Explore the Benefits of Multi-Channel Gate Drivers in Automotive Power Seats

    Unless you're on a spaceship, a power car seat is probably the most sophisticated chair you've ever sat in. More adjustable than an airplane seat—and more comfortable than a dentist's chair—power car seats offer luxurious comfort, convenience, and safety.

    Whether we're moving the seat up or down, forward or backward, or adjusting the lumbar support, motors make these operations effortless. Besides the ease of movement, powerful electric car seats offer other advantages. For example, in-seat functions like fans and heaters can even extend the vehicle's range by reducing the electrical load on the overall cabin climate system.

    Power Seat Design Trends

    Given the plethora of power seat options available, manufacturers are looking for ways to power multiple motors. Designers have traditionally used relays to switch motor power, but relays consist of several tiny mechanical parts and have limited speed capabilities. These shortcomings, combined with the noise generated by relays each time they switch, make them unsuitable for pulse-width modulation (PWM) control of motor speed.

    There is a trend away from electromechanical relays and toward integrated circuits (ICs) given their advantages, including smaller size, quiet operation, speed control, and reliability. The fast, silent switching of ICs allows for speed control using PWM to provide smooth and synchronized motion of multiple motors.

    Relay-driven seats typically use only one relay for each motor, and the first IC designs followed this pattern. With only a few motors, the complexity penalty was tolerable. However, combining the drive circuitry for multiple motors into a fully functional car seat offers significant benefits, including a reduced bill of materials and smaller board space due to the design approach of using multi-axis integrated drives.

    Multi-channel gate driver solutions

    TI offers multi-motor automotive gate drivers designed specifically for power seats. The DRV8714-Q1 and DRV8718-Q1 are four- and eight-channel half-bridge gate drivers, respectively. They integrate a charge pump, current sense amplifiers, and logic for multiple loads. A single IC can control up to seven different motors or a combination of motors and heaters, reducing the number of components required in the seat module.

    Figure 1 shows the DRV8714-Q1 controlling three motors using a shared half-bridge. Any motor can be operated individually or in specific combinations simultaneously. Metal-oxide semiconductor field-effect transistors (MOSFETs) can be selected to best match the motor characteristics, and the DRV8714-Q1's registers can be adjusted to optimize performance.

    Figure 1: A three-motor seat driven by the DRV8714-Q1 automotive gate driver

    Increased flexibility

    With up to 8 channels of half-bridge drivers, the DRV8718-Q1's flexibility enables driving bidirectional motors, unidirectional motors (such as fans), and non-motor loads such as heaters using the correct number of MOSFETs, each tailored for a specific load. 

    Figure 2 shows a DRV8718-Q1 controlling four seat motors, a fan, and a heater. The fan (unidirectional rotation) requires only one half-bridge, while the heater requires only one high-side MOSFET, reducing the number of external transistors—all thanks to the flexibility of the multi-channel gate driver.

    Figure 2: The DRV8718-Q1 automotive gate driver drives motors, heaters, and fans.

    Because each external MOSFET can be flexibly selected to best suit the corresponding load, low drain-source on-resistance MOSFETs can be used to improve thermal performance in high-current applications. The DRV8718-Q1 series controls the switching slew rate by adjusting the gate current, thereby improving electromagnetic compatibility. We can also adapt our seat module to various motors by changing the MOSFETs without changing the overall design.

    Simplified interface

    Another advantage of a single chip is that it significantly reduces the number of signals to the microcontroller (MCU). With only four pins (using the serial peripheral interface [SPI]), the MCU can control up to eight half-bridges, or even more channels in a daisy-chain configuration, from a single DRV8718-Q1, as shown in Figure 3. This control includes setting parameters for each half-bridge, independent or coordinated control of the transistors, and diagnostic monitoring.

    Figure 3: Controlling 16 half-bridges using a daisy-chain configuration from an MCU to the DRV8718-Q1 automotive gate driver

    Unified control reduces the MCU's workload. With a single command, the MCU enables all drivers. By reading a single register, the MCU can determine if any motors have failed. Subsequent SPI reads can identify the fault channel and detailed information, if necessary.

    in conclusion

    Power seats in cars continue to offer greater comfort and convenience. However, the motors that drive them present several design challenges. Multichannel gate drivers help address these challenges by integrating many channels into a single chip, along with features such as diagnostics and current-sense amplifiers. The DRV8718-Q1 multichannel gate driver provides a flexible, compact solution to accommodate the latest features in car seats.