L9678P datasheet(118/203 Pages) STMICROELECTRONICS

118 / 203 page

Deployment drivers

L9678P, L9678P-S

118/203

DocID029274 Rev 3

The condition of two open channels, i.e. without squib 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 is indicated on the open channel.

Squib resistance measurement

During a resistance measurement, a two-step process is performed. At the first step, both

ISRC current generator and ISNK current limited switch are enabled and connected to the

selected SFx and SRx channel, through ISRC, ISNK and RES_MEAS_CHSEL bit fields in

the Loop Diagnostic Request Register (LPDIAGREQ). A differential voltage is created

between the SFx and SRx pin based in the ISRC current and squib resistance between the

pins. The SPI interface will provide the first resistance measurement voltage (Vout1) 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 (Vout2). Microcontroller is then allowed

to calculate the mathematical difference between first and second measurements to obtain

the real squib resistance value. The current sources ISRC and ISNK used for Squib

Resistance measurements are completely controlled by the user via SPI. Optionally, an

automatic control by the IC for current sources switch-off after ADC reading can be

activated by enabling the EN_AUTO_SWITCH_OFF bit in the SYS_CFG register.

Vout1

GIsource

Rleak Rsq



Rleak Rsq

-------------------------------







Rsq

Rleak Rsq

------------------------------- Vgnd VrefSQL

–





GVoffset



Vout2

GRsq



Rleak Rsq

-------------------------------

Vgnd VrefSQL

–





GVoffset



Rsq

Vout



GIsrc



--------------------

(assuming Rleak >> Rsq)

where:

G = differential amplifier gain.

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 resistance

measurement reported in the electrical parameters table.

Table 8. Short between loops diagnostics decoding

Fault condition on

squib 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

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