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Inquiry regarding protection diode for DRV8811

Atsushi Okui
Genius 4015 points
Other Parts Discussed in Thread: DRV8811

Hello, all

We have one inquiry regarding the protection diode for DRV8811 from our customer, please refer to the item below and feedback us with your comment.

When referring the schematic of DRV8811EVM-001 (page 13 of User's Guide), MBRS360 is used.

http://www.ti.com/lit/ug/slva344b/slva344b.pdf

We assume this schottky is used for the protection against counter electromotive force from motor.

However, if the impedance on VM side is higher, we assume this could not effect well.

(We also assume that to put varistor near side of motor would effect more.)

Please let us clarify whether the configuration on EVM is commonly used method.

We thank you in advance for your information.

Best regards,

  • 0
    TI__Guru** 114905 points

    Hi Okui-san,

    The schottky diodes shown are optional. If the device is to be used in asynchronous mode (SRn = HI), current will flow through the body diodes of the FETs. This will cause additonal heating inside the DRV8811, which may lead to overtemperature shutdown.

    The external schottky diodes provide an alternate path to allow the current to decay. This path does not introduce additional heat into the DRV8811.

    Please refer to the post http://e2e.ti.com/support/applications/motor_drivers/f/38/p/61083/219315.aspx#219315 for more information. Also a search for the keywords "DRV8811 external diode" will provide even more discussion.

  • 0 in reply to Rick Duncan
    Genius 4015 points

    Duncan-san,

    Thank you for your reply.

    With regard to the contents above, we have inquired additional 2 inquires from them.

    Please refer to the item below, and feedback us with your comment.

    1) In case of SRn=Low, which is used in synchronous mode, would the current also be flown through the body diodes of FETs? Since the description on the datasheet "decay mode" would refer to be able to flow through them in case SRn=Low as well.

    2) Would these diode could protect against the counter electromotive force from motor, in case that the system is off (Vcc=0V)? Unfortunately, the motor this customer uses does not have any protection to cause the counter electromotive force into DRV8811.

    We assume that we need to have additional protection on AOUT1/2 and BOUT1/2.

    We thank you once again for your information.

    Best regards,

  • 0 in reply to Atsushi Okui
    TI__Guru** 114905 points

    Hi Okui-san,

    Sorry for the delayed responsed. Answers are below.  If I do not respond within a few days, please feel free to ask the question a second time.

    1) In case of SRn=Low, which is used in synchronous mode, would the current also be flown through the body diodes of FETs? Since the description on the datasheet "decay mode" would refer to be able to flow through them in case SRn=Low as well.

    Answer: The current flows through both the FETs and the body diodes when SRn=Low. When SRn=Low the current will flow through the body diodes during the deadtime, when both FETs are off (typically 475ns). After the deadtime, the FETs are enabled and the current flows through active FETs for better efficiency.

    2) Would these diode could protect against the counter electromotive force from motor, in case that the system is off (Vcc=0V)? Unfortunately, the motor this customer uses does not have any protection to cause the counter electromotive force into DRV8811.

    Answer: I am assuming that you are referring to loss of VCC while the motor is spinning. Is that correct? I will have to confirm this, but should have the answer soon.

    We assume that we need to have additional protection on AOUT1/2 and BOUT1/2.

  • 0 in reply to Rick Duncan
    TI__Guru** 114905 points

    Hi Okui-san,

    For the second question, the  body diodes will protect against the counter electromotive force assuming the minimum and maximum voltages of the DRV8811 are not violated.