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DRV8711: DRV8711 SPI problems at higher current

T Kerklaan
Prodigy 50 points
Part Number: DRV8711
Other Parts Discussed in Thread: CSD88537ND

Hi,

We've designed the DRV8711 onto our motor drive board. We are facing problems with SPI communication when the driver current is increased. At lower currents (200 mA) reads/writes of the registers are fine. When increasing the current (600 mA), SPI writes garbage data to the registers causing unexpected behavior (eg motor gets disabled in CTRL register). Readback of the registers also fail. The FAULTn pin is not asserted.

1. Do you know what might be the problem?

Specs:

  • 48V
  • Rsense = 0.05 Ohm
  • FETs: CSD88537ND
  • 600 mA: gain=10, torque = 27 
  • Motor: ACT 23HS8430. 3.0 Amps

2. I'm having trouble getting the optimal DRIVE register settings for the combination of our FETs and sense resistor. Could you make a suggestion? 

These are the current settings:

lDriveReg->Address = 0x06;
lDriveReg->IDRIVEP = 0b10; // High-side gate drive peak current = 150 mA
lDriveReg->IDRIVEN = 0b10; // Low-side gate drive peak current = 300 mA
lDriveReg->TDRIVEP = 0b01; // High-side gate drive time = 500 ns
lDriveReg->TDRIVEN = 0b01; // Low-side gate drive time = 500 ns
lDriveReg->OCPDEG = 0b10; // OCP deglitch time = 4 µs
lDriveReg->OCPTH = 0b01; // OCP treshold = 500 mV

Thanks

  • 0
    TI__Guru** 114905 points
    Hi T Kerklaan,

    Can you confirm the SPI signals transition smoothly from low to high and high to low? Reflections or noise can cause incorrect transactions.
    Can you confirm the SDATI is changing at the proper time with respect to SCLK?

    Please see: www.ti.com/.../slva637.pdf and www.ti.com/.../slva632.pdf for assistance setting the registers.
  • 0 in reply to Rick Duncan
    Prodigy 50 points

    Hi Rick,

    Thanks for your reply. In current design we're not able to monitor SPI signals. Proper testpins are enabled in the next revision, to be delivered in a few weeks.

    We believe that inbound EMI of the FETs are causing the incorrect transactions on the SPI bus.This is a major problem as the DRV8711 can interpret the signals as incorrect settings (eg far too high current).

    In our PCB design, the FETs are placed almost next to the DRV8711 (3 mm apart). In the BOOST/EVM designs the FETs and driver are placed >  1 cm apart. Does the distance might have an effect on the reflections on SPI? What distance do you recommend?

    Kind regards,

  • 0 in reply to T Kerklaan
    TI__Guru** 114905 points
    Hi T Kerklann,

    Noise in the system can affect the SPI operation. There is no recommended distance, though it is recommended to separate digital signals from high voltage signals that create noise.