Datasheet
MAX9921
Dual, 2-Wire Hall-Effect Sensor Interface
with Diagnostics
14 ______________________________________________________________________________________
Applications Information
Hall-Effect Sensor Selection
The MAX9921 is optimized for use with 2-wire Hall-
effect switches or with 3-wire Hall-effect switches con-
nected as 2-wire (Figure 6). When using a 3-wire Hall
sensor, the resistor R is chosen so that the current
drawn by the Hall sensor crosses the MAX9921 current
threshold when the magnetic threshold of the Hall sen-
sor is exceeded.
Table 4 shows a partial list of Hall sensors (primarily 2-
wire) that can be used with the MAX9921. The DC
Electrical Characteristics Table gives tested I
IH
/I
IL
cur-
rent threshold limits for both a lower current range and
a higher current range which correspond to the selec-
tion of the R
ISET
resistor value. This makes the
MAX9921 compatible with a wide array of 2- and 3-wire
Hall sensors.
Hall Input Bypass Capacitor
The MAX9921 is optimized for use with external protec-
tion 0.01µF capacitors from both IN1 and IN2 to BAT.
These are essential to ensure robustness against auto-
motive transients. These capacitors may be tied to
GND instead of to BAT, but the connection to BAT is
recommended.
Low-Voltage Operation
To ensure correct operation of the Hall sensor at low
input voltages, it is important to consider the voltage
drop of the MAX9921 with low battery voltages. This
dropout voltage can be calculated using the formula:
V
DROPOUT
= I
HALL
x R
PU
I
HALL
is the maximum current which must be supplied to
the Hall sensor and R
PU
is the internal resistance of the
MAX9921, nominally 50Ω (see the Input Pullup
Resistance vs. V
BAT
graph in the
Typical Operating
Characteristics
). As an example, assume the use of a
HAL573 sensor, which draws a maximum current of
17mA. The dropout voltage is then 850mV and the
approximate minimum voltage supplied to the Hall sensor
is 6V - 0.85V = 5.15V, which is higher than the minimum
operating voltage of 3.75V specified for the HAL573.
MAX9921
IN_
R
V
CC
x_
GND
Figure 6. 3-Wire Hall-Effect Switches Configured as 2-Wire
PART MANUFACTURER WEBSITE COMMENT
HAL573-6 Micronas www.micronas.com 2-wire
HAL556/560/566 Micronas www.micronas.com 2-wire
HAL581/584 Micronas www.micronas.com 2-wire
A1140/1/2/3 Allegro www.allegromicro.com 2-wire
A1180/81/82/83 Allegro www.allegromicro.com 2-wire
Table 4. Partial List of Compatible Hall Switches
Note: The Hall switches listed above are supported by the MAX9921 using R
ISET
= 63.4k
Ω
.