The flowmeter is powered by lithium manganese dioxide (LiMnO2) and lithium thionyl chloride (lisocl2) batteries. Compared with LiMnO2 battery, lisocl2 battery can achieve higher energy density and better cost per watt, so it is widely used in flowmeter. However, the pulse response of lisoci2 battery is poor, which will lead to a significant drop in voltage during transient current load.

Buffer elements such as hybrid layer capacitor (HLC) or electric double layer capacitor can be combined with lisocl2 battery to improve its pulse load capacity, but the reliable combination of HLC and lisocl2 battery is costly and will affect the total cost of the instrument. Since the battery also affects the maintenance requirements and service life of the flowmeter, the alternative method of combining the step-down / boost converter with the lisocl2 battery will help to reduce the overall solution cost and prolong the service life of the flowmeter.

This paper will introduce five excellent practices when using buck / boost converter and lisoci2 battery, so as to prolong battery life and reduce overall maintenance and cost requirements. First, we discuss some common design challenges.

Key design problems in designing intelligent flowmeter system

A typical flowmeter system consists of five important components: metering front end, communication front end, microcontroller (MCU), power management integrated circuit and protection front end.

In addition to these requirements, the flowmeter is usually small in size, must be operated on site for more than 15 years, and the maintenance cost should be as low as possible.

Power consumption overview of typical flowmeter

Table 1 lists the power consumption profile of the standard flowmeter, which is divided into three working modes.

Table 1: power consumption overview of standard flowmeter

Excellent practice of designing flowmeter with buck / boost converter

To help extend the battery life and performance of smart flowmeter design, please consider the following five excellent practices.

Good practice 1: limit the peak current provided by the battery.

As shown in Figure 1 (data sheet from saft ls17330 battery), lisocl2 battery usually does not support the high dynamic range curve required by the radio communication system used in smart flowmeter. One way to solve this problem is to use tps63900 buck / boost converter and a buffer element to filter the battery current.

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Figure 1: typical discharge curve of saft ls17330 at + 20 ° C

Good practice 2: keep the output and input voltage levels independent.

Achieving an independent voltage level optimizes the input current curve drawn from the battery and the output current supplied to the load. This approach also simplifies the use of buffer elements between input and output.

Excellent practice 3: use a converter with low working current and standby current less than 500na.

In order to optimize the energy use of the system, the average current consumption of the converter must be negligible compared with the current consumption of the system. For example, if the average current consumption of the flowmeter is about 5 µ a, 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.

Treat the system as a resistor powered by the converter. Keeping the power supply voltage low can reduce the standby current consumed by the system.

Excellent practice 5: optimize the voltage load of each working 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.

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Figure 2: Battery front end based on tps63900

Some measurement data

Load transient measurements are performed under the following conditions:

·Standby current (IOUT): 158 µ a within 999ms

·Active current (IOUT): 97.4ma within 1ms

·Input voltage (VIN): 3.6V

·Output voltage: 3.0 V

·Output capacitance: 300 µ f

As shown in figures 3 and 4, the tps63900 can filter the input current drawn from the battery while maintaining excellent efficiency and adjusting the output voltage.

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Figure 3: impulse response of tps63900

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Figure 4: efficiency of tps63900 at input voltage of 3.6V

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 battery, as well as the problems that cannot be solved by traditional more complex and high-cost methods for a long time.

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Figure 5: tps63900 solution area

For more information on designing with tps63900 buck / boost converter, please refer to other resources or comments in this article.

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