L9678P datasheet(28/203 Pages) STMICROELECTRONICS

28 / 203 page

Start-up power control

L9678P, L9678P-S

28/203

DocID029274 Rev 3

4.3

Configurable system power control

The overall operating voltage requirements of the device are different considering the

L9678P device (without VSUP regulator and remote sensor interface) or the L9678P-S

device (with VSUP regulator and remote sensor interface). Performance for the L9678P-S

device is influenced by the PSI-5 remote sensor interfaces. This function requires a minimum

voltage at the channel's input (VSUP) to ensure a proper functionality for the sensor.

An integrated current generator (30 mA nominal) is used to charge the external energy

reserve capacitor connected to VER pin. Any system load (regulators, interfaces, squib

driver diagnostics) operates directly from battery until battery is lost. Upon detecting low or

loss of battery, the crossover switch enables operation from energy reserve.

4.3.1

ERBOOST switching regulator

The L9678P IC uses an advanced energy reserve switching regulator operating at 1.882

MHz nominal. The higher switching frequency enables the user to select smaller less

expensive inductors and moves the operating frequency to permit easier compliance with

system emissions.

The energy reserve boost regulator charges the external system tank capacitor through an

integrated fixed current source significantly reducing in-rush currents typical of large energy

reserve capacitors. The boost circuit provides energy for the reserve capacitor with

assumed run time load of less than 20 mA and to the VSF regulator. Once system shutdown

is initiated or a loss of battery condition is diagnosed, the boost regulator is disabled so that

system power can be taken from the energy reserve capacitor.

The energy reserve boost regulator defaults to 23 V at power-on and can be set to 33 V

nominal by the user through an SPI command. The boost converter can also be disabled by

the user through an SPI command. Enabling, disabling and setting the boost output voltage

are done through the System Control (SYS_CTL) register. Boost converter diagnostics

include over voltage and under voltage. The under voltage condition is reported by the

ER_BST_NOK bit in the POWER_STATE register. The integrated FET featuring the boost

switch is protected against short to battery by means of a thermal shutdown circuit. When

thermal fault is detected the FET is switched off and latched in this state until the related

fault flag ERBST_OT in the FLTSR register is read. In case of loss of ground the FET is

switched off and automatically reactivated as soon as ground connection is restored. Over-

voltage protection from load dump and inductive flyback is provided via an active clamp and

an ER_Boost disable circuitry, see Figure 10.

Figure 10. ERBOOST block diagram

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