[Guide]This article will introduce five good practices when using buck/boost converters and LiSOCI2 batteries to maximize battery life and reduce overall maintenance and cost requirements. First, we discuss some common design challenges.
The flowmeter uses lithium manganese dioxide (LiMnO2) and lithium thionyl chloride (LiSOCl2) batteries. Compared with LiMnO2 batteries, LiSOCl2 batteries can achieve higher energy density and better cost per watt, so they are commonly used in flow meters. However, the impulse response of the LiSOCI2 battery is poor, which will cause the voltage to drop significantly during the transient current load.
Buffer elements such as hybrid layer capacitors (HLC) or electric double layer capacitors can be used in combination with LiSOCl2 batteries to improve their pulse load capacity, but the reliable combination of HLC and LiSOCl2 batteries is costly and will affect the total cost of the meter. Because the battery also has an impact on the maintenance requirements and life of the flowmeter, the alternative method of combining the buck/boost converter with the LiSOCl2 battery helps to reduce the overall solution cost and extend the life of the flowmeter.
This article will introduce five good practices when using buck/boost converters and LiSOCI2 batteries to maximize battery life and reduce overall maintenance and cost requirements. First, we discuss some common design challenges.
Key design problems when designing intelligent flowmeter systems
A typical flow meter system includes five important components: metering front-end, communication front-end, microcontroller (MCU), power management integrated circuit and protection front-end.
In addition to these requirements, flow meters are usually small in size, must be operated on site for more than 15 years, and maintenance costs should be as low as possible.
Power consumption profile of a typical flow meter
Table 1 lists the power consumption profile of standard flow meters, divided into three working modes.
Table 1: Power consumption overview of standard flow meters
Good practice for designing flow meters with the help of buck/boost converters
To help extend the battery life and performance of smart flowmeter designs, please consider the following five good practices.
Good practice 1: Limit the peak current provided by the battery.
As shown in Figure 1 (taken from the data sheet of the SAFT LS17330 battery), LiSOCl2 batteries generally do not support the high dynamic range curve required by the radio communication system used in smart flow meters. One way to solve this problem is to use the TPS63900 buck/boost converter and a buffer element to filter the battery current.
Figure 1: SAFT LS17330 typical discharge curve at +20°C
Good practice 2: Keep the output and input voltage levels independent.
Achieving independent voltage levels optimizes the input current curve drawn from the battery and the output current provided to the load. This approach also simplifies the use of buffer components between input and output.
Good practice 3: Use a converter with a low operating current and a standby current of less than 500nA.
In order to optimize the energy use of the system, the average current consumption of the converter must be negligible compared to the current consumption of the system. For example, if the average current consumption of the flow meter is about 5µA, the standby current of the converter should be less than 500nA.
Good practice 4: Reduce the voltage of the power supply system as much as possible.
Think of the system as a resistor powered by the converter. Keeping the power supply voltage low can reduce the standby current consumed by the system.
Good practice 5: Optimize the voltage load of each operating mode through dynamic voltage regulation.
As shown in Figure 2, the dynamic voltage regulation function of TPS63900 enables the converter to dynamically change its output voltage to supply power to the load at the ideal operating point.
Figure 2: Battery front end based on TPS63900
Some measurement data
●Measure load transient under the following conditions:
●Standby current (Iout): 158µA in 999ms
●Activity current (Iout): 97.4mA within 1ms
●Input voltage (Vin): 3.6V
●Output voltage: 3.0 V
●Output capacitance: 300µF
As shown in Figure 3 and Figure 4, the TPS63900 can filter the input current drawn from the battery while maintaining excellent efficiency and regulation of the output voltage.
Figure 3: Impulse response of TPS63900
Figure 4: Efficiency of TPS63900 at 3.6V input voltage
By combining ultra-low standby current consumption, excellent transient response, output noise level, and dynamic voltage regulation in a 2.5mm x 2.5mm package or 21mm2 total solution size, the TPS63900 can help solve the problems encountered when using LiSOCl2 batteries, and The traditional more complex and high-cost methods cannot solve the problem for a long time.
Figure 5: TPS63900 solution area
(Source: Texas Instruments)
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