L9678P datasheet(21/215 Pages) STMICROELECTRONICS

21 / 215 page

DS11626 Rev 9

21/216

L9678P, L9678P-S

Start-up power control

214

Start-up power control

4.1

Power supply overview

The L9678P IC contains a complete power management system able to provide all

necessary voltages for an entry level airbag or cut-off battery application. Moreover L9678P

power supply is user configurable allowing the design engineer to balance cost and

performance as per their particular application. 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 switching boost

regulator. The design of this boost regulator is intended to be a cost effective solution

with respect to traditional boost regulators because it makes use of a low value

inductor with an operative frequency of 1.882 MHz. Switching output is ERBSTSW pin,

while voltage feedback input pin is ERBOOST. The output voltage could be set to either

23 V±5% or 33 V±5%.



Energy reserve capacitor connected to VER pin. To control in-rush current, a dedicated

current generator is implemented between ERBOOST pin and VER pin.



Capability to drive an external safing FET (n-ch type) by means of an internal voltage

regulator on VSF pin, where a 20 V level is given (configurable to 25V via SPI

command).



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.



One linear regulator VDD5 (5 V nominal, ±4% tolerance) requiring external power

transistor and capacitors. VDD5 is used as micro-controller supply (in case of 5 V

family controllers) and, in any case, as supply for VDD3V3 rail.



One integrated linear regulator VDD3V3 (3.3 V nominal, ±4% tolerance) requiring

external capacitors. VDD3V3 is used as micro-controller supply (in case of 3.3 V family

controllers).



VDDQ pin to provide output voltage rail reference. VDDQ could be connected to either

VDD5 or VDD3V3 to enable 5 V or 3.3 V digital communication between device and

micro-controller.



Capability to drive an external power transistor connected to VIN to provide a 7.2 V rail

on VSUP pin. This voltage rail could be used to supply PSI-5 remote sensor.



Battery voltage sense input comparator with hysteresis connected to VBATMON pin.

Power-up and operation states are carefully handled with respect to the battery level to

provide the most effective power supply configuration.



All voltage rails (VIN, ERBOOST, VER, VRESDIAG, VDD5, VDD3V3, VSUP and VSF)

can be monitored through internal ADC diagnostics.

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