L9965P datasheet(46/104 Pages) STMICROELECTRONICS

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3.7.2.5

Pyro igniter resistance measurement

This step aims at measuring the pyro-fuse igniter circuit resistance and verifying that it lays within safety-limits.

This test requires HWSC (hardware self-check) prior to execution to cover possible faults determining

measurement error out of range. It also requires the VRCM check to verify that test currents are in range. To

achieve safety targets, it is strongly recommended to apply such a strategy before running this test.

Figure 22. Pyro igniter resistance measurement

RPYRO

RWIRE/2

CESD

RWIRE/2

LWIRE/2

LEMI

CESD

CPYRO

IPF_PU

PGND

IPR_PD

The diagnostic works as follows:

•

The IPF_PU and IPR_PD currents are enabled, so that a IPF_PU current flows through PF and PR.

•

The IC waits for the differential voltage settling between PF and PR.

•

The ADC converts the PF vs PR differential voltage and compares the obtained VPYRO result to the

VRES_LOW_TH and VRES_HIGH_TH limits.

•

The IPF_PU and IPR_PD currents are disabled.

The pyro resistance sensitivity can be computed as follows:

RPYRO=VPYROIPFPU ∂RPYRO

∂VPYRO= 1IPFPU=25ΩV≅16.275mΩLSB

(3)

The VRES_LOW_TH and VRES_HIGH_TH thresholds can be programmed in NVM respectively in VRES_LOW_TH

and VRES_HIGH_TH bit fields.

The result is saved in the RES_MEAS_PRE register.

The resistance measurement diagnostic step duration is tRES_MEAS.

L9965P

Functional description

DS14702 - Rev 4

page 46/104

L9965P Datasheet(PDF) 46 Page - STMicroelectronics