L9659 datasheet(28/51 Pages) STMICROELECTRONICS

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Functional description

L9659

28/51

Doc ID 022048 Rev 2

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 will be 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 will

sink or source a current less then 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

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

to another squib loop in the system.

The following table indicated how faults would be interpreted.

Once the command is issued the state of the comparators will be captured on the next

falling edge of CS_D. The results are valid after TSHORTDIAG time, which is mainly

dependant on the external capacitors on the squib lines.

Squib resistance measurement

During a resistance measurement, both ISRC and ISINK are switched on and connected to

the selected SQHx and SQLx channel. A differential voltage is created between the SQHx

and SQLx pin based in the ISRC current and resistance between the pins. The analog

output pin, AOUT, will provide the resistance-measurement voltage based on the scaling

factor indicated in the electrical parameters section. The tri-state output, AOUT, will be

connected to an ADC input of a microprocessor. When not running squib resistance

diagnostics the AOUT pin will be high impedance.

To increase accuracy of the squib resistance measurements the offset of the internal

amplifier can be provided on the AOUT pin. This is done by setting the appropriate

calibration bit, waiting the required time, and reading the converted AOUT voltage

connected to the microprocessor ADC.

The normal measurement method for squib resistance is to take a single ended analog

output measurement for a channel (VAOUT with AMC bit=0) and use the tolerances and

Table 11.

How faults shall be interpreted

Fault condition for

channel(1)

Condition where 4 open channels have the SQHx pins shorted will not be detected. If one squib is open

and the other has a normal squib connection then the fault will be indicated on the channel that is open.

Assumes both pins are tested

Channel leakage diagnostics

results with IPD ON

Channel leakage

diagnostics results with IPD

OFF

No Shorts

No Fault

Short to battery

STB Fault

Loop to loop short

STG Fault

No Fault

Short to ground

STG Fault

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