Infotainment: Right track

Start-stop application schematic operating at 2.1MHz

Bruce Haug, product marketing engineer for Analog Devices, explains how to power infotainment devices in automotive start-stop systems

Car makers continue to tout stop-start systems that help save fuel. As its name implies, a stop-start system shuts off the engine instead of letting it idle at a stop, and then rapidly restarts the engine once it is ready to drive away. For those who do a lot of stop-and-go driving, it reduces emissions and saves fuel by not idling the engine for extended periods of time.

The concept is simple. As an example, a car stopped at a red light or train crossing does not need the engine to be running; if the engine isn’t running, it is not wasting any energy. As a result, the reduction in fuel consumption can be as much as eight per cent in city traffic compared with a car without such a system.

Driving comfort and safety are not compromised by a vehicle start-stop function, since it is not activated until the engine has reached an ideal running temperature. The same applies if the air conditioner has not yet brought the cabin to the desired temperature, if the battery is not adequately charged or if the driver moves the steering wheel.

The start-stop function is coordinated by a central control unit that monitors data from all relevant sensors, including the starter motor and the alternator. If necessary for comfort or safety, the control unit will automatically restart the engine, for example if the vehicle begins to roll, the battery charge falls too low or condensation forms on the windshield.

Furthermore, most systems recognise the difference between a temporary stop and the end of the trip. It will not restart the engine if a driver’s seat belt is undone, or if the door or boot is open. If desired, the start-stop function can be completely deactivated with the press of a button, for now, at least.

However, when the engine restarts and there is an infotainment system turned on or another electronic device requiring greater than 5V, there is a possibility that the 12V battery can dip to below 5V, causing these systems to reset. Some navigation and infotainment systems operate from a 5V and higher input voltage. When the input voltage dips to below 5V during an engine restart, these systems will reset when the DC-DC converter only has the capability to step down the input voltage. Obviously, it is not acceptable to have a music player or the navigation system reset when the car restarts.

 

Controller

Thankfully, a triple output DC-DC controller is available that combines a boost controller and two step-down controllers in a single package. The high efficiency synchronous boost feeds the two downstream synchronous step-down converters, avoiding an output voltage dropout when the car battery voltage droops – useful in automotive start-stop systems. In addition, when the input voltage from the car battery is higher than its programmed boost output voltage, the boost controller runs at 100% duty cycle and simply passes the input voltage directly to the step-down converters, reducing power loss.

The diagram shows a schematic of such a device with the boost converter supplying 10V to the step-down converters. In addition to powering the two step-down converters, which produce 5V/7A and 3.3V/10A, the boost converter can be used as a third output that can provide an additional 2A. This circuit maintains 2.1MHz operation at up to 28Vin and skips cycles above 28V.

The device operates from an input voltage of 4.5 to 38V during start-up and maintains operation down to 2.5V after start-up. The synchronous boost converter can produce output voltages up to 60V and can run with the synchronous switch fully on to pass through the input voltage when it is high enough to increase efficiency.

The two step-down converters can produce output voltages from 0.8 to 24V, with the entire system achieving efficiency up to 95%. Its 45ns minimum on-time enables high step-down conversions while switching at 2MHz, thus avoiding critical noise-sensitive frequency bands such as AM radio and allowing for smaller external components.

It can be configured for burst mode operation, which reduces quiescent current to 28µA per channel (38µA when all three channels are on) while regulating the output voltage at no load, useful for preserving battery run times in always-on systems.

The 1.1Ω on-board all n-channel mosfet gate drivers reduce switching losses and provide output current of more than 10A per channel, limited only by external components. Furthermore, the output current for each converter is sensed by monitoring the voltage drop across the inductor (DCR) or by using a separate sense resistor. The constant frequency current-mode architecture enables a selectable frequency from 320kHz to 2.25MHz or it can be synchronised to an external clock over the same range.

 

Run times

Any battery-powered system that requires an always-on power bus while the rest of the system is turned off must conserve battery energy. This state is commonly referred to as sleep, standby or idle mode and requires these systems to have very low quiescent current. The need for low quiescent current to conserve battery energy is especially important in automotive applications that can have several electrical circuits such as telematics, CD and DVD players, remote keyless entry, and multiple always-on bus lines.

The collective current consumption of these systems during standby mode needs to be as low as possible and the pressure continues to mount for battery energy conservation as cars become more dependent on electronic systems for their operation.

The device in the diagram draws 28µA when in sleep mode with the boost converter and one of the buck converters on. With all three channels on and in sleep mode, it draws 20µA, which significantly extends battery run times when in idle mode.

This is done by configuring the part to enter high efficiency burst mode operation, where the device delivers short bursts of current to the output capacitor followed by a sleep period where the output power is delivered to the load by the output capacitor only.

In sleep mode, much of the internal circuitry is turned off except for the critical circuitry needed to respond quickly. When the output voltage drops enough, the sleep signal is activated and the controller resumes normal burst mode operation by switching on the top external mosfet. Alternatively, there are instances when the user will want to operate in forced continuous or constant frequency pulse skipping mode at light load currents. Both modes are easily configurable and have a higher quiescent current.

 

Efficiency and size

The efficiency for the 5V output as referenced in the schematic is about 90%. A three to four per cent higher efficiency can be attained if the operating frequency is reduced from 2.1MHz to 300kHz.

 

Protection

The device can be configured to sense the output current by using DCR (inductor resistance) or a sense resistor. The choice between the two current sensing schemes is largely a trade-off between cost, power dissipation and accuracy. DCR sensing is becoming popular because it saves expensive current sensing resistors and is more power efficient, especially in high current applications. The sense resistor, however, is a more accurate way of sensing current.

On-board comparators monitor the buck output voltages and signal an overvoltage condition when the output is greater than ten per cent of its nominal value. When this condition is sensed, the top mosfet is turned off and the bottom mosfet is turned on until the overvoltage condition is cleared. The bottom mosfet remains on continuously for as long as the overvoltage condition persists. If the output voltage returns to a safe level, normal operation automatically resumes.

At higher temperatures, or in cases where the internal power dissipation causes excessive self heating on-chip, the over-temperature shutdown circuitry will shut down the device. When the junction temperature exceeds approximately +170˚C, the over-temperature circuitry disables the on-board bias LDO, causing the bias supply to drop to 0V and effectively shutting down the entire device in an orderly manner. Once the junction temperature drops back to approximately +155˚C, the LDO turns back on.

 

Conclusion

Automotive start-stop systems allow for fuel savings that will continue to evolve over the next several years. Care must be taken with regards to powering on-board infotainment and navigation systems that need up to, or can exceed, 5V. These systems can reset when the car battery voltage droops to less than 5V with an engine restart.

The device described here boosts the battery voltage to a safe operating level. This, combined with two step-down controllers, makes it suitable for powering many automotive electronic devices in cars that have a start-stop system.

Bruce Haug is product marketing engineer for Analog Devices

www.analog.com

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