This paper presents a custom integrated circuit (IC) on which circuit functions necessary for “Active Hydraulic Brake (AHB) system” are integrated, and its key component, “Current-to-Digital Converter” for solenoid current measurement. The AHB system, which realizes a seamless brake feeling for Antilock Brake System (ABS) and Regenerative Brake Cooperative Control of Hybrid Vehicle, and the custom IC are installed in the 4th-generation Prius released in 2015. In the AHB system, as linear solenoid valves are used for hydraulic brake pressure control, high-resolution and high-speed sensing of solenoid current with ripple components due to pulse width modulation (PWM) is one of the key technologies. The proposed current-to-digital converter directly samples the drain-source voltage of the sensing DMOS (double-diffused MOSFET) with an analog-to-digital (A/D) converter (ADC) on the IC, and digitizes it. The conversion characteristic is compensated for the DMOS onresistance variation by an ADC reference compensation technique. A hybrid Active-Passive ΔΣ A/D converter is adopted and realizes small chip area and low power dissipation. A post-processing digital CIC (cascaded integrator-comb) decimation filter removes the ripple components which synchronize with the PWM switching efficiently. Through the design optimization of these, a 12-bit resolution and a response speed necessary for the solenoid current control have been achieved. The IC chip is fabricated in Toyota in-house SOI 0.35μm BiCDMOS process.
