L9660 datasheet(26/49 Pages) STMICROELECTRONICS

26 / 49 page

Functional description

L9660

26/49

DocID024714 Rev 2

Figure 9. Squib diagnostics block diagram

Short to battery/ground and loop to loop diagnostics

The leakage diagnostic includes short to battery, short to ground and a short between loop

tests. The test has to run for each SQHx and SQLx pin so that shorts can be detected

regardless of the resistance between the squib pins.

Normal short to battery/ground diagnostics.

For the test the internal VRCM is switched on and connected to the selected pin (SQHx or

SQLx) pin. The IPD bit selected OFF deactivates the pull-down current on the channel

under test and all other channels. During the test with no leakage present the voltage on the

selected SQHx or SQLx pin is equal to VBIAS and no current is sunk or sourced by VRCM.

If a leakage to ground, battery or to SQLy is present, the VRCM sinks or sources a current

less then ISVRCM trying to keep VBIAS. Two current comparators, ISTB and ISTG, detect the

abnormal current flow.

Loop to loop diagnostics

For this test the same procedure is followed except the pull-down current (IPD) is selected

to be ON that deactivates the pull-down current only on the channel under test with all other

channel pull-down currents active. If a short to ground fault is active, assuming it was not

active during normal short to battery/ground diagnostics, then that particular channel has a

short to another squib loop. To detect loop to loop shorts between ASICs in the system the

Stop diagnostics command with IPD enabled needs to be sent to the other ASICs before

running the loop to loop diagnostics on the channel to be monitored. If the channel being

monitored has a short to ground fault active, assuming it was not active during normal short

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