Now, more and more smart sensors are added to smart building automation, such as smart cat eyes, doorbells, portable cameras, and smoke detectors. These products are usually powered by batteries, such as common 18650 lithium batteries, AA dry batteries. So in battery-powered products, how to design a reasonable power supply scheme is the key issue to prolong the battery life.

Author: Betty Guo

Now, more and more smart sensors are added to smart building automation, such as smart cat eyes, doorbells, portable cameras, and smoke detectors. These products are usually powered by batteries, such as common 18650 lithium batteries, AA dry batteries. So in battery-powered products, how to design a reasonable power supply scheme is the key issue to prolong the battery life.

The article will first analyze the characteristics of commonly used batteries, and then provide a technical design scheme with a visual doorbell as an example.

1. Analysis of different battery characteristics

In smart home products, the commonly used battery types are mainly dry batteries and lithium batteries. Lithium batteries are divided into 18650 cylindrical batteries, polymer lithium batteries, nickel-hydrogen batteries and so on. Since the battery voltage will decrease as the battery is used, requirements are placed on the design of the post-stage circuit. The following are the characteristics of these three lithium batteries:

Battery Type 18650 Cylindrical Battery Polymer Lithium Battery NiMH Battery

Features – High energy density.

– High working voltage platform.
-Long cycle life.
– Small self-discharge – Pro-weight, thin volume, high capacity density
– Good charge and discharge characteristics
– no memory effect
-low cost
– Good fast charging feature
– Long life, no memory effect
– Wide operating temperature range

No matter what type of lithium battery it is, its discharge characteristics and power curve are basically the same. Figure 1 below shows the relationship between the voltage and the capacity of the lithium battery at different discharge rates.

Analysis of battery characteristics in smart home and sharing of design solutions
Figure 1 The relationship between the voltage and the capacity of the lithium battery at different discharge rates

It can be seen from Figure 1 that for different discharge rates, the relationship between battery voltage and power is different, and it is also related to operating temperature. Therefore, it is impossible to judge whether the battery has power or not based on the voltage of the battery. Under normal circumstances, when the lithium battery voltage drops to 3V, there is basically no remaining power in the battery. At this time, even through the boost chip, it cannot provide enough energy for the load to use.

At the same time, Figure 2 below shows the relationship between battery voltage and power under different load conditions. Under different load conditions, the same battery voltage will have a large difference in the remaining battery power. SOC (State of Charge) is the state of available electrical energy in a lithium battery, usually expressed as a percentage. The relative state of charge ranges from 0% to 100%, 100% when the battery is fully charged and 0% when fully discharged. The load current is divided into pulse current and continuous current. In the case of pulse current and continuous current discharge, the discharge curve of the battery will also be different. For example, MCU, Sub-1G type loads will generate pulse current when starting, and for LEDs, etc., it is continuous. current.

Analysis of battery characteristics in smart home and sharing of design solutions
Figure 2 The relationship between battery voltage and power under different loads

Another type of widely used batteries are button cells and dry cells. The common alkaline button battery has a nominal voltage of 1.5V, and the lithium button battery is 3V. The working voltage of the dry battery is 0.7V~1.5V. When the battery is discharged to a voltage lower than 0.7V, it will not work normally. At the same time, its constant current discharge curve is also related to external environmental factors.

As can be seen from the above two figures, taking the 3V system as an example, for the button battery, when the battery voltage drops to 2.5V, the battery still has remaining power for the system to use. But because the battery voltage is too low to provide enough voltage, in order to further prolong the battery life, the 2.5V voltage needs to be boosted to meet the system working level.

2. Examples of power supply design applications

Taking into account the current battery-powered smart home products, data transmission is usually carried out wirelessly, such as WIFI and Bluetooth. As mentioned above, when the communication device starts up instantaneously, there will be a large startup current (usually 1A~2A) that instantly pulls down the battery voltage. When the battery voltage is too low, the subsequent power chips enter UVLO and the system stops working. Combined with the above applications of analyzing battery voltage and power, buck-boost chips can be used to meet common applications when designing a power supply.

Combined with the need for low battery power consumption, ICs with lower quiescent currents are required, typically at the nA/uA level. Combined with the use of WIFI/BLE including speakers and LEDs in smart homes, a large operating current will be required, generally about 1A~2A. The low Iq 2A output current buck-boost chip TPS63802 is very suitable for this type of application.

TPS63802 is a buck-boost chip with a quiescent current of 11uA and 2A output. The input voltage range is from 1.3V to 5.5V, which can cover most smart home product applications and help users to further reduce the design volume. According to TI’s reference design, the overall design size can be reduced to ~28mm. For details, please refer to TI’s official website, TPS63802 technical information.

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