33797 Datasheet, PDF (28/34 Pages) Freescale Semiconductor, Inc – Four Channel Squib Driver IC

FUNCTIONAL DEVICE OPERATION
PROTECTION AND DIAGNOSIS FEATURES
SQB_HI_XX to the SENSE_XX pin. When resistive
measurement legs to comparators located in the SENSE_XX
pin are activated, a fault on either side of the squib can be
easily detected. An external current path that causes the
SQB_LO_XX, SQB_HI_XX, or SENSE_XX pin to be pulled
below the minimum threshold, will be indicated as a “Short to
Ground”.
If the SQB_LO_XX, SQB_HI_XX, or SENSE_XX pin has
an external current path that causes the pin to be pulled
above the maximum threshold, a “Short to Battery” will be
indicated.
SQUIB SHORT-TO-BATTERY/GROUND
DIAGNOSTIC DATA RESULTS
If SQB_LO_XX, SQB_HI_XX, or SENSE_XX pin is
shorted to battery, the bit NO_SH_BATT will be set to “0”. If
a SQB_LO_XX, SQB_HI_XX, or SENSE_XX pin is shorted to
ground, the bit NO_SH_GND will be set to “0”. During
standard operation, both NO_SH_BATT and NO_SH_GND
will be set to “1”.
Note This diagnostic circuit uses an internal 2.7 mA
current source connected to the SQB_HI_XX pin as a bias. If
the SQB_LO_XX and SQB_HI_XX pins have any
capacitance (due to discrete capacitors or parasitic loading),
the diagnostic condition will require a settling time based on
the RC time constant.
SQUIB HARNESS SHORT-TO-BATTERY / GROUND
DIAGNOSTICS WITH AN OPEN SQUIB ($C3
COMMAND)
This diagnostic function is to be used with no squib present
(open squib condition) in the wiring harness. For an open
squib condition, the function must monitor the voltage on the
SQB_HI_XX and SQB_LO_XX pins for “Short to Ground”
and “Short to Battery” conditions.
This function monitors the voltage on the SENSE_XX pins.
The voltage is compared to two thresholds (minimum and
maximum) and will provide two bits of data to indicate if the
pin voltage is above, below, or in between the predetermined
threshold levels.
When enabled, a pair of opposing N-channel CMOS
transistors are activated, creating roughly a 500Ω resistance
between the SQB_HI_XX and SQB_LO_XX pins together.
A 2.7 mA current source located in the SQB_HI_XX pin is
activated, sourcing current from the SQB_HI_XX to the
SQB_LO_XX pin to the SENSE_XX pin. When resistive
measurement legs to comparators located in the SENSE_XX
pin are activated, a short to BAT / GND fault can easily be
detected. An external current path that causes the
SQB_LO_XX, SQB_HI_XX, or SENSE_XX pin to be pulled
below the minimum threshold, will be indicated as a “Short-
to-Ground”.
If the SQB_LO_XX, SQB_HI_XX, or SENSE_XX pin has
an external current path that causes the pin to be pulled
above the maximum threshold, a “Short-to-Battery” will be
indicated.
SQUIB SHORT-TO-BATTERY/GROUND
DIAGNOSTIC DATA RESULTS
If SQB_LO_XX, SQB_HI_XX, or SENSE_XX pin is
shorted to battery, the bit OPEN NO_SH_BATT will be set to
“0”. If a SQB_LO_XX, SQB_HI_XX, or SENSE_XX pin is
shorted to Ground, the bit OPEN NO_SH_GND will be set to
“0”. During standard operation, both OPEN NO_SH_BATT
and OPEN NO_SH_GND will be set to “1”.
Notes
1. This diagnostic circuit uses an internal 2.7 mA current
source connected to the SQB_HI_XX pin as a bias. If
the SQB_LO_XX and SQB_HI_XX pins have any
capacitance (due to discrete capacitors or parasitic
loading) the diagnostic condition will require a settling
time based on the RC time constant.
2. With an OPEN NO_SH_GND or OPEN_NO_SH_BATT
indicated, the SQB_HI_XX or SQB_LO_XX line
contains the fault condition. The standard squib short-
to-battery / ground diagnostics ($C1) can be executed
to determine if the fault condition is on the SQB_HI_XX
pin or the SQB_LO_XX pin.
CONTINUITY TEST for the LOW-SIDE DRIVER
SQB_LO_XX CONNECTION ($C2 COMMAND)
(LOW-SIDE DRIVER CONTINUITY STATUS)
Low-side driver continuity is checked during the continuity
test diagnostics. This function is used to check continuity at
the SQB_LO_XX pin connection. When enabled, a 2.0 mA
current source located in the SQB_HI_XX pin is activated
sourcing current from the SQB_HI_XX to the SQB_LO_XX
pin.
For a standard connection, the SQUIB_LO_XX_CON bit
will be set to “1”. With an open circuit connection, the
SQUIB_LO_XX bit will be set to “0”. The driver continuity
information will be cleared after the information is transmitted
on the next valid SPI command.
SQUIB RESISTANCE DIAGNOSTICS ($D0–$D3
COMMAND)
This function monitors squib resistance. When enabled, a
diagnostic current derived from R_DIAG is passed through
the selected squib. The resulting voltage across the squib is
amplified and passed to an 8-bit voltage level detector. The
eight bits of data will indicate if the selected squib has a
resistance value above or below predetermined thresholds.
The value of R_DIAG can be varied to allow the detection
range to be altered. Increasing the value of R_DIAG will
reduce the diagnostic current; thus, a higher squib resistance
will be needed to reach the same R_TH points. In the case that
R_DIAG is a short-to-ground, the diagnostic current through
the squib resistance will typically be less than 50 mA.
33797
28
Analog Integrated Circuit Device Data
Freescale Semiconductor