Rainbow Electronics MAX9921 - Manual (Page 10)

Detailed Description
The MAX9921 connects two 2-wire Hall-effect sensors to
a low-voltage µP. This device supplies current through
IN1 and IN2 to Hall sensors and monitors the current
level drawn by the Hall sensors. The MAX9921 outputs a
high or low logic-level to the corresponding open-drain
output (OUT1 or OUT2). If the current flowing out of
either IN1 or IN2 exceeds the high input current thresh-
old, the corresponding output goes high. If the current
flowing out of either IN1 or IN2 is lower than the low input
current threshold, the corresponding output goes low.
Hall Input Diagnostic
Normal Mode
In normal mode, DIAG is driven low. In this case, if ERR
is high, the outputs (OUT1 and OUT2) indicate the high
or low state of the corresponding Hall sensors (IN1 and
IN2). In normal mode, a low ERR indicates a fault. If
ERR is low, the outputs may be pulled low and may not
indicate the high or low state of the Hall sensors. This
can happen during the power-up, restart, or blanking
cycles of the Hall inputs, or due to a fault on one or
both of the Hall inputs, or when V
BAT
is out of range,
while the error output is low. If one output continues
signaling and the other output is low, the output with
the low logic-level indicates a fault or a restart and
blanking cycle on the corresponding Hall input. Table 1
summarizes normal mode operation.
Diagnostic Mode
When DIAG is driven high, the MAX9921 enters diagnos-
tic mode. In this mode, OUT1 and OUT2 output diagnos-
tic information. IN1 takes precedence over IN2. IN2’s
diagnostics remain masked until a fault on IN1 is cleared.
For diagnostics and troubleshooting, when IN1 or IN2
shuts off due to an input short to ground, it continues to
source 50µA. A falling edge at DIAG restarts a Hall input
that has been shut off due to a short to ground.
Diagnostic indications are never latched internally and
they indicate the real-time state of IN1 or IN2. Table 2
summarizes diagnostic mode operation.
Hall Input Fault Detection
If a fault is detected, the ERR output is asserted low to
notify the µP. This condition can occur due to the Hall
input being shorted to ground, shorted to battery,
or open.
MAX9921
Dual, 2-Wire Hall-Effect Sensor Interface
with Diagnostics
10 ______________________________________________________________________________________
Functional Diagram
OE
ERR
DIAG
OUT1
OUT2
BAT
IN1
IN2
REF
GENERATION,
DIAGNOSTICS,
AND
OVERVOLTAGE
DETECT
ISET
CONTROL
FILTER
FILTER
GND
MAX9921
INPUT OUTPUT
DIAG
ERR
OUT1 OUT2
DIAGNOSIS COMMENT
0 1 0 or 1 0 or 1
OUT1 and OUT2 indicate state of IN1
and IN2, respectively Normal mode: No fault indication (outputs
indicate Hall sensor high or low status)
0 0 0 0
Fault on IN1 and/or IN2, or V BAT out
of range, or power-up or restart
blanking (unknown current level of
IN1 and IN2)
Normal mode: ERR asserted low indicates
fault (outputs may no longer indicate the
high or low state of the Hall sensors)
0 0 0 0 or 1 Fault on IN1 or restart blanking of IN1
Normal mode: ERR asserted low indicates
fault; Hall output 2 continues signaling
0 0 0 or 1 0 Fault on IN2 or restart blanking of IN2
Normal mode: ERR asserted low indicates
fault; Hall output 1 continues signaling
Table 1. Diagnostic Truth Table (Normal Mode)