Another high voltage event is jump-start. Some tow trucks use two batteries in series to assure effective jump starts to revive a dead car battery, so an automobile’s circuits must survive the doubled nominal battery voltage of 28 V for a couple of minutes. Many Power by Linear™ high voltage step-down regulators, such as the Silent Switcher® and Silent Switcher 2 families, including the LT8650S and LT8640S (Table 1) operate up to 42 V, exceeding this requirement. In contrast, lower voltage rated options would require a clamp circuit, adding cost and lowering efficiency. Some Power by Linear regulators, such as the LT8645S and LT8646S, are rated for 65 V to accommodate truck and airplane applications, where a 24 V system is the norm.
Table 1. Silent Switcher and Silent Switcher 2 Monolithic Buck Regulators for Automotive Applications
Another voltage transient occurs when a driver starts an automobile and the starter draws hundreds of amperes of current from the battery. This pulls down the battery voltage for a short period of time. In a traditional automobile, this happens only when the car is started by the driver—for instance, when one starts a car to drive to the supermarket and starts it again to drive back home. In modern automobiles with start-stop features to save fuel, start-stop events can occur a number of times on that supermarket trip—at every stop sign and every red light. The additional start-stop events put significantly more strain on the battery and starter than in a traditional automobile.
Furthermore, if a start event happens on a cold morning, the starter draws more current than at higher ambient temperatures, pulling the battery down to 3.2 V or lower for around 20 ms—this is called cold crank. There are functions that must remain active even in cold crank conditions. The good thing is that, by design, such critical functions typically do not require significant power. Integrated solutions, such as the LT8603 multiple channel converter, can maintain regulation even if their inputs drop below 3 V.
ISO 7637-2 and TL82066 define many other pulses. Some have higher positive or negative voltages but also higher source impedances. Those pulses have relatively low energy compared to the events described above, and can be filtered or clamped with proper selection of input TVS.
The active rectifier controller LT8672, featuring high input voltage rating (+42 V, −40 V), low quiescent current, ultrafast transient response speed, and ultralow external FET voltage drop control, provides protection in 12 V automotive systems with extremely low power dissipation.
Battery Reverse Polarity
Whenever the battery terminals are disconnected, there is a chance the car battery polarity is reversed by mistake and the electronic systems can be damaged from the negative battery voltage. Blocking diodes are commonly placed in series with supply inputs to protect against supply reversal, but blocking diodes feature a voltage drop, resulting in an inefficient system and reducing the input voltage, especially during a cold crank.