L9654 datasheet(23/59 Pages) STMICROELECTRONICS

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L9654

Functional description

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

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

When VRES is connected to the ignition, VRES voltage is 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.

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, L9654 latches ARM signal from the shift register and starts 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 is

initiated by ARM signal



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

initiated by deployment command

During a deployment, the device turns on the respective high-side (SQH) and low-side

(SQL) drivers for duration of tDEPLOY. When a deployment is initiated, it can’t 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

command is sent before the timer for the previous command expired, the timer is refreshed.

Sending a deployment-disable command terminates the pulse stretch timer operation.

ONLY a timer operation started by a deployment-enable command can be terminated.

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