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Automakers have long touted start-stop systems as fuel-saving devices. As the name suggests, a start-stop system shuts off the engine when the car is stopped, instead of letting it idle, and then quickly restarts it when needed. If your driving involves frequent stop-and-go traffic, avoiding prolonged engine idling reduces emissions and saves fuel. The concept is very simple. For example, if you stop at a red light or when a train is approaching, the engine shouldn't run; if the engine doesn't run, no energy is wasted. Fuel consumption in urban traffic can be reduced by up to 8% compared to cars without such a system.
Driving comfort and safety are not affected by the automatic start-stop function, as this function only activates when the engine reaches the ideal operating temperature. It will also not activate if the air conditioning has not yet reached the desired cabin temperature, the battery is not fully charged, or the driver is still turning the steering wheel.
The automatic start-stop function is coordinated by a central control unit that monitors data from all relevant sensors, including the starter motor and alternator. If comfort or safety requires, the control unit will automatically restart the engine—for example, if the wheels start to turn, the battery level drops too low, or condensation forms on the windshield. Furthermore, most systems can recognize the difference between a temporary stop and the end of a trip. The system will not restart the engine if the driver's seatbelt is unfastened, or if a door or trunk is opened. If needed, the automatic start-stop function can be completely disabled by pressing a button (at least for now).
However, when the engine restarts, the 12V battery may have dropped below 5V. If the infotainment system or other electronic devices require a voltage higher than 5V, this could cause these systems to reset. Some navigation and infotainment systems operate with an input voltage of 5V or higher. If the input voltage drops below 5V during engine restart and the DC-DC converter only has input voltage step-down functionality, these systems will reset. Clearly, a music player or navigation system resetting when the car restarts from a start-stop position is unacceptable.
Solution
Analog Devices (ADI) recently introduced the PowerbyLinear® LTC7815, a three-output DC-DC controller that integrates a boost controller and two buck controllers in a single package. The high-efficiency synchronous boost converter feeds two downstream synchronous converters, preventing output voltage drops when the vehicle battery voltage decreases—a highly useful feature in automotive start-stop systems. Furthermore, when the vehicle battery input voltage is higher than its programmed boost output voltage, the boost controller operates at 100% duty cycle, passing only the input voltage directly to the buck converter, thus minimizing power consumption.
Figure 1 shows the schematic of the LTC7815 boost converter supplying 10V to the buck converter. In addition to powering the two buck converters (producing 5V/7A and 3.3V/10A respectively), the boost converter can also be used as a third output, providing an additional 2A of current. The circuit maintains an operating frequency of 2.1MHz up to 28VVIN and skips cycles above 28V.
The LTC7815 can operate with an input voltage from 4.5V to 38V during startup and remains operational after startup until the input voltage drops to 2.5V. The synchronous boost converter can produce an output voltage up to 60V, and when the input voltage is high enough, it allows the synchronous switch to be fully turned on to pass the input voltage for maximum efficiency. Two buck converters can produce an output voltage from 0.8V to 24V, and the entire system achieves an efficiency of up to 95%.
The shortest on-time of 45ns enables high buck ratio switching in 2MHz switching operation, thus avoiding noise-sensitive critical frequency bands (such as AM radio) and allowing the use of smaller external components.
The LTC7815 can be configured for Burst Mode® operation, reducing quiescent current to 28μA per channel (38μA with all three channels on) while regulating output voltage under no-load conditions. This feature is useful for saving battery runtime in continuously on systems. A powerful 1.1Ω built-in all-N-channel MOSFET gate driver minimizes switching losses and provides over 10A of output current per channel, limited only by external components. Furthermore, output current sensing for each converter is performed by monitoring the voltage drop across an inductor (DCR) or using a separate current-sensing resistor. The LTC7815's constant-frequency current-mode architecture offers selectable frequencies from 320kHz to 2.25MHz, or can be synchronized to an external clock of the same range.
Figure 1. Schematic diagram of LTC7815 start-stop application, operating frequency is 2.1MHz.
Figure 2. Burst mode operating voltage diagram of LTC7815.
Extend battery life
Any battery-powered system that requires a constantly active power bus when the rest of the system is off must conserve battery energy. This state is often referred to as sleep, standby, or idle mode and demands very low quiescent current. This requirement for low quiescent current to conserve battery energy is particularly important for automotive applications, which may include multiple electrical circuits such as telematics, CD/DVD players, remote keyless entry, and multiple always-on bus circuits. The total current consumption of these systems in standby mode must be as low as possible, and the pressure to conserve battery energy continues to grow as automotive operation becomes increasingly reliant on electronic systems.
In sleep mode, the LTC7815 consumes only 28 μA of current when the boost converter and one of the buck converters are on. With all three channels in sleep mode, the LTC7815 consumes only 20 μA, significantly extending battery runtime in idle mode. This is achieved by configuring the LTC7815 in efficient Burst mode, where the device provides a short burst of current to the output capacitor before entering sleep mode, during which time output power is delivered to the load solely through the output capacitor. Figure 2 shows a conceptual timing diagram for this operating mode.
In sleep mode, most of the internal circuitry is shut down, except for the critical circuitry required for rapid response. When the output voltage drops sufficiently to activate the sleep signal, the controller resumes normal Burst mode operation by turning on the external MOSFET at the top. Alternatively, in some cases, users may prefer to operate in a forced continuous or constant frequency pulse-skipping mode under light load current. Both modes are easy to configure and offer higher quiescent current.
Efficiency/Solution Size
The 5V output efficiency shown in Figure 1 is approximately 90% (as shown in Figure 3). If the operating frequency is reduced from 2.1MHz to 300kHz, the efficiency can be improved by 3% to 4%.
Figure 3. Efficiency and load current of LTC7815 in different converter sections.
Figure 4 shows the LTC7815 demonstration board (schematic diagram shown in Figure 1), with the widest part being 48mm.
Figure 4. Dimensions and layout of the top and bottom layers of the LTC7815 demonstration board.
Protective features
The LTC7815 can be configured to sense the output current using either a DCR (inductor resistance) or a current sensing resistor. The choice between the two current sensing methods primarily involves a trade-off between cost, power consumption, and accuracy. DCR sensing is increasingly popular because it eliminates the need for an expensive current sensing resistor and offers higher power efficiency, especially in high-current applications. A current sensing resistor, on the other hand, is a more accurate current sensing method.
The on-chip comparator monitors the buck output voltage and issues an overvoltage condition signal when the output exceeds the nominal value by 10%. Upon detection, the top MOSFET turns off and the bottom MOSFET turns on until the overvoltage condition is cleared. The bottom MOSFET remains on as long as the overvoltage condition persists. Normal operation automatically resumes once the output voltage returns to a safe level.
In cases of high temperature or excessive on-chip self-heating due to internal power consumption, the over-temperature shutdown circuit will shut down the LTC7815. When the junction temperature exceeds approximately 170°C, the over-temperature circuit will disable the on-chip bias LDO, thereby reducing the bias supply to 0V and sequentially and effectively shutting down the entire LTC7815. Once the junction temperature drops back to approximately 155°C, the LDO will turn back on.
in conclusion
Automotive start-stop systems save fuel and will continue to evolve in the coming years. Powering onboard infotainment and navigation systems requires careful handling, as these systems need voltages as high as 5V or even higher. If the car battery voltage drops below 5V when the engine restarts, these systems may reset. The LTC7815 addresses this issue by boosting the battery voltage to a safe operating level. This feature, combined with two buck controllers, makes it ideal for powering numerous automotive electronics in vehicles equipped with start-stop systems.
