L9662 datasheet(35/80 Pages) STMICROELECTRONICS

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L9662

Functional description

Figure 9. Squib diagnostics block diagram

Short to battery/ground and loop to loop diagnostics

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

loop tests. The test will be 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). The IPD bit will be selected to be OFF which will deactivate the pulldown current on

all the channels. During the test if no leakage is present the voltage on the selected SQHx

or SQLx pin will be equal to VBIAS and no current is sunk or sourced by VRCM. If a leakage

to ground or battery, the VRCM will sink or source a current less than ISVRCM trying to

keep VBIAS. Two current comparators, ISTB and ISTG, will detect the abnormal current

flow.

Loop to loop diagnostics

For this test the same procedure is followed except the pulldown current (IPD) is selected to

be ON which will deactivate the pulldown current only on the channel under test with all

other channel pulldown 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

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