Extending Battery Life in Smart E-Locks

Introduction

New smart e-lock power architectures greatly increase battery life by reducing system standby power consumption. Power management is a key design challenge in Internet of Things (IoT) and smart home products. For smart electronic locks, product downtime due to dead batteries or frequent battery changes can lead consumers to abandon the product. Malfunctions can also result in users being locked out. Smart e-locks face high peak current demands from radios and motors, coupled with long periods of low-power standby, necessitating advanced power architectures to extend battery life.

System Overview

Smart e-lock systems typically include integrated circuits (ICs) such as light-emitting diode (LED) drivers, Wi-Fi® communications, and more. This document focuses on three key ICs: 1) a microcontroller with wireless connectivity (e.g., Bluetooth® low energy), 2) a motor driver, and 3) power management.

The term "events" refers to the motor's activity during locking or unlocking. For example, locking and then unlocking the front door counts as two events. A common benchmark for performance comparison is 24 events per day.

Wireless Microcontroller

In a smart e-lock, the wireless microcontroller (MCU) communicates wirelessly with a phone to lock and unlock the door. To minimize lag, the MCU must periodically send an advertising event signal and then return to a low-power standby state, typically consuming single-digit micro-amps (μA). This low standby current is crucial for long battery life.

Advertising events, distinct from locking/unlocking events, occur when the wireless microcontroller wakes up to transmit identifying information and listen for incoming connection requests from peer devices like smartphones. The advertising period is programmable, ranging from 20 ms to 10.24 seconds. A longer period reduces power consumption but increases connection time. A 500 ms advertising period offers a good balance between power consumption and connection speed.

Figure 1 shows a current consumption waveform during a Bluetooth low energy advertising event. It depicts periodic high current spikes (around 6.1mA) for advertising events, separated by a 500ms period, with a low standby current of approximately 1.2μA.

Motor

All electronic locks require a motor and motor driver to wirelessly lock and unlock the door. The motor's current profile varies based on the torque needed for different door locks, often peaking around one amp. Motor driver efficiency is significantly impacted by the on-resistance of its MOSFETs; lower on-resistance yields higher efficiency. A motor driver, such as the DRV8833, must be compatible with the e-lock's power source and motor. Motor driver voltage is typically around 5V.

Power Management

Power management is essential for converting varying battery voltages to the specific voltages required by the wireless microcontroller, motor driver, and other subsystems. While it adds cost, size, and inefficiency, it must be integrated into the overall system design. Power management efficiency is critical, especially in standby modes where micro-amps (μA) of current are drawn. Achieving efficiency at both light and heavy loads is challenging and requires specialized ICs.

Power management systems rely on user-installed batteries. The choice of battery type, number, and configuration is closely linked to the system's power architecture and management selection. AA-size alkaline batteries are common due to their availability and low cost. With an average per-cell voltage of 1.25V, four AA cells can provide over four years of battery life. Newer, cost-effective power management solutions using switching DC/DC converters (boost or buck) offer significantly higher efficiency and longer battery life compared to traditional low drop-out (LDO) linear regulators, often more than doubling battery life with minimal added cost.

Linear Regulator (LDO)

In an LDO-based system, four AA batteries are typically connected as 4s1p (four cells in series, one parallel) to create a 5V supply for the motor. An LDO then converts this 5V down to the 2.5V required by the wireless microcontroller. LDOs typically achieve around 50% efficiency at higher loads, but efficiency drops dramatically in standby mode due to quiescent current. For instance, the TPS76625, converting 5V to 2.5V, shows only 2% efficiency at a 1.2μA standby load due to its 35μA quiescent current, leading to higher standby power consumption and reduced battery life.

Figure 2 illustrates an LDO-based smart e-lock block diagram. Four AA batteries (4s1p) supply 5V to a motor driver and a power management LDO. The LDO converts 5V to 2.5V for the wireless microcontroller. The diagram notes LDO efficiencies of 50% at full load and 2% at standby, with 100% motor driver efficiency.

