L9679P datasheet(27/272 Pages) STMICROELECTRONICS

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L9679P

Start-up and power control

271

Start-up and power control

6.1

Power supply overview

The L9679P IC contains a complete power management system able to provide all

necessary voltages for a high feature airbag system or integrated safety system. A general

block diagram is shown in Figure 3. The power supply block contains the following features:



Two 3.3 V internal regulators for operating internal logic (CVDD) and analog circuits

(VINT3V3). An external CVDD pin is used to provide filtering capacitance to digital

section supply rail.



Energy reserve supply (ERBOOST) achieved through an integrated 1.882 MHz

switching boost regulator. The energy reserve capacitor is charged using an internal

constant current source controllable through SPI. Besides, a second current source is

available to discharge the capacitor. The primary function for the second current source

is to diagnose the integrity of the energy reserve capacitor, value and ESR. During

system shutdown, the device can enable the discharge current source via SPI

command to quickly dissipate the remaining energy stored in the energy reserve

capacitor.



Sync pulse supply (SYNCBOOST) is achieved through an integrated 1.882 MHz

switching boost regulator. The SYNCBOOST regulator ensures a minimum voltage is

available for operating the satellite sync signal and also provides the input voltage to

the remote sensor buck regulator. The sync pulse boost regulator is disabled for battery

voltage levels resulting in an output voltage above the set regulation point.



The integrated current limited ER switch requires no external components. This switch

is controlled through the integrated power control state machine and is enabled either

once a loss of battery is detected or a shutdown command is received. Under the same

conditions also the discrete digital pin COVRACT is activated allowing the control of an

external optional cross-over switch.



Two 1.882MHz synchronous buck regulators for remote sensor supply and VCC. The

SATBUCK regulator, remote sensor buck supply, is sourced from the SYNCBOOST

regulator and can be selected to be either 7.2 V or 9 V nominal. The VCC regulator is

sourced from the SATBUCK regulator and is user selectable through the VCCSEL pin

to either 5 V or 3.3 V nominal voltage.



Battery voltage sense input comparator with hysteresis and wake-up input are the

primary control signals for the power supply control state machine.



Based on own mission profile and ECU total current consumption, the user must

evaluate if the activation of fast slope option of each ERBoost, SyncBoost and SatBuck

regulator (bit 8/9/10, $3F SW_REGS_CONF SPI register) is needed to increase the

overall efficiency.

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