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DRV8428: External control input when VM is not supplied, and recommended value of VM bulk capacitor

Takanori Kikuchi
Prodigy 30 points
Part Number: DRV8428

Tool/software:

Hi, 

We are considering using separate power supplies for the driver and the external controller.

  1. If the EN pin is pulled up to high when VM is not supplied, is there a possibility of failure, malfunction, or voltage leakage?

  2. If there are some input to STEP, DIR, SLEEP, M0/1, or VREF when VM is not supplied, is there a possibility of failure, malfunction, or voltage leakage?

  3. Is there a recommended value and type for the bulk capacitor to be placed on VM pin?
    Since 30 drivers will be powered from the same DCDC, we are concerned that the capacitance will become large and the inrush current will become large.
    The drive current is 160mA/channel, and only up to 6 devices will be driven at the same time, others will be in SLEEP mode.

Best regards,

  • +1
    TI__Mastermind 19400 points

    Hello,

    Using separate power supplies for driver and external controller is not an issue. 

    1.

    If the voltage on the VM pin ever drops below the UVLO threshold for the power supply, all outputs are disabled, and the EN/nFAULT pin is pulled low. The EN pin is shared with the nFAULT pin and if fault reporting is being used, the following configuration should be used.

    "When a fault is detected, EN/nFAULT pin is forced low by turning on Q1 - which discharges the capacitor C1. The H-bridges are disabled when the voltage on the EN/nFAULT pin falls below the VIL threshold. The bridges stay disabled till the fault condition is removed or a second MCU pin directly applies a voltage higher than VIH to the EN/nFAULT pin. Thereafter, Q1 is turned off and C1 charges back through the resistor R1."

    2.

    The STEP, DIR, SLEEP, M0, and VREF pins all have pulldown resistors and will be fine if supplied while VM is not. 

    3. 

    Bulk capacitance selection is specific to the design and desired operation. Please see the following app note to help with bulk capacitor selection.

    Bulk Capacitor Sizing for DC Motor Drive Applications

    Best,

    David