L9679ETR datasheet(75/127 Pages) STMICROELECTRONICS

75 / 127 page

Table 11. Short between loops diagnostics decoding

Fault condition on squib/

pyroswitch channel

Channel leakage diagnostics with

PD_CURR on (for other channels than the

one under test)

Channel leakage diagnostics with

PD_CURR off (for all channels)

No shorts

No fault

Short to battery

STB fault

Short to ground

STG fault

Short between Loops

STG fault

No fault

The condition of two open channels, i.e. without squib/pyroswitch resistance connecting SFx to SRx, that have a

short between loops on SFx cannot be detected. If only one of the two shorted SFx pins is open, the fault will be

indicated on the open channel.

Squib/Pyroswitch resistance measurement

During a resistance measurement, a two-step process is performed.

At the first step, both the ISRC current generator and the ISNK current limited switch are enabled and connected

to the selected SFx and SRx channel, through ISRC, ISRC_CURR_SEL, ISNK and RES_MEAS_CHSEL bit fields

in the Loop Diagnostic Request Register (LPDIAGREQ). The ISRC current can be configured to either 40 mA or 8

mA nominal value through the ISRC_CURR_SEL bit in the LPDIAGREQ register providing the user with two

different measurement range options. A differential voltage is created between the SFx and SRx pin based on the

ISRC current and squib/pyroswitch resistance between the pins. The SPI interface will provide the first resistance

measurement voltage (Vdiff1) based on the amplifying factor of the differential amplifier and a 10 bit internal ADC

conversion.

The second measurement step (bypass measurement) is performed redirecting ISRC to the selected SRx pin,

while keeping ISNK on; this way, the differential amplifier and following ADC will output the offset measurement

through SPI (Vdiff2). Microcontroller is then allowed to calculate the mathematical difference between first and

second measurements to obtain the real squib/pyroswitch resistance value.

Vdiff1=GRSQ× ISRC_*× RLKG_SF×RSQ

RLKG_SF+RSQ + RSQ

RLKG_SF+RSQ× VLKG_SF−VSRx_RM +Voff_RSQ

Vdiff2= GRSQ×RSQ

RLKG_SF+RSQ× VLKG_SF−VSRx_RM +Voff_RSQ

RSQ=Vdiff1−Vdiff2

GRSQ×IRSC_* (assumingRLKG_SF⪢RSQ)

The simplification in the calculation method reported above can result in some amount of error that is already

incorporated in the overall tolerance of the squib/pyroswitch resistance measurement reported in the electrical

parameters table.

Values of each measurement step can be required addressing the proper ADCREQx code in the Diagnostic

Control command (DIAGCTRLx) on Table 14. Diagnostics control register (DIAGCTRLx).

This calculation is tolerant to leakages and, thanks to a dedicated EMI low-pass filter, also to high frequency

noises on squib/pyroswitch lines. Moreover, L9679E features a slew rate control on the ISRC current generator to

mitigate emissions.

The ISRC current generator is connected to VSYNC input pin. Special care shall be considered in programming

the duty cycle of the squib/pyroswitch measurements to keep the power dissipation under control, in the event

VSYNC line is supplied with high voltage.

High squib/pyroswitch resistance diagnostics

With this test, the device is able to understand if the squib/pyroswitch resistance value is below 200 Ω, between

500 Ω and 2000 Ω or beyond 5000 Ω. During a high squib/pyroswitch resistance diagnostics, VRCM and ISNK

are enabled and connected respectively to SFx and SRx on the selected channel. VREF voltage level will be

output on SFx. Current flowing on SFx will be measured and compared to ISRlow and ISRhigh thresholds to identify

if the resistance is above or below RSRlow or RSRhigh levels. The results are reported in the LPDIAGSTAT register.

The relative flags (HSR_HI and HSR_LO) are not latched and reflect the current status of the comparators.

L9679E

Deployment drivers

DS12485 - Rev 8

page 75/127

L9679ETR Datasheet(PDF) 75 Page - STMicroelectronics

L9679ETR Datasheet(HTML) 75 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 ...More

Datasheet Download

Link URL