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DRV8305: Radiated Emissions releated to DRV8305 Internal Clock

Kesokeso
Prodigy 20 points
Part Number: DRV8305

I use two DRV8305NPHPRs on my custom pcb for two BLDC motors. 

I tested radiated emissions about the pcb.

Then I found peak emissions around 120MHz.

The source of peak emissions are most likely caused by DRV8305 internal clock.

Because when EN_GATE=Hi, the emissions occurs and SPI communication is stopped.

Also I checked peak emissions with EMC probes and found the silmillar peak emissions around DRV8305.

The emissions propagate from the driver through the MOSFETs to the motor.

Emission countermeasures are not easy.

Because it propagates from the gate line as well as the power supply line.

How should I deal with emissions?

  • 0
    TI__Genius 12231 points

    Hi Kesokeso,

    Thank you for your inquiry.

    If possible, would you be able to provide your PCB layout and schematic as well? 

    The high radiated emissions may be caused by inductive interference near the oscillator.

    Best Regards,

    -Joshua

  • 0 in reply to Joshua Maize
    Prodigy 20 points

    Hi, Joshua

    Thank you for your reply.

    My pcb layout is here. The schematic around DRV8305 is the same as the DRV8305 datasheet.

    I understand that broad spectrum noise comes from PWM (green mark in the fig) and sharp spectral noise comes from clock (blue mark in the fig).

    The motor control makes PWM noise. 

    But there are no clocks which make sharp spectrum noise, except SPI clock.

    MPU works with a crystal oscillator.

    In addition, power source of 3.3V/5V is not switching DCDC but linear regulators.

    Main power source is 24V lead battery.

    When EN_GATE=Hi & WAKE=Hi, the emission occurs even though SPI is stopped.

    I checked peak emissions with EMC probes and found the similar peak emissions around DRV8305.

    Also, I checked each pin voltage with an oscilloscope and found the similar peak in DVDD of DRV8305.

    Then the peak emissions are most likely caused by DRV8305 internal clock.

    Do you have any idea about the emissions?

  • +1 in reply to Kesokeso
    TI__Genius 12231 points

    Hi Kesokeso,

    Thank you for the extra information. 

    Considering DVDD is experiencing spikes along with the internal clock, it may be that the 8305 external components (like capacitor C-DVDD) need to be grouped closer to DVDD/internal clock as any parasitic losses (such as inductive losses caused by longer trace distances) can cause radiated emission from the fast-switching of the clock. 

    Hope this helps lead to a solution.

    Thank you,

    -Joshua

  • 0 in reply to Joshua Maize
    Prodigy 20 points

    Hi, Joshua

    Thank you for your reply.

    Finally, I found the solution.

    The external components (such as capacitors) have already been placed closer to each Pin and GND.

    The solution is to change DVDD capatitor from 1uF to 2.2uF.

    The emissions dropped over 3dBuV/m.

    A 10V rated MLCC(1005/0402 size) was used, but the capacity may not be sufficient because of DC bias characteristics.

    Thank you

    Regards,

    Kento