A more elegant approach is to use the LT8672 active rectifier controller, which can toggle an N-channel MOSFET fast enough to rectify the input voltage at frequencies up to 100 kHz. An active rectifier controller is an ideal diode controller with two important additions: a large reservoir of charge boosted up from the input voltage and a powerful gate driver to toggle the N-channel MOSFET both on and off quickly. This method can reduce power loss by more than 90% compared to a Schottky. The LT8672 also protects downstream circuitry from a reverse battery condition just as an ideal diode controller would.
The engine starting profile (ISO 16750-2: test 4.6.3) is an extreme undervoltage transient, sometimes referred to as the cold crank pulse because the worst-case battery droop occurs at lower temperatures. Specifically, when the starter turns over, the 12 V battery voltage can momentarily drop to 8 V, 6 V, 4.5 V, or 3 V, depending on severity level classification (I, IV, II, III, respectively).
In some systems, a low dropout (LDO) linear regulator or switching buck regulator is sufficient to allow power rails to ride through this transient, provided that ECU voltages are less than the lowest input voltage. For example, if the highest ECU output voltage is 5 V, and it must satisfy severity level IV (minimum input voltage of 6 V), then a regulator with a dropout voltage less than 1 V is enough. The lowest voltage segment of an engine starting profile lasts for only 15 ms to 20 ms, so a rectifying element (Schottky diode, ideal diode controller, active rectifier controller) followed by a large bypass capacitor may be able to ride through this portion of the pulse if the voltage headroom briefly dips below the regulator dropout.
If, however, the ECU must support voltages higher than the lowest input voltage, then a boost regulator is required. Boost regulators can efficiently maintain a 12 V output voltage from inputs less than 3 V at high current levels. There is one problem with a boost regulator, though: the diode path from input to output prevents disconnect, so current is not naturally limited at startup or due to a short. To prevent current runaway, specialized boost regulators, such as the LTC3897 controller, incorporate a surge-stopper front end to allow output disconnect and current limiting, as well as provide reverse voltage protection when back-to-back N-channel MOSFETs are used. This solution can address the load dump, engine start, and reverse battery conditions with a single integrated circuit, but available current is limited by the SOA of the surge-stopper MOSFET.
A 4-switch buck-boost regulator lifts this constraint by combining a synchronous buck regulator and a synchronous boost regulator through a shared inductor. This approach can satisfy both the load dump and engine starting profile tests without MOSFET SOA limitations on current level or pulse duration, while retaining the ability to disconnect the output and limit current.