RDAIRPABPSI5UG , Rev. 2.0
8
Freescale Semiconductor, Inc.
Understanding the System
3.1.2
MC33789 - Airbag System Basis Chip
This device implements all vehicle sensor interfaces and the airbag system support functions:
3.1.2.1
Power Supply Block
•
A switched-mode power supply DC-DC converter in a boost configuration to generate the high voltage level (33 V), in
which energy is stored in the autarky capacitor, and used to allow continued operation of the airbag system for a defined
time following a collision, which leads to disconnection of the battery
•
A switched-mode power supply DC-DC converter in a buck configuration, to efficiently step down the boost supply to a
level suitable for supplying the satellite sensors interfaces (9.0 V) and further regulators, for the local ECU supplies
•
A switched capacitor charge pump to double the output of the buck converter, for use in supplying the necessary voltage
for the PSI5 sync pulse generation (18 V)
•
A linear regulator to provide the local logic supply (5.0 V) for ECU devices i.e. microcontroller, local sensor, squib driver
3.1.2.2
Safing Block
This block includes a SPI monitor which inputs all inertial sensors (PSI5 satellites and onboard sensors) read by the microcontroller over
the sensor SPI interface, and compares it to pre-defined threshold acceleration values for each local and vehicle collision sensor. Based
on this comparison, where the threshold is exceeded in three consecutive acquisition cycles, the system is armed by enabling the safing
outputs, which in turn enables the squib drivers, so that the application can fire the necessary squibs based on the airbag algorithm results.
3.1.2.3
DC Sensors Interface
A low speed (DC) interface which connects to resistive, simple switch and hall effect sensors which are used to check whether seat belts
are being worn through seat belt switches and seat position through seat track sensors.
3.1.2.4
PSI5 Satellite Sensors Interface
Four Satellite sensors interfaces, which connect to collision sensors distributed around the vehicle. The interfaces are implemented based
on the PSI5 V1.3 specification, and can operate in synchronous modes. It detects current drawn by the satellite and translates the
current-modulated satellite messages into digital data, which the MCU retrieves via the SPI interface.
3.1.2.5
LIN Physical Layer
For connection to vehicle diagnostic interface (K-line) or Occupant Classification System.
3.1.2.6
Lamp Driver
A flexible high or low-side driver which can be configured in hardware which supports PWM driven LED or warning lamp driver.
3.1.2.7
Diagnostics
A number of measures which allow diagnosis of implemented functions on the system basis chip, e.g. all voltage supplies including power
transistor temperature monitors, autarky capacitor ESR, etc.
3.1.2.8
Additional Communication Line
MC33789 is designed to support the Additional Communication Line (ACL) aspect of the ISO-26021 standard, which requires an
independent hardwired signal (ACL) to implement the scrapping feature.
