Auto Electronics

A Battery of New Challenges for Voltage Regulators

The increasing amount of electronic circuitry in new cars is influencing the development of so many integrated circuits, even humble voltage regulators. While under-the-hood temperatures, varying battery voltages and transients, and general concerns about reliability have always presented challenges to developers of voltage regulators for automotive, these days the challenges are getting tougher.

For example, the input voltage range of a regulator has always been an issue because under cold-cranking conditions the battery voltage is going to drop. So, as one chip vendor explained, in the past it would have been sufficient for some customers if the voltage regulator operated down to 8 V and a regulator capable of 5 V operation would be deemed a good design. However, now automotive customers are requesting regulators that can operate at 4 V or even lower to be sure that critical electronics will operate reliably at all times.

Meanwhile, regulators also face challenges on the high end of their input range. Though the car uses a 14 V electrical system, the rating of the voltage regulator must account for the possibility of a double battery voltage (28 V) during a jump start. Then, there's transient suppression in the vehicle's electrical system, which must be above 28 V, to squelch the effects of load dump. Consequently, automotive customers may request a rating of 36 V, 40 V or even higher. Furthermore, if the same regulator is to be used in cars and trucks, then the input range of the regulator may need to be extended to 60 V or higher.

These requirements have been further complicated by demand for low power consumption. When the engine is shut off, the vehicle's electronics are not permitted to drain the battery. The example frequently cited is the case where the driver leaves his or her vehicle at the airport and then returns after a lengthy trip. The car has to start and this requirement helps the carmaker to formulate the spec for the total current drawn from the battery when the engine is off. There's also the requirement that certain circuits like remote keyless entry be kept alive. A typical requirement for an electronic control unit (ECU) is that its current draw be less than 100 µA in the key-off condition.

Some of the recently introduced switching regulators and linear regulators have been developed to meet these challenges, while also achieving other goals like small design size and low cost. One such part is the LT3481, which was unveiled by Linear Technology in May. This 2 A buck regulator employs the company's burst-mode operating technique to limit quiescent current to less than 50 µA typical, while maintaining very low output ripple — less than 15 mVpp regardless of output current. In shutdown IQ is less than 1 µA. This is a 36 V rated part capable of operating at inputs down to 3.6 V. At the same time, it allows for a compact design since it switches at up to 2.8 MHz and comes in a 10-pin MSOP.

In May, National Semiconductor introduced another buck regulator. Rated for 1.5 A output, the LM26001operates at input voltages ranging from 4 V to 38 V or at voltages as low as 3 V during cold-crank transients. National specifies the LM26001's quiescent current as 10 µA typical in shutdown and less than 40 µA typical in sleep mode. This TSSOP-housed device also offers 1.5% voltage accuracy.

There have also been linear regulators developed with the latest automotive criteria in mind. The MAX6791-MAX6796 family of single and dual 300 mA linear regulators from Maxim Integrated Products is an example. Designed for 5 V to 72 V input, these regulators consume 68 µA typical. In shutdown, IQ reduces to 27 µA typ.

Over time, we may expect expanded offerings of these types of regulators, which will combine the low input voltage capability and low current consumption with improvements in other areas — perhaps more integrated design, higher current ratings, or higher switching frequencies. But given the very specialized nature of these designs, it may be some time before second sourcing becomes one of the available options.

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