Boost Converter

To improve standby efficiency, a boost converter architecture can be used. Here, the battery configuration is often rearranged to 2s2p (two series, two parallel cells), providing a 2.5V output that is a direct match for the wireless MCU, resulting in 100% efficiency for that connection. However, a boost converter is still required to step up the 2.5V to the 5V needed by the motor. A typical boost converter, like the TPS61030, offers around 85% efficiency when boosting. Due to the boost ratio and efficiency, these converters draw higher currents from the battery, increasing losses.

Figure 3 presents a boost converter-based smart e-lock block diagram. Four AA batteries (2s2p) provide 2.5V directly to the wireless microcontroller (100% efficient). A power management boost converter steps this up to 5V for the motor driver, with 85% full-load efficiency.

Buck Converter

A buck converter can be used in place of an LDO to significantly increase efficiency. An ultra-low power buck converter, such as the TPS62745, can achieve approximately 90% efficiency. The motor subsystem remains nearly 100% efficient as it connects directly to the battery pack. Standard buck converters often have high quiescent current (I_Q), reducing standby efficiency. However, ultra-low power buck converters designed for IoT applications feature very low I_Q, enabling over 67% efficiency at typical standby-mode load currents with a 2.5V output.

Figure 4 outlines a buck converter-based smart e-lock block diagram. Four AA batteries (4s1p) supply 5V to both a motor driver and a power management buck converter. The buck converter steps down 5V to 2.5V for the wireless microcontroller, showing 90% full-load and 67% standby efficiency for the buck converter.

Figure 5 displays the efficiency of an ultra-low power buck converter across different input voltages and output currents. It demonstrates that efficiency remains high, exceeding 67%, even at typical standby-mode load currents for a 2.5V output.

Power Management Architecture Comparison

The efficiency of the power architecture is critical for extending smart e-lock battery life, as power management components consume battery energy. Figure 6 compares the daily power consumption of three architectures (LDO, Boost, Buck) using pie charts and bar charts. The pie charts illustrate the power distribution among the wireless MCU, motor driver, and power management for each architecture, with chart size indicating total power consumption. Figure 7 plots battery life in years against the number of events per day for these architectures. For applications with fewer than 36 events per day, both Buck and Boost architectures offer improved battery life over LDO. The Buck architecture provides the best battery life for higher event rates, while the Boost architecture's performance degrades relative to LDO at very high event rates due to increased motor power requirements.

Figure 6 compares the daily power consumption of three power architectures (LDO, Boost, Buck) via pie charts and bar charts. Pie charts show the percentage breakdown of power usage for the wireless MCU, motor driver, and power management for each architecture. Bar charts indicate the total power consumption for each architecture.

Figure 7 plots battery life in years against the number of events per day for LDO, Boost, and Buck architectures. It highlights that for fewer than 36 events per day, both Buck and Boost offer improved battery life over LDO. The Buck architecture provides the best battery life for higher event rates, while the Boost architecture's performance degrades relative to LDO at very high event rates due to increased motor power requirements.

Conclusion

New power architectures in IoT-connected devices like smart e-locks enable significantly higher battery life compared to traditional LDO-based implementations. Switching power converters, either boost or buck, increase battery life for smart locks with fewer than 36 lock/unlock events per day. An ultra-low power buck converter can more than double battery life for lower event systems and nearly double it for higher event systems. The ultra-low quiescent current (I_Q) of such a buck converter is key to extending battery life by vastly improving efficiency during lengthy standby modes. Designers of connected and IoT products should re-evaluate their power management architectures to ensure optimal battery life.

References

• Joakim Lindh, Christin Lee, and Marie Hernes. Measuring Bluetooth Low Energy Consumption, Texas Instruments Application Report (SWRA478), December 2016
• Smart e-lock Reference Design Enabling 5+ Years Battery Life on 4× AA Batteries, TI Design (TIDA-00757)
• Chris Glaser. I_Q: What it is, what it isn't, and how to use it, TI Application Note (SLYT412), 2Q11
• Product folders: CC2640, DRV8833, TPS76625, TPS61030, TPS62745

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