L9658 datasheet(23/64 Pages) STMICROELECTRONICS

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L9658

Functional description

Doc ID 14219 Rev 3

23/64

The following power-up conditions is considered as normal operations. VRES input can be

connected to either a power supply output or an ignition voltage. VDD is connected to 5 V

output of power supply. When VRES is connected to the power supply, VDD voltage will

reach its regulation voltage before VRES voltage is stabilized. In this condition, the device

has the control o f its internal logic and that prevent an inadvertent turn-on of the drivers.

When VRES is connected to the ignition, VRES voltage will be stabilized before VDD

reaches its regulation voltage. In this condition, all drivers are inactive. A pull-down on the

gates of high side drivers (SQH) is provided to prevent these drivers from momentarily

turning-on. Any loop driver fault conditions do not turn on the SQH and SQL drivers. Only a

valid deployment condition can turn on the respective SQH and SQL drivers. Refer to

section for valid deployment conditions.

4.9.1

Arming interface

The arming interface is used as a fail-safe to prevent inadvertent airbag deployment. Along

with deployment command, these signals provide redundancy. Pulse stretch timer is

provided for each channel/loop. Either ARM signal or deployment command shall start the

pulse stretch timer.

Arming interface has a dedicated 8-bit SPI interface.

When CS_A is negated, L9658 latch ARM signal from the shift register and start the pulse

stretch timer for the respective channel/s. The device can deploy a channel, ONLY when

DEPEN is asserted and any of the following conditions are satisfied:

–

the respective deployment command is sent during a valid pulse stretch timer,

which initiated by ARM signal

–

the respective SPI ARM command is sent during a valid pulse stretch timer, which

initiate by deployment command

During a deployment, the device turn on the respective high side (SQH) and low side (SQL)

drivers for duration of tDEPLOY. When a deployment is initiated, it cant be terminated, except

during a reset event.

Figure 11.

Deployment sequence

When a deployment-enable command is sent through SPI, the pulse stretcher shall be

initiated immediately following the falling edge of CS_D. When another deployment-enable

Pulse Stretch Timer

DEPLOY

Deploy

Valid

Deployment

Window

CS _D

Deploy Success Flag

1: Deployment-enable Command 2: Clear Deploy Success Flag

SPI CS_D Notes:

PULSE

Valid

Deployment

Window

Valid

Deployment

Window

t < t

PULSE

Valid

Deployment

Window

t<t

PULSE

3: Deployment-disable Command

ARM Status Flag

CS_A

ARMEN Notes:

A: Arming-enable Command

B: Arming-disable Command

DEPEN input is assumed to be active in this sequence.

SPI Deploy Command

Status Flag

PULSE

t<t

PULSE

DEPLOY

PULSE

Deploy Status Flag

Monitor Mode Command bit

D8 = 0

Deploy Status Flag

Monitor Mode Command bit

D8 = 1

t<t

PULSE

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