What is a smart gate-drive architecture? - Part 1

Control, efficiency, protection … these are all terms you hear regarding new integrated circuits, but what do they really mean? While I can’t speak to all of the devices, I can talk about a new technology that Texas Instruments is introducing with its motor gate drivers. Our motor gate drivers for brushed DC, stepper and brushless DC motor applications are using a new architecture called smart gate drive. In this blog series, I’ll give a quick overview about what it is, which motor gate drivers are using it, and where you can learn more.

TI’s smart gate drive architecture is a combination of protection features and gate-drive configurability provided by the gate driver itself through two features called IDRIVE and TDRIVE. In the first installment of this two-part series, I’ll give an overview on IDRIVE. In the next installment, I’ll cover TDRIVE.

IDRIVE is the ability to dynamically adjust the gate driver’s output drive current. Getting into the specifics of how this works is outside the scope of this series, but you can refer to this application report about IDRIVE/TDRIVE to learn how gate-drive current affects the power MOSFET. In short, IDRIVE enables control of the MOSFET VDS slew rate, an important parameter in switching power designs, through a simple register write or analog voltage setting. Figure 1 shows this feature in action. The figure is a persistence scope capture of the VDS slew rate on the DRV8305-Q1EVM, ranging from 10-70mA of drive current.

Figure 1: DRV8305-Q1 IDRIVE example

MOSFET VDS slew rate is important because it is a key parameter for achieving optimal switching efficiency and minimal parasitic effects. Switching efficiency as it relates to slew rates is relatively well understood, but many of the parasitics effects are not as obvious. Two common parasitic side effects related to MOSFET VDS slew rate are switch-node ringing and electromagnetic interference (EMI).

Switch-node ringing occurs due to high dV/dt (slew rates) and parasitic inductances and capacitances of the power MOSFETs and PCB layout. This ringing can cause the switch-node voltage to drop below ground or rise above the supply, often violating specifications of the gate driver or power MOSFETs and thus leading to catastrophic breakdowns. Figure 2 shows an example of switch-node ringing (in yellow) causing a large negative spike and violating the gate driver’s absolute maximum rating.

Figure 2: Switch-node ringing example

While you can tackle switch-node ringing with external components such as Schottky diodes or resistor/capacitor (RC) snubbers, often the best method is to adjust the VDS slew rate and reduce the dV/dt component of the equation. IDRIVE gives you the ability to quickly make this decision in the design phase and ensure that it remains constant over the system’s lifetime.

Another subtle problem faced in switching power designs is EMI, often attributed to switch-node ringing but with a different failure mode. Instead of violating an absolute maximum rating, the switch-node ringing introduces high-frequency components that radiate to nearby components and systems. This can show up in compliance testing when a product exceeds acceptable interference levels.

Figure 3 shows another example of switch-node ringing, but this time the high-frequency oscillation and its harmonics are translating into higher RF emission levels, as shown in Figure 4.

Figure 3: Switch-node ringing EMI example

Figure 4: EMI scan example

Adjusting IDRIVE enables modification of the MOSFET slew-rate and removes the high-frequency ringing, as shown in Figure 5. Figure 6 shows the corresponding RF scan with largely reduced RF emission levels.

Figure 5: Switch-node ringing removed

Figure 6: EMI scan ringing removed example

 

With these topics in mind, Texas Instruments has just released the DRV8305-Q1 brushless DC motor gate driver with smart gate drive, designed specifically for automotive applications such as pumps, valves, fans and more. Automotive applications are the perfect fit for a smarter gate driver due to high reliability and stringent electromagnetic compliance (EMC) requirements. To learn more, check out the DRV8305-Q1 data sheet.

In my next post, I’ll cover TDRIVE, the other half of TI’s smart gate drive architecture, and how it makes motor systems more reliable and efficient.

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