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DRV8912-Q1: Noise from driver supply (VM) coupling into logic supply (VDD)

Oliver Tedder
Intellectual 505 points
Part Number: DRV8912-Q1

Hi,

We are using the DRV8912 in some of our designs for driving motors and other inductive loads.  We are seeing that some noise on the logic VDD rail (3.3V in our case) when driving certain loads, and turning them on and off.

For example, image1 is an AC coupled trace of the 3.3V net when we are driving a motor with PWM but full duty cycle.

image2 is the same motor but driven at ~50% duty cycle.

image3 shows a transient on the 3.3V net when the motor is initially turned on (on the left of the plot where the supply is stable, nothing is being driven by the DRV.

PWM frequency is 2kHz in each case.  We see noise on the 3.3V net at around that frequency, but also at a lower frequency as shown in image4 which is presumably something related to the motor speed (since its centre position increases as the PWM duty cycle is increased):

My question is, is there any particular isolation or filtering that we shold pay attention to, or you can suggest, to prevent this noise coupling back into the VDD supply?

Many thanks,

Ollie.

  • 0
    Intellectual 505 points

    The above is my first question (1).

    A related question (2):
    Is it possible that any large overcurrent events on the VM supply could feedback and damage the VDD supply in any way?  E.g. a short on one of the motor driver supplies causing a short on the VDD supply?

    Thanks.

  • 0 in reply to Oliver Tedder
    TI__Mastermind 44835 points

    Hi Oliver,

    Thank you for your questions. 

    A couple of possibilities.

    1. First I suggest to check VM and make it stable. If your VM ripple is large (~>500mV), you could try to increase Bulk cap on VM line.(e.g adding ~470uF). It could help.

    2. Recommend to have 1uF cap close to VDD pin.

    3 PCB layout. Here is example on datasheet. If VM or some of OUTx cross over VDD line, it may cause coupling.

    VM supply does not damage VDD supply as long as lower than Vin max.

    regards

    Shinya Morita

  • 0 in reply to Shinya Morita
    Intellectual 505 points

    Hi Shinya,

    Thanks for your response.
    1.  I will check the ripple on VM.  We have 10uF and 100nF decoupling on each VM input but from your suggestion it seems that this could be insufficient (there is 100uF bulk on that net but not directly close to the chip).

    2. We have 100nF close to VDD, we can increase this.

    3. We will check this, thanks.

    Regarding "VM supply does not damage VDD supply as long as lower than Vin max." I assume you are talking about the maximum ratings in the datasheet (40V for VM, 5.75V for VDD)?

    Many thanks,

    Ollie.

  • 0 in reply to Oliver Tedder
    TI__Mastermind 44835 points

    Hi Ollie,

    Thank you for your feedback. 

    3. Yes, I meant abs max.

    regards

    Shinya Morita