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DRV8305: charge pump external load current

user3327517
Prodigy 30 points
Part Number: DRV8305

I'll design the bridge circuit to spin the BLDC in FOC, and I will plan to use DRV8305.
The driver needs to drive the 12 MOSFETs (each by 2 of High and low side) in total.
The Qg of MOSFET is 139nC per piece and switching frequency is 30kHz, so Gate Drive RMS Current is calculated 50mA.
Here's question. If the capacitance CPxH/L would be doubled  (0.047uF -> 0.1uF) , then can DRV8305 drive 50mA?

  • 0
    TI__Genius 14485 points
    Hi user3327517,

    The DRV8305 was not originally designed for this operating condition, nor have we tested this case during characterization. Because we do not have test data, we cannot guarantee the reliability of this device for operating conditions outside of those mentioned in the datasheet.
  • 0 in reply to James Lockridge19
    Prodigy 30 points
    Hi, James

    Thank you for your reply, and could you answer the another question?
    I consider to use two DRV8305 to drive the 12 MOSFETs. Is that OK?

    Regards
    HIsakazu
  • 0 in reply to user3327517
    TI__Genius 14485 points

    Hisakazu,

    If you are trying to drive two separate motors with separate triple half-bridges, then that should be fine.

    However, if you are trying to drive one motor with two high-side and two low-side FETs on each half bridge, then I would not recommend that. The two DRV8305 drivers may not be perfectly synchronized even if you provide the same control signals to both. Both FETs on the high side and low side may not turn on or off at exactly the same time. This can cause operation to be inefficient and shoot-through on the half bridge FETs may occur.

    If you are trying to drive only one motor, here are some suggestions:

    • Find FETs with a lower gate charge
    • Re-calculate RMS gate current depending on the commutation algorithm you are using. In the expressions below, I assume a single FET on the HS and LS, but you can double this quantity if there are two FETs on the HS and LS.
      • Asynchronous Trapezoidal: I_RMS = Q_G * 2 FETs * F_SW
      • Synchronous Trapezoidal: I_RMS = Q_G * 4 FETs * F_SW
      • Sinusoidal: I_RMS = Q_G * 6 FETs * F_SW
    • Double check to see if any of the gate drive current settings on the DRV8305 can still work for your application.

  • 0 in reply to James Lockridge19
    Prodigy 30 points

    Hi, James

    Thank you for your reply.

    I reconsidered your suggestion, but I have no way. So unfortunately I would use other device.

    Regards

    Hisakazu