SLVAF66 June   2021 DRV3255-Q1 , DRV8300 , DRV8301 , DRV8302 , DRV8303 , DRV8304 , DRV8305 , DRV8305-Q1 , DRV8306 , DRV8307 , DRV8308 , DRV8320 , DRV8320R , DRV8323 , DRV8323R , DRV8340-Q1 , DRV8343-Q1 , DRV8350 , DRV8350F , DRV8350R , DRV8353 , DRV8353F , DRV8353R

 

  1. 1 Introduction to High-Power Motor Applications
    1. 1.1 Effects of a Poorly-Designed High-Power Motor Driver System
    2. 1.2 Example of the High-Power Design Process
  2. 2 Examining a High-Power Motor Drive System at a High Level
    1. 2.1 Anatomy of the Motor Drive Power Stage and How to Troubleshoot
    2. 2.2 Troubleshooting a High-Power System
  3. 3 High-Power Design Through MOSFETs and MOSFET Gate Current (IDRIVE)
    1. 3.1 MOSFET Gate Current
      1. 3.1.1 How Gate Current Causes Damage
      2. 3.1.2 Gate Resistors and Smart Gate Drive Technology
        1. 3.1.2.1 Gate Resistors
        2. 3.1.2.2 Smart Gate Drive and Internally-Controlled Sink and Source Gate Currents
        3. 3.1.2.3 Summary for Gate Resistors and Smart Gate Drive Technology
      3. 3.1.3 Example Gate Current Calculation for a Given FET
  4. 4 High-Power Design Through External Components
    1. 4.1 Bulk and Decoupling Capacitors
      1. 4.1.1 Note on Capacitor Voltage Ratings
    2. 4.2 RC Snubber Circuits
    3. 4.3 High-Side Drain to Low-Side Source Capacitor
    4. 4.4 Gate-to-GND Diodes
  5. 5 High-Power Design Through a Parallel MOSFET Power Stage
  6. 6 High-Power Design Through Protection
    1. 6.1 VDS and VGS Monitoring
      1. 6.1.1 Turning Off the FETs During an Overcurrent, Shoot-Through, or FET Shorting Event
    2. 6.2 Passive Gate-to-Source Pulldown Resistors
    3. 6.3 Power Supply Reverse Polarity or Power Supply Cutoff Protection
  7. 7 High-Power Design Through Motor Control Methods
    1. 7.1 Brake versus Coast
      1. 7.1.1 Algorithm-Based Solutions
      2. 7.1.2 External Circuit Solutions
      3. 7.1.3 Summary of Brake versus Coast
  8. 8 High-Power Design Through Layout
    1. 8.1 What is a Kelvin Connection?
    2. 8.2 General Layout Advice
  9. 9 Conclusion
  10. 10Acknowledgments

6.2 Passive Gate-to-Source Pulldown Resistors

Figure 6-2 Example Passive Gate-to-Source Pulldown Resistors

The primary purpose of the passive pulldown resistor is to ensure that there is a known relationship between the gate and source if the gate driver fails. Specifically, if the gate driver is stuck in a sink or source current state, or if the gate driver goes into a high impedance state, the resistor ensures there is a path to keep the FET from conducting.

The passive gate-to-source pulldown resistors offer a path for the charge to equalize the gate and source voltages so the FET turns off more quickly. In reality, if the gate driver was damaged, some other protection or commutation logic circuit notices there is a problem, and the system detects it. The importance of these pulldown resistors is to make sure the shoot-through condition does not occur before other protection circuits can identify a problem has occurred. This is the difference between replacing the gate driver IC to fix the system and dealing with a melted motor, blown FETs, or irreversible damage to the PCB.

It is important to note that some gate drivers have passive, hundreds of kΩ pulldown resistors integrated into the device to fill this protective role. However, some designers might want a stronger pulldown located near the gate and source of the FETs so the charge at the gate does not need to travel through a potential gate resistor and inductive traces to equalize the gate and source voltage. Another benefit is that the external pulldowns have no dependency on the gate driver which also helps in the context of adding redundancies to allow the system to fail in a known state.

As a final note, every pulldown resistor needs to be considered in the final power loss calculation. However, pulldowns usually contribute less than a milliwatt of total power dissipation which is much less than the tens of milliwatts produced by the RDS(on) or sense resistor. Remember that any current through these pulldown resistors must be accounted for when considering the capability of the VGLS, charge-pump, or bootstrap capability.

In summary:

  • External passive pulldown resistors provide a path for charge at the gate-to-move to the source so that a FET can turn off if the active pulldowns fail
  • These pulldowns range from tens of kilo-ohms to hundreds of kilo-ohms
  • These external passive pulldowns contribute a lot less power dissipation compared to major sources of loss in a gate driver circuit
  • Many gate drivers integrate the passive pulldown within the device