With the increasing use of start-stop vehicle technology (which automatically turns off the engine while it is idling), more and more automotive systems must operate at low input voltages. These low input voltages occur during a warm crank (when the battery voltage can drop as low as 6V) or a cold crank (when the battery voltage can drop as low as 3V).
This article illustrates an intermediate, 8V switching power supply that withstands the complete automotive input voltage range (including cold crank and load dump conditions). The power supply assures a stable 8V supply for common subsystems such as a CD driver, LCDs, and a radio module in modern infotainment systems. To avoid disturbance in the AM and FM bands, the switching power supply runs at a fixed frequency of 2MHz, enabling an ideal solution for radio systems.
Importance of Low Input Voltage Functionality and EMI Requirements
Figure 1 illustrates common automotive systems that require different architecture solutions.
In systems where the main power supply is 3.3V, a front-end buck converter with low dropout voltage may be sufficient (Case 1). Additionally, a boost converter can operate off the 3.3V to regulate 5V (e.g., for the CAN transceiver) or other higher voltages (Case 2). Systems operating with 5V or higher voltage rails require a front-end "preboost" to ensure that the input voltage to a buck converter never falls below a specific voltage (Case 3).
Figure 1. Automotive power-supply solutions.
Low electromagnetic radiation (EMI) is also a critical requirement for automotive power supplies, especially in the sensitive AM band. The design presented here meets this challenge by operating the switching converters above the AM band, i.e. at frequencies guaranteed to be above 1.71MHz (the high end of the Medium Wave band). Operating switching converters at high frequencies also reduces the size and cost of external passive components.
Essential Design Parameters for the Automotive Switching Power Supply
Figure 2 shows the switching power-supply schematic. It incorporates a 4.5V to 40V step-up controller (IC1) and a 36V step-down controller (IC2), with additional circuitry for proper operation. Both ICs are synchronized with an external 2MHz square-wave logic signal, provided by a microcontroller or dedicated IC. This approach allows great flexibility in choosing the optimal switching frequency for the power supply.
During normal battery conditions, IC1 is disabled and IC2 regulates the 8V on the OUTB node. When the battery voltage decreases during cold crank, IC1 is enabled and boosts the voltage on the OUTA node. This allows IC2 to regulate the 8V on the OUTB node. Because of the robustness of both ICs, the entire design can survive up to a 40V automotive load dump. The system has been set up and tested to provide 20W of power (8V at 2.5A) on its main output (OUTB), although the external components can be modified to reach even higher output power. (See discussion about optimizing external components for both IC1 and IC2 below.)
Figure 2. The switching power-supply schematic features a step-up
boost controller (IC1, MAX15005) and a step-down controller (IC2,