Rainbow Electronics MAX9621 - Manual (Page 10)

Dual, 2-Wire Hall-Effect Sensor Interface with
Analog and Digital Outputs
MAX9621
10
Hall Input Short-to-Battery Condition
The MAX9621 interprets a short to battery when the volt-
age at IN1 or IN2 is higher than V
BAT - 100mV. The digi-
tal outputs go low and the analog outputs are set to zero
output current. If IN1 or IN2 is more than 1V above V
BAT ,
it back-drives current into BAT. The MAX9621 restarts
the Hall inputs when the Hall input is loaded again.
Hall Input Short to Ground
The Hall input short-to-ground fault is effectively a
latched condition if the input remains loaded by the Hall
switch. The current required to power the Hall switch is
shut off and only a 50µA pullup current remains. The Hall
input can be manually reenergized or it can be reener-
gized by the µP. A 10µs to 20µs negative pulse at SLEEP
restarts with a blanking cycle any Hall input that has
been shut down due to the short-to-ground condition.
During startup or restart, it is possible for a Hall input
to charge up an external capacitance of 0.02µF without
tripping into a short-to-ground latched state. During
the short-to-ground fault, DOUT1 and DOUT2 are high
impedance (pulled high by the pullup resistors), while
AOUT1 and AOUT2 are set to zero-output current.
Manual Method for Reenergizing Hall
Sensor and Means for Diagnosing an
Intermittent Hall Sensor Connection
Figure 3 shows the behavior of the MAX9621 when a
Hall input is open. Figure 4 shows the behavior of the
MAX9621 when the open input is reconnected to a Hall
sensor. Figures 3 and 4 demonstrate how a short-to-
ground Hall input can be reset. Resetting a short-to-
ground Hall input involves three steps:
1) Relieve the short to ground at the Hall sensor.
2) Disconnect the Hall input from the Hall sensor (open-
input fault condition).
3) Reconnect the Hall input to the Hall sensor.
The MAX9621 restarts the Hall input with a blanking
cycle. If the Hall input is disconnected from the Hall
sensor for 10ms, it allows the Hall input to be pulled up
by the 50FA pullup current to register the open-input
fault condition. Reconnecting the Hall input to the Hall
sensor restarts the Hall input with a blanking cycle. This
provides a manual means of reenergizing a Hall input
without having to resort to the FP to restart it. This also
demonstrates the behavior of an intermittent connection
to a Hall sensor.
Table 1. AOUT_/DOUT_ Truth Table
Figure 3. Hall Input Ramps to Open-Circuit Fault When a Short to Ground Is Relieved
*If IN_ is already shorted to BAT or open during power-up,
DOUT_ goes to high-Z until IN_ is loaded.
14V
V IN_
V BAT - 25mV
HALL INPUT
OPEN-CIRCUIT
FAULT
5mV/ms
I IN_
50 µA
0V
0A TIME
TIME
HALL INPUT
SHORT-TO-
GROUND FAULT
HALL INPUT
DISCONNECTED
FROM SENSOR
CONDITION AOUT_ DOUT_
IN_ Short to GND 0 High-Z
IN_ Short to BAT or IN_ Open 0 Low*
SLEEP Low 0 High-Z
V BAT > 18V 0 Low*