Dual, 2-Wire Hall-Effect Sensor Interface

with Diagnostics

14

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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 IIH/IIL cur-

rent threshold limits for both a lower current range and

a higher current range which correspond to the selec-

tion of the RISET 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:

VDROPOUT = IHALL x RPU

IHALL is the maximum current which must be supplied to

the Hall sensor and RPU is the internal resistance of the

MAX9921, nominally 50Ω (see the Input Pullup

Resistance vs. VBAT 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

VCC

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 RISET = 63.4kΩ.