An effective way to save fuel in cars: the "start/stop" system

The Auto Start/Stop function helps reduce fuel consumption and emissions by shutting off the engine and automatically restarting it every time the car comes to a complete stop. In city traffic, fuel consumption can be reduced by up to 8% compared to cars not equipped with this system. An added benefit is lower CO2 emissions.

The principle is simple: if the engine is not running, no fuel is consumed. The Auto Start/Stop system automatically switches the engine off when it is not needed. In traffic jams or even in stop-and-go traffic, simply put the car in neutral and take your foot off the clutch to activate the function. A Start/Stop message in the information display will indicate that the engine has been switched off. To restart the engine, depress the clutch, engage gear, and the car will quickly resume operation and you can continue driving in no time.

The Auto Start/Stop function does not affect driving comfort and safety. For example, the function will not be activated until the engine has reached a certain ideal operating temperature. The same principle applies if the air conditioning has not yet adjusted the cabin to the desired temperature, the battery is not fully charged or the driver turns the steering wheel.

The auto start/stop function is coordinated by a central control unit that monitors the data from all relevant sensors, including the starter motor and alternator. The control unit can also automatically restart the engine if comfort or safety requires it. For example: if the vehicle starts moving, the battery charge level drops too low or condensation forms on the windscreen. In addition, most systems can distinguish between a brief stop and the end of the journey. The system will not restart the engine if the driver's seat belt is unfastened or the doors or trunk are open. If necessary, the auto start/stop function can be completely deactivated at the touch of a button.

However, when the engine is restarted and an infotainment system is on or any other electronic device that requires more than 5V, the 12V battery may drop below 5V, causing such systems to reset. Some infotainment systems operate with an input voltage of 5V and 8.5V, which is fed by a buck converter that runs off the car battery. If the input voltage drops below 5V during an engine restart, these systems will reset when the DC/DC converter can only step down the input voltage. Obviously, it would be unacceptable for users to have these audio-visual systems automatically reset every time the car is restarted while watching a video or listening to a CD.

A new solution

Fortunately, Linear Technology has introduced a triple-output DC/DC controller, the LTC3859A, which integrates a synchronous boost controller and two synchronous buck controllers in a single package. The synchronous boost converter output feeds the buck converter to maintain a high enough voltage to prevent electronic systems that require an operating voltage above 4V from resetting during the engine restart process. In addition, when the input voltage from the car battery to the boost converter is higher than its programmed output voltage, it will operate under 100% duty cycle conditions and simply pass the input voltage directly to the buck converter, thereby minimizing power losses. Figure 1 shows the schematic diagram of the LTC3859A, where the synchronous boost converter provides 10V to the synchronous buck converter when the battery voltage drops below 10V. In addition to powering the two buck converters (5V/5A and 8.5V/3A in this case), the boost converter can also be used as a "third output" to provide an additional 2A output.

Figure 1: Typical LTC3859A start/stop application schematic

The LTC3859A is a low quiescent current, current mode control, triple output synchronous DC/DC controller using all N-channel MOSFETs. At startup, the LTC3859A operates from an input voltage range of 4.5V to 38V and remains operational down to 2.5V after startup. The two step-down controllers (Channels 1 and 2) operate 180° out of phase and can generate output voltages from 0.8V to 24V, ideal for powering navigation, infotainment systems, processors and memory. The boost controller (Channel 3) operates in phase with Channel 1 and can generate output voltages up to 60V. Powerful 1.1Ω built-in gate drivers for each channel minimize MOSFET switching losses. The operating frequency can be set from 50kHz to 900kHz or synchronized to an external clock with a frequency range of 75kHz to 850kHz using an internal phase-locked loop. The LTC3859A differs from the LTC3859 in that it has an internal clamp circuit on the INTV _{CC pin. This clamp circuit provides a fail-safe means to protect the INTV CC} pin from excessive voltage if the user inadvertently uses a leaky Schottky limiting diode .

Other features of the device include built-in LDOs for IC power and gate drive, programmable soft-start, power good signal and external V _CC control. V _REF accuracy is ±1% over the -40°C to 85°C operating temperature range, and the LTC3859A is available in a 38-lead SSOP package or a 38-lead 5mmx7mm QFN package.

Extend battery life

Conserving battery energy is a necessity for any battery-powered system that requires an "always-on" power bus while the rest of the system is shut down. This state is often referred to as "sleep," "standby," or "idle" mode and requires only very low quiescent current from the system.

