Review the challenges of designing an electronic control unit buck converter, from low quiescent current and low noise to high reliability.
High-end cars require close to a hundred electronic control units (ECUs), each taking power from the car battery with the intermediation of an on-board buck converter. As an example, an engine control unit is illustrated in Figure 1. Many ECUs must remain in standby mode even when the ignition key is off. Their standby currents add up, increasing the rate of discharge of the car battery. Accordingly, the quiescent current specification for these units is getting tougher to meet. The ECU buck converter must meet many other challenges inherent to the automotive environment. This article reviews the challenges of designing an ECU buck converter, from low quiescent current and low noise to high reliability.
Figure 1 Engine control unit
ECUs in the car
ECUs are small computing modules that control most of the systems in a modern car. They are connected via a controller area network (CAN) and control the car's engine, power windows, brakes, airbags, lights, entertainment system, steering functions, and more (Figure 2). To minimize the battery discharge, car manufacturers are specifying quiescent currents for ECUs as low as 100µA.
Figure 2 CAN-connected ECUs in a car
Powering the ECU
The block diagram of a typical ECU is shown in Figure 3. The on-board buck converter powers the MCU, CAN, and I/Os while interfacing directly to the battery. The buck converter must withstand the battery voltage, which can be as high as 14.7V on a fully charged battery. Vehicles employing start/stop technology experience large voltage dips when the engine starts, so the lower limit for the power source is well below the typical 12V and can be as low as 6V or lower. A high and well-controlled PWM switching frequency, above the AM band, is required to reduce radio frequency interference while spread spectrum is necessary to meet electromagnetic interference (EMI) standards. With only 100µA of quiescent current at the ECU disposal, every microamp spared by the on-board buck converter is one more microamp that is usable for the module’s microcontroller, memory, or CAN. Finally, a high-efficiency buck converter will reduce the ECU heat generation, improving its reliability.
Figure 3 ECU block diagram
Low quiescent current solution
With the ECUs in standby mode when the ignition key is off, the electronics standby currents add up, increasing the rate of discharge of the car battery. To reduce its quiescent current, the switching regulator, which normally switches at high frequency to reduce passives size, must now slow down, and enter skip mode. Skip mode allows the regulator to skip cycles when they are not needed, which greatly improves efficiency at light loads. With this mode of operation, the quiescent current is reduced to a few microamps as shown in Figure 4.
Figure 4 IQ vs. VIN curves
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Chintan Parikh is an executive business manager at Maxim Integrated focusing on automotive power management solutions. Chintan holds a master’s degree in Engineering Management from Santa Clara University.
Nazzareno (Reno) Rossetti, PhD EE at Maxim Integrated, is a seasoned analog and power management professional, a published author, and holds several patents in this field. He holds a doctorate in Electrical Engineering from Politecnico di Torino, Italy.
Pankaj Kashikar is an engineering director for high voltage power products in the automotive business unit of Maxim Integrated. He holds a master’s degree in Electrical Engineering from University of Colorado, Boulder and a bachelor’s degree from University of Mumbai, India.