L9660 datasheet(27/49 Pages) STMICROELECTRONICS

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L9660

Functional description

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 is 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, provides the resistance-measurement voltage based on the scaling factor

indicated in the electrical parameters section. The tri-state output, AOUT, is connected to an

ADC input of a microprocessor. When not running squib resistance diagnostics the AOUT

pin is in high impedance state.

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

equation shown in the parametric table. The L9660 is also capable of improving the

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

comparator. This method works 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

High squib resistance diagnostics

During a high squib resistance diagnostic, VRCM and ISINK are switched ON and

connected to SQHx and SQLx on the selected channel. Current flowing on SQHx is

measured and compared to IHR threshold to identify if resistance is above or below

RSQHZ. The results are reported in the next SPI message. Once the command is issued

Table 11. How faults shall be interpreted

Fault condition for

channel(1)

Condition where 2 open channels have the SQHx pins shorted is not detected. If one squib is open and the

other has a normal squib connection then the fault is 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

L9660 Datasheet(PDF) 27 Page - STMicroelectronics

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