Power supplies for automotive Start /Stop systems

September 17, 2013 // By Mark Scholten
Mark Scholten of ON Semiconductor looks at various solutions available to the designer, including low dropout linear regulators, reverse battery protection schemes, and different boost options for Start/Stop systems.

In order to curb fuel consumption, many automobile makers are implementing the “Start/Stop” function into their next generation of vehicles and significant numbers of such vehicles are already on the road. These systems turn off the engine when the vehicle comes to a stop and then automatically turn it back on when the foot is moved from the brake pedal to the accelerator pedal – or, in the case of a manual transmission, when the clutch pedal is depressed to re-engage gear. This helps reduce fuel consumption in city driving and stop-and-go rush hour traffic.

Such systems introduce some unique engineering challenges to the vehicle’s electronics however, since the battery voltage can drop to 6.0 V or lower when the engine re-starts. In addition, typical electronic modules have a reverse polarity diode included to protect the electronics in the event the car is jump-started (bopsted) with the jumper cables accidentally reversed. This diode causes another 0.7 V drop in the battery voltage, leaving 5.3 V or less for down-stream circuitry. With many modules still requiring a 5 V supply, power sources are simple running out of headroom to operate properly.

BOOST SUPPLY
One approach is a boost supply. This takes a lower input voltage and creates a higher voltage on the output. Many suppliers are currently employing some type of boost supply at the front end of electronic modules in order for them to operate properly through dropout conditions caused by the Start/Stop system.

As with most engineering problems, there are a number of ways to solve an issue. If the battery voltage only drops down to  6 V at the input, then the first and simplest solution is to find an extremely low drop-out linear regulator that requires less than 0.3 V of headroom. This can work for modules with lower current requirements, but one soon runs out of options for modules with higher current needs.

An alternative approach

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