Low quiescent current is particularly important in automotive applications to conserve battery energy. In standby mode, the total current consumption of such systems must be as low as possible, and as cars rely more and more on electronic systems to operate, the pressure on automakers to conserve battery energy continues to increase.

In sleep mode (boost and one of the two buck converters are on), the LTC3859A draws only 75μA. When all three channels are on and in sleep mode, the LTC3859A draws only 100μA, significantly extending battery run time in idle mode. This is achieved by configuring the device to enter a high-efficiency Burst Mode operation, in which the LTC3859A delivers a brief pulse of current to the output capacitor, followed by a sleep cycle where only the output capacitor delivers output power to the load. Figure 2 shows a conceptual timing diagram that illustrates how it works.

Figure 2: LTC3859A Burst Mode Operation Voltage Plot

The Burst Mode output ripple is independent of the load, the only thing that will vary is the length of the sleep interval. In sleep mode, most of the internal circuitry is turned off, except for critical circuits for fast response, further reducing its quiescent current. When the output voltage drops enough, the sleep signal level goes low and the controller resumes standard Burst Mode operation by turning on the top external MOSFET. On the other hand, there are cases where the user wants the device to operate in forced continuous mode or constant frequency pulse skipping mode at light load currents. Both modes are easy to configure and have higher quiescent current and lower peak-to-peak output ripple.

Load Dump / Efficiency / Solution Size

The term “load dump” refers to the inductive surge that occurs after the starter motor is turned off. For an automotive 12V lead-acid battery system, this surge voltage is typically clamped at 36V (maximum). This surge requires the controller, MOSFETs, and associated components to operate at the clamping voltage. These higher voltage devices (e.g., 40V MOSFETs) cause efficiency degradation, and care must be taken to minimize this undesirable effect. When using the circuit in Figure 1, the efficiency of each voltage rail is greater than 92% (as shown in Figure 3). For clarity, the efficiency of each buck and boost section is shown separately. In addition, Figure 4 shows the layout and size of this circuit, with the tallest component reaching 4.8mm.

Figure 3: LTC3859A efficiency vs. load current (for different converter sections)

Figure 4: Dimensions and layout of the LTC3859A demo board. (a) Top side (b) Bottom side.

Startup and shutdown

The three channels of the LTC3859A can be shut down individually using the RUN1, RUN2 and RUN3 pins. Pulling any of these pins below 1.2V shuts down the main control loop for the corresponding channel. Pulling all three pins below 0.7V disables all controllers and most internal circuitry, including the built-in LDO. In this state, the LTC3859A draws only 8μA of quiescent current.

Soft start or tracking

The TRACK/SS1 and TRACK/SS2 pins of the two buck controllers can be used to adjust the soft-start on-time or to perform “coincident” or “ratiometric” tracking of two or more supplies during startup. These correlation curves are shown in Figure 5 with a resistor divider placed between the TRACK/SS pins of the master and slave supplies.

Figure 5: LTC3859A output voltage tracking: (a) coincident tracking (b) ratiometric tracking

Protection function

The LTC3859A can be configured to sense the output current using either DCR (inductor resistance) or a sense resistor. The choice of the two current sensing schemes depends largely on a trade-off between cost, power consumption and accuracy. DCR is becoming increasingly popular because it eliminates the need for expensive current sense resistors and is more efficient, especially in high current applications. The LTC3859A has a current foldback feature for the buck channel to help limit the load current when the output is shorted to ground.

An internal comparator monitors the buck output voltage and indicates an overvoltage condition when the output is greater than 10% of its nominal output voltage. When this condition is detected, 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 as long as the overvoltage condition persists. Normal operation automatically resumes if the output voltage returns to a safe level.

At higher temperatures, or when internal power dissipation causes excessive self-heating within the chip, the thermal shutdown circuit shuts down the LTC3859A. When the junction temperature exceeds approximately 170°C, the thermal protection circuit disables the built-in bias LDO, causing the bias supply to drop to 0V and effectively shutting down the entire LTC3859A in an orderly manner. Once the junction temperature drops back to approximately 155°C, the LDO turns back on.

in conclusion

Fuel-saving automotive start/stop systems will continue to evolve in the coming years. Care must be taken to power in-car infotainment and navigation systems, as well as disk drives that require voltages up to or even above 5V to operate correctly. Such systems reset when the input voltage drops out of regulation due to engine restart. The LTC3859A provides a solution that boosts the battery voltage to a safe operating level using its built-in synchronous boost controller. The LTC3859A combines a synchronous boost controller with two synchronous buck controllers, making it ideal for powering a wide range of automotive electronics, maintaining regulation for all output voltages when the engine is restarted.