Gate-Driver IC for Automotive Three-Phase Brushless Motor: TB9083FTG

Toshiba Electronic Devices & Storage Corporation ("Toshiba") has launched “TB9083FTG,” a gate driver IC^[3] for automotive three-phase brushless DC motors compliant with ISO26262 and supporting safety level ASIL-D^[4]. The device drives external MOSFETs to a rotate three-phase brushless DC motor (Figure 1) and can drive the upper and lower MOSFETs for all three-phases, i.e. six MOSFETs. Examples of applications for which the device is suitable are Electric Power Steering or EPS, Electric brakes and Shift-by-Wire.

In August 2017, Toshiba previously launched “TB9081FG”, a gate driver compliant with ISO 26262. TB9083FTG was developed by improving “TB9081FG”.
The key improvements are robust functionality through redundancy, ABIST (Analog Built-in Self-test) and LBIST (Built-in Self-test) circuits. These are indispensable for compliance with ISO 26262. In terms of redundancy of functions, two band gap reference voltage circuits are built in as in the TB9081FG, and two clock oscillation circuits (OSC) are newly built in the TB9083FTG. The purpose is to minimize the occurrence of dependent failures. On the other hand, ABIST and LBIST take a role to check whether internal diagnostic functions operate properly. These check results can be read out via SPI communication.  

There are three major improvements made to the TB9083FTG. First is, the inclusion of only three channels of safety relays compared to the five in TB9081. Safety relays take a role to electrically cut off abnormal point of motor or external MOSFET and so on. Based on customer feedback, it was determined that three would be enough to meet their system requirements. In the case of EPS, two channels could be applied to the power supply relay and the reverse polarity protection relay and the other channel could be applied to the motor relay to cut off the control circuit from the motor. (Figure 2) 

Second feature is the adoption of a VQFN 48pin package with a small PCB mounting area of 7.0mm x 7.0mm (Table 1). Since TB9081FG was a 64-LQFP with 12.0mm x 12.0mm mounting area which includes outer leads, the area is reduced by around half. Selection for the small package is deeply related to ISO 26262, because ISO 26262 might require redundant devices for robust functionality in order to ensure the higher safety level. This redundancy enables the case that a motor can be driven by the other TB9083FTG even if one TB9083FTG has a failure. However, using two devices doubles the mounting area on the PCB. TB9083FTG prevents the increase by adopting a small package. 

In addition, to further reduce required PCB area, the TB9083FTG has built-in resistors to set the gain and the reference voltage of the motor current sense amplifier. These resistor was required externally in TB9081FG. The gain can be set via the serial interface SPI. 

The third feature is a built-in sequencer and verification circuit to detect open and short faults of the external MOSFETs. TB9083 monitors midpoints of the upper and lower arms of each of the three phases (Figure 3). The microcontroller can read out the diagnostic results via SPI. To implement the same diagnostics, the TB9081FG needed to monitor the motor terminal voltage with a built-in A-D converter (ADC) in the microcontroller and external bias resistors. In addition to eliminating the need to use an A-D converter in the microcontroller (MCU) for this diagnosis, it also has the advantage of reducing the amount of software code. It also eliminates the need for external resistors for diagnostics.

The adopted QFN package employs a wettable flank structure^[5]. This enables visual inspection of solder joints using an automatic optical inspection (AOI) system, and contributes to improved solder joint reliability. In addition, Toshiba has verified the package through 3,000 cycles in the mounting temperature cycling test.

Toshiba continues to strengthen our line-up of ISO 26262 2^nd edition capable gate-driver IC for automotive three-phase brushless DC motor applications Toshiba will continue to contribute to the electrification and safety-improvement of automotive equipment by bringing to market products with optimized functions and performance built into the ICs to meet customer needs.