Analysis of three typical cases of power supply for vehicle electronic systems

ADI's DC-DC converters are widely used in the industry and can achieve voltage conversion with very high efficiency in different scenarios of automotive electronic systems, thereby saving energy while minimizing heat dissipation design issues. This article takes ADI's automotive DC-DC converters in three different scenarios as examples to explain how ADI uses its rich experience in analog technology research and development to meet the needs of automotive designers for a balance between automotive power, power, and energy consumption.

LED headlight clusters can be innovative and artistic. High and low beams can be “wrapped” with stylish and distinctive daytime running lights (DRL). Because the DRLs are only needed when the high and low beams are off, a single LED driver can be used to power the high and low beam LEDs or the DRLs. This only works if the LED driver has a flexible input-to-output ratio and can step up and down the input-to-output voltage. A buck-boost design can meet this requirement.

The multi-beam LT8391A buck-boost LED driver shown below is capable of driving LED string voltages from 3V to 34V. This allows it to drive a low-beam string and create a high-beam by adding LEDs to the low-beam string. The same driver can be switched to drive a higher voltage, but lower current DRL. Switching from low-beam-only LEDs to a combined low/high beam string does not produce spikes in the output voltage or LED current. The LT8391A smoothly transitions between the boost, 4-switch buck-boost, and buck operating regions. Changing from a low-beam string to a high-beam string without LED spikes can be a tricky challenge for the converter, but this multi-beam LED circuit makes it easy.

LT8391A Multi-beam LED Headlight Cluster Solution for Low Beam, High Beam and DRL

As can be seen, the LT8391A 2MHz, 60V buck-boost LED driver controller can power LED strings in automotive headlights, and features of the device include its low EMI four-switch architecture and spread spectrum frequency modulation to meet CISPR 25 Class 5 EMI specifications. In addition, the device's unique high switching frequency allows it to operate at frequencies above the AM band, requiring very little EMI filtering.

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. With the support of efficient battery technology and corresponding engine management programs, the start-stop system can also work properly at lower temperatures and only requires a short warm-up process before activation. In addition, most systems can recognize the difference between a temporary stop and the end of a journey. If the driver's seat belt is unfastened, or the door or trunk is opened, the system will not restart the engine. However, when the engine is restarted using the start-stop system, the 12 V battery voltage may have dropped below 5 V, which may cause these systems to reset if the in-vehicle infotainment system is turned on or other electronic devices require a voltage higher than 5 V. Even some navigation systems require a higher input voltage to work. Therefore, when the input voltage drops below 5 V during the engine restart, the navigation or music player system needs to be reset if the DC-DC converter only has the input voltage step-down function. For the electronic control unit ECU to work stably, a buck-boost converter is required.

ADI's triple output synchronous DC/DC controller LTC7815 integrates a boost controller and two buck controllers in a single package. The high-efficiency synchronous boost converter feeds two downstream synchronous converters, which is a very useful feature in automotive start-stop systems.

LTC7815 start-stop application schematic, operating at 2.1 MHz

The LTC7815 can operate from an input voltage of 4.5V to 38V during startup and remains operational until the input voltage drops to 2.5V after startup. The synchronous boost converter can generate 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 to maximize efficiency. The two buck converters can generate output voltages from 0.8V to 24V, and the entire system can achieve up to 95% efficiency. The minimum on-time as low as 45ns enables high step-down ratio conversion in 2MHz switching operation, avoiding critical noise-sensitive frequency bands (such as AM radio) and using smaller external components.

A newly proposed automotive standard, called LV 148, will merge the secondary 48 V bus with the existing automotive 12 V system. This new standard requires that the 12 V bus continue to power the ignition, lighting, infotainment, and audio systems. The 48 V bus will power active chassis systems, the air conditioning compressor, adjustable suspension, electric superchargers/turbochargers, and will even support brake energy recovery. Adding a 48 V power supply network to the vehicle is not without significant impact. Electronic control units (ECUs) will be affected and will need to adjust their operating range to the higher voltage.

LT8228 is a bidirectional DC-DC controller recently launched by ADI. It can improve the performance, control function and simplify the design of 48V/12V dual-battery DC-DC automotive systems by using the same external power components for buck and boost. It can work in 48V bus to 12V bus buck mode or 12V to 48V boost mode as needed. When starting the car or requiring additional power, LT8228 allows two batteries to supply power to the same load at the same time. With this versatile bidirectional converter, power conversion designers can easily configure the 12V and 48V battery systems required for future fully autonomous vehicles.

The LT8228 uses the same external power components for step-up conversion as for step-down conversion and is a 100 V bidirectional constant current or constant voltage synchronous buck or boost controller with independent compensation network. The direction of power flow is automatically determined by the LT8228 or controlled externally. Input and output protection MOSFETs are used to prevent negative voltages, control inrush currents, and provide isolation between terminals under fault conditions such as switching MOSFET shorts. In step-down mode, MOSFET protection at the V1 terminal prevents reverse current. In step-up mode, the same MOSFET controls output inrush current and protects itself through an adjustable timer circuit breaker.

LT8228 in a Simplified Bidirectional Battery Backup System Configuration

In addition, the LT8228 features bidirectional input and output current limiting and independent current monitoring. Masterless, fault-tolerant current sharing allows any paralleled LT8228 to be added or removed while ensuring current sharing accuracy. When a single LT8228 is disabled or under a fault condition, it stops delivering current to the average bus, making the current sharing scheme fault-tolerant.

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