CS2082
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7
Each SHx pin is pulled up to V
BAT
while each SLx pin is
pulled down to GND through separate nominal 10 kΩ
resistors, thus biasing each normal fire path to about 1/2
V
BAT
. An open fire path has been detected if both the SBx
and SGx bits are set for that path. To detect faults between
fire paths and to test driver function, each driver should be
activated individually. The activated driver should cause its
respective fault bit to be set. If an activated driver does not
set its respective fault bit, a driver fault has been detected. If
an activated driver causes the fault bit of an inactivated
driver to be set, a fault between fire paths has been detected.
Table 4 defines the implied ranges over which the various
types of faults can be detected.
Table 4. Implied Resistive Fault Detection Ranges
Fault Min Nom Max Unit
Short to Ground 1 5 10 kΩ
Short to Battery 1 5 10 kΩ
Open 5 20 40 kΩ
Driver Open 1 5 10 kΩ
Driver Shorted 1 5 10 kΩ
Squib to Squib 1 5 10 kΩ
Squib Resistance Measurement – $3d
The $3d command activates squib resistance
measurement for the selected firing path. The respective
active–high bit definitions are shown in Table 5. At
power–up, the default path is ‘None.’
Table 5. Squib Resistance Path Select
D3 D2 D1 D0 Path
x x 0 0 NONE
x x 0 1 SQUIB 1
x x 1 0 SQUIB 2
x x 1 1 NONE
Squib resistance is measured by forcing 50 mV nominal
(proportional to V
CC
) across the squib. The resulting squib
current is passed to an external load resistor at the MR pin,
converting the current back into a voltage. This voltage may
be read directly at the MR pin, or passed through the analog
multiplexer to be read at the A
OUT
pin. The known values of
the squib differential voltage (V
DIFF
) and the MR resistance
(R
MR
), and the measured MR voltage (V
MR
) indicate squib
resistance such that:
R
SQUIB
R
MR
V
DIFF
V
MR
Typical MR voltage response for R
MR
= 50 Ω over a squib
resistance range of 0.6 Ω to 6.0 Ω is illustrated in Figure 2.
Figure 2. Typical MR Voltage Response
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0.6 1.4 2.2 3.0 3.8 4.6 5.4
R
SQUIB
V
MR
Measurement accuracy of the CS2082 with combined
tolerances and with and external 1% load resistor at the MR
pin can be defined by the equation:
R
SQ(E)
V
DIFF(IDEAL)
R
MR(IDEAL)
V
DIFF
12%
R
SQ(A)
1% R
MR
1%
R
SQ(A)
12.5%15.94%
where V
DIFF(IDEAL)
and R
MR(IDEAL)
are the assumed
values for the squib resistance solution algorithm, R
SQ(A)
is
the actual squib resistance, and R
SQ(E)
is the result of the
solution algorithm. An additional error may be added if the
MR voltage is measured through the analog multiplexer.
In operation, current is sourced from V
BAT
to the SHx pin,
through the squib to the SLx pin, and returned to ground
through the MR load resistor. Current clamps are provided
for both the SHx and SLx pins and a voltage clamp is
provided for the MR pin. These clamps along with the
resolution of the ADC are the constraining factors for the
minimum and maximum measurable squib resistance
values.
The minimum measurable squib resistance can be defined
as:
V
DIFF(MIN)
I
LIM(MAX)
R
SQUIB(MIN)
V
DIFF(MIN)
R
MR(MIN)
V
CLAMP(MAX)
The maximum measurable squib resistance can be
defined as:
R
SQUIB(MAX)
V
DIFF(MAX)
R
MR(MAX)
(2
n
1)
V
CC(MIN)
In the above equations, V
DIFF
is the SHx–SLx forced
differential voltage, I
LIM
is the SHx resistive measure
current limit, V
CLAMP
is the MR clamp voltage, R
MR
is the
toleranced MR load resistor value and n is the number of bits
of resolution of the ADC.
It should be noted that during resistive measurements,
faults to GND or BAT (dependent on V
BAT
voltage and
