L9678P datasheet(114/203 Pages) STMICROELECTRONICS

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Deployment drivers

L9678P, L9678P-S

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DocID029274 Rev 3

6.1.4

Deployment success

Deploy success flag is set when the deploy timer elapses. This bit (CHxDS) is contained in

the Deploy Status Register. Within the Global Status Word register (GSW), a single bit

(DEPOK) is also set once any of the four deployment channels sets a deploy success flag.

6.2

Energy reserve - deployment voltage

One deployment voltage source pin is used for channels 0 and 1 (SS01) and one for

channels 2 and 3 (SS23). These pins are directly connected to the high side drivers for each

channel.

6.3

Deployment ground return

There are dedicated power ground connections for deployment current, SGx pins. One

ground connection is sufficient for two deployments occurring simultaneously.

6.4

Deployment driver protections

6.4.1

Delayed low-side deactivation

To control voltage spikes at the squib pins during drivers deactivation at the end of a

deployment, the low side driver is switched off after tDEL_SD_LS delay time with respect to

the high side deactivation.

6.4.2

Low-side voltage clamp

The low side driver is protected against overvoltage at the SRx pins by means of a clamping

structure as shown in Figure 23. When the Low side driver is turned off, voltage transients at

the SRx pin may be caused by squib inductance. In this case a low side FET drain to gate

clamp will reactivate the low side FET allowing for residual inductance current recirculation,

thus preventing potential low side FET damage by overvoltage.

6.4.3

Short to battery

The low side driver is equipped with current limitation and overcurrent protection circuitry. In

case of short to battery at the squib pins, the short circuit current is limited by the Low side

driver to ILIM_SR. If this condition lasts for longer than TFLT_ILIM_LS deglitch filter time then

the low and high-side drivers will be switched off and latched in this state until a new

deployment is commanded after SPI_DEPEN is re triggered.

6.4.4

Short to ground

The squib driver is designed to stand a short to ground at the squib pins during deployment.

In particular, the current flowing through the short circuit is limited by the high side driver

(deployment current) and the high-side FET is sized to handle the related energy.

In case the short to ground during deployment occurs after an open circuit, a protection

against damage is also available. The high side current regulator would have normally

reacted to the open circuit by increasing the Vgs of the high side FET. Thanks to a dedicated

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