L9679ETR datasheet(72/127 Pages) STMICROELECTRONICS

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7.2

Deployment voltage

One deployment voltage source pin is used for adjacent channels (e.g. SS23 for channels 2 and 3). These pins

are directly connected to the high side drivers for each channel.

7.3

Deployment driver protections

7.3.1

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 TDEL_SD_LS delay time with respect to the high side deactivation.

7.3.2

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 12. Analog deployment block diagram. 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.

7.3.3

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 shortcircuit current is limited by the low-side driver to ILIMSRx. 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 retriggered.

7.3.4

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.

7.3.5

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 TFLT_OS_LS

then the high side is turned off for TOFF_OS_HS and then reactivated. By this feature, intermittent open squib/

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

7.4

Diagnostics

The L9679E provides the following diagnostic feedback for all deployment channels:

•

High-voltage leakage test for oxide isolation check on SFx and SRx

•

Leakage to battery and ground on both SFx and SRx pins with or without a squib/pyroswitch

•

Short between loops diagnostics

•

Squib/Pyroswitch resistance measurement with leakage cancellation and selectable range (10/50 Ω)

•

High squib/pyroswitch resistance with range from 500 Ω to 2000 Ω

•

SSxy, SFx voltage status

•

High- and low-side FET diagnostics

•

Loss of ground return diagnostics

•

High-side safing FET diagnostics

L9679E

Deployment drivers

DS12485 - Rev 8

page 72/127

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