Door lock power topology design that can be used for more than 5 years with AA batteries

What power supply topology should be used to design a smart door lock's battery to last longer?

Power device topologies commonly used in smart door locks include linear regulators, boost (step-up), and buck (step-down).

The success of IoT (Internet of Things) devices depends on user convenience. Basically, user convenience means that electronic devices are easy to connect and control. The less maintenance required for these connected electronic devices, the better. Who would put up with the inconvenience of having to stop their electronic devices frequently to replace batteries?

The power supply of a smart door lock must support a wireless microcontroller (MCU) such as the SimpleLink™ Bluetooth low energy CC2640R2F solution, a motor driver such as the DRV8833 for opening and closing the door lock, and other peripherals such as LEDs. There are three ways to convert the voltage from the battery to the load: stepping down using a low dropout (LDO) linear regulator, stepping up using a boost DC/DC converter, or stepping down using a buck DC/DC converter.

Figure 1 shows a basic block diagram of a smart door lock using an LDO such as the TPS76625. Engineers choose LDOs because of their price. In most cases, the IC price of an LDO is lower than that of a buck (step-down) or boost (step-up) converter IC. However, linear circuits are less efficient and therefore shorten battery life.


Figure 1: Block diagram of a smart door lock using LDO

Figure 2 is a block diagram using a boost converter such as the TPS61030. The four AA batteries are rearranged to support the boost topology for the motor driver, and the wireless MCU is connected directly to the batteries. Although this is very efficient, converting to high power using a boost converter for the motor driver leads to a higher power loss in absolute terms. Let's find a more efficient way.


Figure 2: Smart door lock block diagram using boost converter

Figure 3 is a block diagram when using a buck converter. The efficiency figures are provided when using the TPS62745 ultra-low power buck converter. Using an ultra-low power converter can significantly improve the efficiency in the system's standby mode. The smart door lock is in standby mode most of the time except when the door lock is opened and closed.


Figure 3: Smart door lock block diagram using buck converter

Figure 4 compares the battery life of each topology. This graph assumes that the door lock is opened and closed 12 times a day. As you can see from the graph, the battery life is proportional to the frequency of connection from the wireless MCU. The more frequently the wireless MCU connects to detect someone trying to open the door lock, the more power it consumes. When the interval between connections is 500ms, a battery life of 60 months (5 years) is possible using 4 AA alkaline batteries.


Figure 4: Battery life comparison with different power topologies

So far, we have briefly looked at different power topologies. The white paper “Extending Battery Life with Smart Door Locks” and the smart door lock reference design that provides 5+ years of battery life using 4 AA batteries are available on the TI website for more information.