The emergency call (eCall) system is an updated electronic subsystem; you will see it more and more in the coming years. The growing automotive safety standards have led many governments to investigate the need for eCall systems. In 2018, the EU will require an eCall device to be installed on all new vehicles. The eCall system automatically contacts the emergency contact in the event of a serious traffic accident and wirelessly sends the airbag pop-up, collision sensor information, and GPS coordinates to the local emergency agency. Since the eCall system is a new automotive subsystem that must adhere to strict regulations, a complete and dedicated power reference design will make it simpler to design.
Let's take a look at Figure 1, each of the boxes in the TI Designs Automotive eCall Power Reference Design (PMP9769.1) with low intermediate voltage.
Figure 1: Block diagram of an eCall power supply for an eCall system powered by a low-voltage backup battery
Backup battery selection
Since the car already has a 12V battery, what is the significance of the backup battery? Think about it, what if you don't have a 12V battery and you need to make an emergency call? If there is no backup battery in the eCall system, nothing will happen - no phone calls, GPS can't find you, and emergency vehicles can't find you. You may be trapped for a few days, or worse. This is why backup batteries play a key role in the eCall system: to ensure the reliability of this life-and-death system.
The eCall reference design uses a low voltage backup battery with a voltage between 2.5V and 4.5V to meet the needs of the eCall system. Common battery chemistries include lithium ions, lithium polymers, lithium iron phosphate (LiFePO4), and different nickel chemical batteries. The three nickel cells produce a battery pack voltage in the proper range, and only one lithium battery can produce the same voltage. After balancing the cost, energy density, size, and cell voltage, the lithium iron phosphate battery is directly supported by the bq25071 linear charger and the bq28Z610 fuel gauge used in the reference design. Other power management devices support different chemistries, but you must find the best battery management solution to use batteries with different chemistries.
Car battery to 5V voltage rail
A major requirement in any automotive system is to protect electronic components with lower voltages from the voltage offset of the main vehicle battery. In systems where the system uses a lower voltage backup battery, the 12V nominal car battery must also be stepped down to a level that is more available and closer to the backup battery voltage. This enables efficient charging without generating excessive heat. The LM43603-Q1 synchronous buck converter can generate a 5V system voltage rail from a car battery. Although the LM43603 is a 36V input rating device and meets automotive requirements, it also supports operation above 2MHz to avoid interference from the AM radio band.
Power path selection
This circuit ensures an eCall system that is always powered, either by the LM43603-Q1 (if the car battery is available) or by a backup battery. Three CSD25402Q3A p-channel NexFETTM power MOSFETs controlled by the TL331-Q1 comparator provide the highest available voltage for the system.
The power path control block powers the remaining DC/DC converters in the system. These converters convert the intermediate voltage rail to the exact voltage required for each load.
GSM module power supply
The Global System for Mobile Communications (GSM) subsystem typically requires approximately 3.8V to communicate between the car and the cellular network. Since the backup battery voltage fluctuates around this level, there must be a buck-boost converter. The TPS63020-Q1 buck-boost converter supports the 2A current required by GSM and the switching frequency above the 2.4MHz AM radio band.
MCU power supply
Each eCall system requires a microcontroller (MCU) to take care of and monitor the eCall subsystem and communicate with the car. Other logic circuits typically operate at the same voltage, which is typically 1.8V. Since this voltage is lower than the intermediate rail voltage, you can use a buck converter. The TPS62290-Q1 is a compact, simple buck converter that operates above the 2.25MHz AM radio band.
GPS module power supply
The GPS module requires 5V, but the current is lower than the current provided by the GSM power supply. However, the voltage in the intermediate rail is 5V, so a buck-boost converter is also required. Reusing the TPS63020-Q1 in the design keeps the overall bill of materials (BOM) at a low level.
Audio amplifier power supply
The audio amplifier supply voltage and current requirements are highly dependent on the choice of audio amplifier and the location of the speakers inside the car. The speaker position has a large effect on the required output power, which in turn affects the voltage and current required by the audio amplifier. In almost all cases, the boost converter powers the audio amplifier while the audio amplifier is running from a low voltage intermediate rail.
The TAS5411-Q1 audio amplifier delivers up to 8W of power and accepts supply voltages up to 18V. The TPS61175-Q1 boost converter provides 9V to the audio amplifier. The TPS61175-Q1 integrates the boost stage into the main power MOSFET and can be set to operate up to 2.2MHz above the AM radio band.
These are all you need to know to power your low-voltage eCall system. How do you shorten your design time with the TI Designs automotive eCall power reference design with low intermediate voltage?
Other resources:
Why do you need a 5.5Vin buck-boost converter in your car?
The importance of battery management to the eCall system
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