L9662 datasheet(36/80 Pages) STMICROELECTRONICS

36 / 80 page

Functional description

L9662

36/80

DocID028325 Rev 1

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

another squib loop in the system.

The following table indicates how faults would be interpreted.

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

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

dependent 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. In test mode, Aout pin can be used as digital/analog

output for testing deployment and diagnostics comparators and thresholds. Test mode is

reserved for supplier use only.

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

equation shown in the parametric table. The device is also capable of improving the

tolerance at resistances below 3.5 Ohms by removing the offset of the differential

comparator. This method involves taking the single ended analog output results for a

channel (VAOUT with AMC bit=0) and subtracting the internal comparator offset

measurement of VAOUT_CAL (VAOUT with AMC bit=1). The summary of the equation for this

is as follows:

AOUT_CAL = (VAOUT – VAOUT_CAL)/VDD

AOUT_CAL typical = 0.08 x RSQUIB

Table 12. How faults shall be interpreted

Fault condition for

channel(1)

1. Condition where 2 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

L9662 Datasheet(PDF) 36 Page - STMicroelectronics

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