Hall-Effect Latch for High Temperature Operation
A1225, A1227
and A1229
9
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
FUNCTIONAL DESCRIPTION AND APPLICATION INFORMATION
SWITCHING BEHAVIOR
The output of the A1225, A1227, and A1229 devices switches
low (turns on) when a magnetic field perpendicular to the Hall
element exceeds the operate point threshold, B
OP
(see figure 1).
After turn-on, the output is capable of sinking 25 mA and the
output voltage is V
OUT(sat)
. Notice that the device latches; that is,
a south pole of sufficient strength towards the branded surface
of the device turns the device on, and the device remains on with
removal of the south pole.
When the magnetic field is reduced below the release point, B
RP
,
the device output goes high (turns off). The difference between
the magnetic operate point and release point is the hysteresis,
B
HYS
, of the device. This built-in hysteresis allows clean switch-
ing of the output, even in the presence of external mechanical
vibration and electrical noise.
When the device is powered-on in the hysteresis range, less than
B
OP
and higher than B
RP
, the device output goes high. The correct
output state is attained after the first excursion beyond B
OP
or
B
RP
.
APPLICATION INFORMATION
The simplest form of magnet that will operate these devices is a
ring magnet, as shown in figure 2. Other methods of operation
are possible.
In three-wire applications the device output is connected through
a pull-up resistor to the supply pin or separate battery voltage
(figure 3). Switching of the output signal indicates sufficient
change of the magnetic field.
B
OP
B
RP
B
HYS
V
CC
V
OUT
V
OUT(sat)
Switch to Low
Switch to High
B+B–
V+
0
Figure 1. Output switching characteristics
Figure 2. Typical magnetic target configuration using a ring magnet
Figure 3. Typical 3-wire application circuit
A122x
VOUT
GND
C
BYPASS
C
L
(Optional)
V
PULLUPV+
R
PULLUP
VCC
Device
Output
