FEATURE SENSING TECHNOLOGY
ADVANCED POWER AT YOUR FINGERTIPS C output
Wade Bussing systems engineer in the advanced sensor technologies division of Allegro MicroSystems, LLC reports on advanced power management using a 3D Hall-effect sensor IC with I2
W
ith the proliferation of human interface devices there is a growing
need for robust, non-contact sensing solutions that combine low cost, low power and low form factor. The Allegro ALS31300 3D Hall-effect sensor in a small DFN10 package is ideally suited for trigger, pushbutton, rotation, joystick and 2D slider joystick applications. The highly configurable power management options including low-power duty-cycle mode, sleep mode and wake on motion make this device ideally suited to battery-powered applications such as drones and camera gimbals, as well as console and mobile gaming controllers. This IC offers the ability to sense magnetic
fields in three different axes, for sensing either linear motion on any one axis or rotational motion using magnetic data from two axes. The sensor operates on supply voltages from 2.65 to 3.5V, and features highly configurable power management to maximise efficiency. The available power modes and typical supply currents for the
ALS31300 are as follows: • Active mode: The device continuously updates magnetic and temperature data. Supply current is a constant 3.4mA.
• Sleep Mode: The device is in a near powered-off state, with no magnetic or temperature data updates. Supply current is constant at 14nA. Sleep mode is valuable in applications where the supply voltage cannot be disabled but minimal power consumption is required.
• Low-power duty-cycle mode (LPDCM): The device toggles between fully active and inactive states, and periodically wakes up to refresh magnetic and temperature data. Supply currents are 3.4mA in the active state and 12μA in the inactive state.
LOW-POWER DUTY-CYCLE MODE In this mode, the ALS31300 toggles between active and inactive states, reducing overall current consumption. The average ICC mode varies depending on the settings used, and may range between 12μA and 2mA. Figure 1 shows the profile of the current as the ALS31300 toggles between active and inactive states. This IC offers eight discrete time frames for tINACTIVE
ranging from 0.5 to 1000ms; the supply current during tINACTIVE
12μA. The duration of tACTIVE
is is dependent on two
settings: the bandwidth selection (BW Select) and the number of active channels.
32 SEPTEMBER 2017 | ELECTRONICS The resulting current profile is shown in
the oscilloscope plot in Figure 2. Note that the I2
C commands are still processed even
while the ALS31300 returns to the inactive state. This is possible because the I2
processed in a separate domain from the main system clock. The interrupt feature enables further
system level power savings for applications requiring long battery life. This technique allows a system’s microcontroller to enter a low-power state and wait for an interrupt. Consider a system that is monitoring for
the presence of an applied magnetic field. For example, an electricity power meter may become inaccurate in the presence of large external magnetic fields. Assume this meter is sensitive to magnetic fields greater than 300Gs (30mT) and that there is a need for maximum current reduction in the system while on battery power due to a power blackout. The interrupt thresholds may be configured
independently for all three axes (X, Y, and Z). For this example, the threshold for each axis is set to a value equivalent to 300Gs. During normal operation of the meter, the
ALS31300 will be used in its full active mode since power consumption is not so much of a concern. In this mode, the device is consuming its typical active ICC at all times and continuously updating magnetic and temperature data. Assume that the electricity meter detects a
Figure 1:
The Allegro ALS31300 3D linear Hall-effect sensor IC
Figure.2:
Configuring low-power duty-cycle mode using maximum low-power mode count and sleep field
Figure.3:
Measured supply current profile in low-power duty-cycle mode
LOW-POWER DUTY-CYCLE MODE Users should consider the goals of the specific application while configuring low-power operation for the ALS31300. For example, assume that this device is used in a system that requires new magnetic data from two channels, X and Y, approx. every 500μs with full resolution. First the X and Y magnetic channels are
enabled and the Z channel is disabled. The bandwidth select value is set to code ‘0’ for full measurement resolution. Next, the value for the maximum low-power mode count, which controls the duration of tINACTIVE
, is set.
The device may then be put into low-power duty-cycle mode by setting the sleep field to a value of ‘2’.
loss of power from the grid and reverts to battery backup, but it is still necessary to monitor for tampering events or large external fields. Since these events are of interest but are not dangerous, it is possible to put this device in its most efficient low- power duty-cycle mode. The ALS31300 is then configured for one of times by setting the
its longest tINACTIVE
maximum low-power mode count to code 6, corresponding to a tINACTIVE
time of 500ms. The system’s microcontroller may now be
put into a deeper sleep state, from which it will be woken up by an active low interrupt signal from the ALS31300 in the presence of a field of more than 300Gs. The interrupt signal may be used as a wake-up event for the meter’s microcontroller, alerting the system to handle the tampering event.
Allegro MicroSystems
www.allegromicro.com e:
allegroeurope@allegromicro.com
C clock is
/ ELECTRONICS
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