This design is powered from either two AAA batteries (default configuration) or an external 5-V to 30-V power supply. To translate the external power supply voltage to the 3.3-V rail utilized by the reference design, the TPS709 LDO is used. This ultra-low quiescent current, low-dropout (LDO) linear regulator allows the design to be powered from the higher voltage batteries typically seen in cordless power drills.

When the trigger is pressed, a magnet attached to the trigger moves along it. The magnet selected is a 3/16 in × 3/16 in cylinder magnet made of neodymium grade 52. The DRV5056 translates the sensed magnetic flux density from this magnet into an output voltage. When interfacing to an external power drill, this output voltage can be connected to the ADC of an integrated motor driver or microcontroller for changing the speed of the drill. The design has resistor dividers to scale the output voltage from the DRV5056 to a voltage input range suited for the ADC.

To maximize battery lifetime, the design is placed in a sleep mode until a wake-up event by pressing the trigger. A wake-up DRV5032 Hall-effect switch detects when the trigger is pressed by measuring the magnetic flux density of the trigger magnet. The DRV5032 device variant selected for the wake-up sensor is a unipolar sensor that responds to positive magnetic flux density readings. When the trigger is pressed, the sensed magnetic flux density at this Hall-effect switch is greater than the magnetic operating point (B_OP ), resulting in the output of the DRV5032 to be asserted low and the design to be awakened from sleep mode into active mode. The magnetic flux density at this sensor drops below the switch release point (B_RP) when the trigger returns back to its original position, resulting in the output of the DRV5032 to be asserted high and the design to be placed back in sleep mode. Since this sensor must always be ON, it is powered directly from VCC.

The output of the wake-up Hall-effect sensor can be falsely triggered to be asserted low when exposed to a positive magnetic flux from a strong magnet that is external to the system. This external magnet is not to be confused with the trigger magnet, which is internal to the system. The design has three optional, additional DRV5032 devices to prevent the system from entering active mode in the presence of external magnetic fields, even if the wake-up Hall sensor is falsely triggered to be asserted low by the external magnetic fields. Each of these three additional Hall sensors can be individually disabled, which allows selection of the desired level of protection against external magnetic fields.

One of the additional DRV5032 devices is referred to as the stray field sensor. The stray field sensor is a unipolar sensor that responds to the negative magnetic flux. To reduce current consumption, the stray field sensor is only powered when the wake-up Hall sensor is asserted low, which occurs when the trigger is pressed or if the wake-up Hall sensor detects a strong, positive magnetic flux density reading. During the entire 10-mm trigger displacement path, the stray field sensor should detect negative magnetic flux density readings from the trigger magnet and the sensed magnetic flux density of the stray field sensor should exceed the absolute value of the B_OP of that sensor. Regardless of the state of the wake-up or other Hall sensors, the system is in sleep mode if the stray field sensor detects positive magnetic flux density readings or if the absolute value of the sensed magnetic flux density does not exceed the absolute value of B_OP. As a result, if a strong external magnet is applied to this design and it causes the stray field sensor to detect a net positive magnetic flux density reading, the system will still be placed in sleep mode even if the output of the wake-up Hall sensor is falsely triggered to be asserted low, thereby helping to prevent the design from turning ON due to the wake-up Hall sensor being accidentally triggered by external magnets.

Even with the stray field sensor, it is possible to fool the design into waking up from sleep mode using a strong, external magnet if the external magnet causes the wake-up sensor to detect a positive magnetic flux density while the stray field sensor detects a negative magnetic flux density. To make this design robust to this scenario, two other optional DRV5032 sensors can be used in this design. These other DRV5032 devices are referred to as the tamper Hall-effect sensors and are powered from VCC. The tamper Hall sensor variants used in this design are omnipolar devices that respond to both positive and negative magnetic flux density readings. Regardless of the state of the other Hall sensors, the system is put in sleep mode if any of the two tamper Hall sensors detects a strong, external magnetic field.

When any of the tamper sensors are enabled, a logic gate is needed to combine the outputs from the different Hall sensors to produce one signal that informs the design when the system should be in sleep mode or active mode. The SN74HCS00 quadruple 2-input NAND gate combines the outputs from the different Hall sensors to produce an active-low signal that is asserted low when the system should be awake and asserted high when the system should be in sleep mode. If tamper Hall sensors are not needed, the reference design can be redesigned to not use the SN74HCS00 by using the following signals as the active-low wake-up signal instead of the SN74HCS00 output:

• The output stray field's Hall-effect switch's output if the stray field sensor is not needed
• The wake-up Hall-effect switch if the stray field is also not enabled.

The TPS22917 in this design is a load switch that can disconnect power from the DRV5056 when the system is asleep, thereby reducing current consumption. Since the TPS22917 has an active high switch control input and the SN74HCS00 produces an active low input, the output from the SN74HCS00 must be inverted before connecting to the switch control input of the TPS22917. The SN74AUP1G00 single 2-input NAND gate is configured to do this signal inversion. Note that the design can also be redesigned to replace the active-high TPS22917 and SN74AUP1G00 with just one active-low TPS22916 device.

As an alternative to powering the DRV5056 from the output of the TPS22917, the DRV5056 in this design can also be powered from a voltage-regulated, external 3.3-V power supply. If reducing the current consumption is desired, the power to the DRV5056 can be switched OFF externally when the system is in sleep mode.

In addition, there are two enable signal options in the design that can be used to provide information to any external systems about whether the design is in sleep mode or active mode. The first enable signal comes from the output of the SN74HCS00. This enable signal is at 0 V when the design is in sleep mode up and 3.3 V when in active mode. For external signals that require an enable signal with a higher voltage, select the second enable signal option.

The second enable option is created by taking the first enable signal and feeding it into an NPN transistor switch circuit. Connect the voltage source input to the NPN circuit to a drill battery to produce a wake-up signal that is at the voltage level of the drill. As an example, if the voltage input of the circuit is connected to an 18-V drill battery, the second enable option will have an output voltage of 18 V when the system is in active mode and 0 V when the system is asleep (note that this is the opposite polarity of enable signal option 1). If enable signal option 2 is not needed, remove the NPN transistor circuit from the design. By default, the NPN transistor circuit is not populated on the design.

An example use-case for the enable signals is for triggering any external systems connected to the design to go to sleep or active mode along with the design. To trigger the external systems for sleep mode, one option is to remove power from these external systems using an external eFuse, load switch, or hot swap controller that has an enable pin that connects to one of the enable signals from this design. Based on the voltage generated by the enable signal of the design, the power to the external system is either connected or disconnected accordingly.

The design also has a TLV9061 op amp; however, this op amp is just for driving one of the LEDs to change its brightness based on the output voltage of the DRV5056. This LED is for showcasing the movement of the trigger when the design is in standalone mode. Since it is mainly for demonstration purposes, this circuit is not needed in final system implementations.

Note that this design has components that were added to demonstrate this design in a standalone mode. Figure 2-2 shows a simplified version of this block diagram that excludes the components in the design that are for showcasing standalone mode, so they are not needed in a final system implementation. In the simplified block diagram, it assumes that there is an external device that turns OFF power to the system based on the output enable signal and that the DRV5056 is powered from an external power supply that is automatically disabled when the system is in sleep mode. In addition, if protection against external magnetic fields is not needed, the tamper DRV5032, stray field DRV5032, and SN74HCS00 can be removed from this simplified block diagram.