Design of smart door lock motor driver IC with dynamic overcurrent detection function

This article introduces a smart door lock motor driver integrated circuit (IC) design with dynamic overcurrent detection function, which can support different power supply voltages and loads.

Currently, most smart door locks are powered by batteries. The battery life is usually about 6 months, up to a year. The length of the battery life depends on the wireless technology used (Wi-Fi, Bluetooth, ZigBee) and how often the door lock is opened and closed.

The motor in this design example is powered by four AA batteries.

Smart door lock manufacturers use different ways to detect the completion status of the bolt opening or closing: limit switches, accelerometers fixed on the shaft, Hall sensors, and magnet groups on the gears, etc. They all require corresponding external components and motor driver ICs.

One solution for detecting the position of the deadbolt is to measure the motor current and turn off the motor when the deadbolt is locked and the motor current rises to a defined threshold (see Figure 1). This approach does not require additional components. However, the threshold value must be determined based on a specific supply voltage, usually the battery voltage at full charge.

Figure 1: Motor current waveform

An improvement to this design is to measure the root mean square (RMS) current of each motor and set different current thresholds to compensate for different battery voltages (see Figure 2). This article describes how to configure the internal logic resources of the high-voltage GreenPAK™ IC for this design.

Figure 2: Motor current waveform with compensation

Configuration and operation principles

1. Operating principles

The design is divided into three parts, as shown in Figure 3:

• Motor stall detection: If the motor current is too high 100 ms after the motor starts, the motor driver IC shuts down its internal mechanism and measures the corrected motor current.

• Current protection threshold setting: Vref (internal logic resource of GreenPAK™ IC) of current CMP depends on motor operating current (set to higher than measured value).

• Overcurrent wait: If the motor operating current is higher than the selected value during this period, the motor will be turned off.

Figure 3: Design run

2. HV GreenPAK internal resource allocation/design

Figure 4: HV GreenPAK design

The register file (RegFile) of the current CMP is used to measure the motor current. There are 16 values, which switch from high to low (see Figure 5).

Figure 5: Register File (RegFile) Data

After 250 ms, the register file switches two values ​​upward (for example, it reaches the value of Byte8 before 250 ms and switches to the value of Byte10 after 250 ms) to set the new current threshold, as shown in Figure 6. When the motor current increases to this new current threshold, the mechanism will shut down (see Figure 7).

Figure 6: Register file usage

Figure 7: Motor shutdown process

For different power supply voltage and load, the motor current will be different. For higher motor current, the "Motor Shutdown Protection Current Level" will become higher.

Application Circuit

Figure 8: Typical application circuit

• PIN#2 Motor ON —> Rising edge turns on the motor

• PIN#3 Motor direction—> Motor rotation direction: HIGH —> forward rotation, LOW —> reverse rotation

• VDD range: 2.3 V – 5.5 V

• VDD2 range: 3.6 V – 6.0 V

Motor testing

Table 1: Motor parameters

When the supply voltage is 6.0 V, the peak value of the motor starting current is about 2A and decreases to the nominal value after 200 ms. The specific value depends on the supply voltage (see Figure 9-12).

Figure 9: Motor starting current waveform, supply voltage 3.6 V

Figure 10: Motor no-load current, supply voltage 3.6 V

Figure 11: Motor starting current waveform, power supply voltage 6.0 V

Figure 12: Motor no-load current, supply voltage 6.0 V

Design running waveform

normal operation

• Supply voltage: 6.0 V

• Motor root mean square (RMS) current: 170 mA

• Motor shutdown protection current: 620 mA

Figure 13: Unloaded motor, supply voltage 6.0 V

• Supply voltage: 3.6 V

• Motor RMS current: 127 mA

• Motor shutdown protection current: 460 mA

Figure 14: Unloaded motor, supply voltage 3.6 V

• Supply voltage: 3.0 V

• Motor root mean square (RMS) current: 310 mA

• Motor shutdown protection current: 670 mA

Figure 15: Load motor, supply voltage 3.0 V

Motor stalls during startup

The motor stall detection time is 100 ms. If the motor current is high within 100 ms after starting, the motor drive will be automatically shut down.

Figure 16: Stalled motor, supply voltage 3.6 V – 6.0 V

Summarize

This article describes a specific example of how to use the Dialog high voltage GreenPAK chip to illustrate the custom design of an integrated circuit for a specific motor and battery pack. This is a very flexible motor control and drive solution that uses configurable internal logic to support the designer's preferences. The integration of the motor drive in the GreenPAK chip means that the entire circuit can be packed into a very small physical space.

Designers can customize the circuit when the motor current or supply voltage changes. GreenPAK chips can also be used to design constant current and constant voltage motor drive control solutions with embedded protection functions such as over-current, under-voltage, and over-temperature protection.