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 VBAT 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.

Dual, 2-Wire Hall-Effect Sensor Interface

with Diagnostics

10

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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)

00

0

0

Fault on IN1 and/or IN2, or VBAT 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)