L9679P datasheet(160/279 Pages) STMICROELECTRONICS

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Deployment drivers

L9679P

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DS11627 Rev 7

8.4

Deployment driver protections

8.4.1

Delayed low-side deactivation

To control voltage spikes at the squib/pyroswitch pins during drivers deactivation at the end

of a deployment, the low side driver is switched off after tdepl_ls-dly delay time with respect to

the high side deactivation.

8.4.2

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 30. When the Low side driver is turned off, voltage

transients at the SRx pin may be caused by squib/pyroswitch 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.

8.4.3

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/pyroswitch pins, the short circuit current is limited by the

Low side driver to ILIMSRx. If this condition lasts for longer than tLIM 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.

8.4.4

Short to ground

The squib/pyroswitch driver is designed to stand a short to ground at the squib/pyroswitch

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

fast comparator detecting the open condition, the driver is able to discharge the FET gate

quickly in order to reduce current overshoot and prevent potential driver damage when the

short to ground occurs.

8.4.5

Intermittent open squib/pyroswitch

A dedicated protection is also available in case of intermittent open load during deployment.

In this case, if load is restored after an open circuit, due to slow reaction of the high-side

current regulation loop, the current through the squib/pyroswitch is limited only to ILIMSRx by

the low side driver. If this condition lasts for longer than tLIMOS then the high side is turned

off for tHSOFFOS and then reactivated. By this feature, intermittent open squib/pyroswitch

and short to battery faults may be distinguished and handled properly by the drivers.

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