normal operation. A ferrite bead designed for very high frequency is NOT recommended. A minimum impedance of 10 Ω or higher is recommended at 10 MHz or lower frequency to effectively limit the current rising rate during short circuit condition.

  • All those things I got from the datasheet. Main problem is how to protect against the connection of an external power supply.

  • Denis,

    DRV8412 provides internal ESD protection and the output short to power and ground protection. Please follow the datasheet to design the protection circuit to against the connection of an external power supply.

    For ESD protection, An ESD diode can be added to the output to protect the output pin if the ESD event is out of the datasheet spec.

    For short protection, So a ferrite bead or inductor is recommended to use the short-circuit protection feature. A ferrite bead designed for very high frequency is NOT recommended. A minimum impedance of 10 Ω or higher is recommended at 10MHz or lower frequency to effectively limit the current rising rate during short circuit condition.

    DRV8412EVM use's guide provides a reference design. https://www.ti.com/lit/ug/slou287/slou287.pdf

  • Yeah, use's guide is greate. And thank you so much for explaning ESD and short circuit protection one more time. But still there is no answer for my question: how to protect against connecting 9V battery to 2 output pins of drv8412. May be this question sounds a bit crazy, but we really have such problem. 

  • Denis,

    "how to protect against connecting 9V battery to 2 output pins of drv8412."

    I count "connecting 9V battery to 2 output" as "the output short to power". DRV8412 provides the output short to power and ground protection. For short protection, a ferrite bead or inductor is recommended to use the short-circuit protection feature. A ferrite bead designed for very high frequency is NOT recommended. A minimum impedance of 10 Ω or higher is recommended at 10MHz or lower frequency to effectively limit the current rising rate during short circuit condition.

    Maybe I miss understand your question. Would you draw a picture to show the differences between "connecting 9V battery to 2 output" and "the output short to power"?

  • May be my question was unclear, sorry for that.

    Scheme looks like this:

    R1 is internal resistance of the battery.

    I guess there will be continuous current ~1A on reverse diodes in DRV8412 and nothing can stop it.  

  • Ok, I thought your input was also that 9V battery. Since the 9V battery and 12V input share the same ground. And, the short current goes through same FETs as DRV8412's "the output short to power and ground protection".

    I would think DRV8412's output short to power and ground protection will work. We just need to add a series inductance. "For short protection, a ferrite bead or inductor is recommended to use the short-circuit protection feature. A ferrite bead designed for very high frequency is NOT recommended. A minimum impedance of 10 Ω or higher is recommended at 10MHz or lower frequency to effectively limit the current rising rate during short circuit condition."