Automotive Power Management – A Look at Very Low Standby Currents

by Frederik Dostal, National Semiconductor

As more electronic components are introduced into the automobile for convenience, safety and over all efficiency, power management becomes more critical. The days of linear regulators as voltage supply for automotive electrical systems are almost over. Switch mode power supplies are more efficient but also more complex in many ways, especially in automotive applications.

One challenge is the reduction of power consumption during standby of these ever increasing electronic systems. How can all these electronic applications be implemented without draining the battery after a few days without recharging and leaving the driver stranded?

Loosing charge by the minute
Ideally all power devices in an automobile are completely turned off when the car is parked, keeping the car battery charged and ready until the car is driven again. The only energy loss in this circumstance is the self-discharge of the lead acid battery which is relatively small, about 5 to 20% per month.

Unfortunately energy consumers, such as key-less locking and theft protection systems, cannot be completely turned off due to standby requirements. And, applications such as navigation systems, camera systems, dashboard systems and many others are adding new requirements. The use in these applications is that some small current is constantly required to keep some memory values and other logic states alive.

Automotive manufacturers define different classes of current consumption for different systems while the car is parked. For many systems a maximum current of only 100uA is allowed. The current is typically averaged over a long time period such as 24 hours. This means that for some amounts of time more current may be consumed as long as there are time periods where the current consumption is less. In order to provide enough current to such needs as memory, clocks or sensors, the power conversion from the 12V automotive voltage supply to the required voltage rails (such as 5V, 3.3V or lower) needs to be as efficient as possible to stay within the car manufacturers’ constraints.

Reducing power consumption enormously
One solutions is switching regulators with a built in sleep mode designed for very low output power applications. Within this mode many parts of the switch mode power supply are turned off. The output voltage regulation resumes in the most efficient way possible. An example is National Semiconductor’s LM26003 step down voltage regulator wit integrated power FET. With loads lower than a few milli-amps, the PWM regulator switches to a burst mode. In this mode, the regulation loop detects if the output voltage is too low and the integrated N-FET switches for a few cycles until the output capacitor of the circuit is charged up again. Then the switching stops, the power consumption is very low, until the load current partially de-charges the output capacitor again.

Important things to consider
When designing for very low current consumption during low load operation, a special IC needs to be chosen. Power regulators not specifically designed for such applications have a relatively high quiescent current or even a minimum load requirement for the circuit to work properly.
Besides the power regulator, there are two elements that influence the current consumption greatly. One is the feedback resistor divider R1 and R2 and the other one is the free wheeling diode D1. Both of these currents are taken from the regulated output voltage.

The resistor divider will consume some of the output current. The larger the impedance of the divider, the lower is the current consumption. However, very large impedance values cause noise sensitivity on a power regulator’s feedback pin. If the feedback pin is a very high impedance node, system noise such as switching noise will couple into the regulation loop and the output voltage is not well regulated. Many automotive manufacturers have a guideline of how high the impedance of such nodes may be. A typical resistance limit is between 100k? and 200k?.

The diode D1 is usually a Schottky diode. This is necessary for switching efficiency, low electro magnetic interference (EMI) and protection of the switch pin of the power supply IC. Schottky diodes have almost no reverse recovery effect and reduce the switch node ringing dramatically. For low quiescent current applications however, Schottky diodes show quite a bit of leakage current that varies with different Schottky diode manufacturers and with operating temperature and reverse voltage. The higher the diode junction temperature and the higher the reverse voltage, the higher the leakage current.

It is important to keep the diode leakage current in mind when designing a very low power standby system and when selecting a Schottky diode. Luckily, many automotive manufacturers specify their average power consumption over 24 hours at relatively low temperatures so that the extreme leakage currents at hot temperatures are not relevant. Table 1 shows the leakage current of a typical 40V, 5A Schottky diode with different reverse voltages at room temperature.

How to measure the total current consumption
Since some regulators use burst mode to operate as efficiently as possible, when measuring the input current, averaging needs to be performed. Though some input capacitance does perform some averaging, it is not enough to get a clean DC input current. It is essential to use a current meter that gives very precise readings in the micro-amp range and also does good averaging over many samples. Besides researching the manuals of available digital multi-meters, it is a good idea to do some measurements on known loads to really see the capabilities of the measurement equipment. Different measurement equipment is likely to show much different results. After some research and evaluation, it will become clear which measurement equipment can be trusted for such a measurement.
   
The car battery must not be afraid
Until integrated regulators specifically for high efficiency low load automotive applications became available designers had to build suitable circuits with many discrete components. Now with integrated solutions, the design has been greatly simplified. As more and more electronics are employed in the automobile, design engineers will be able to enjoy the advantages of integrated solutions and their advanced efficiencies and power control.

Frederik Dostal joined National Semiconductor in 2001 and is a power managementapplications engineer at National's Design Center in Phoenix, Arizona. Dostal holds an Electrical Engineering Diploma from the Friedrich-Alexander-Universität in Erlangen and is a member of the IEEE.