Application of INA series current detection devices in TWS battery box

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Application of INA series current detection devices in TWS battery box [Copy link]

With the explosive growth of TWS headphones, higher requirements are placed on the battery life of headphones and battery boxes. Generally, the battery life can be improved from the following points:

1) Use larger capacity batteries to improve battery life. The disadvantage is that the small size of the earphones and battery box makes it difficult to accommodate large batteries, and large capacity batteries will be too heavy.

2) Improve work efficiency and reduce device leakage current. The headphone chip can use a linear charging chip with only 400-nA leakage current, such as BQ25150A. The charging box chip can use a switching charging chip with only a few microamperes leakage current, such as BQ25619, to increase charging speed and improve thermal issues.

3) Optimize the system, reduce losses and increase battery life.

Next, we will introduce the application of TI's current sampling chip in optimizing systems and reducing losses from the third aspect.

Figure 1 is a typical TWS headset system block diagram. The red current sampling part is used to turn off the boost circuit when the current is small, generally less than 5mA, to reduce losses.

Figure 1

Current sampling circuits generally use two methods:

1) Using a sampling resistor of about 1 ohm is simple and convenient. The voltage on the resistor can be directly read to the MCU to control the switch of the boost circuit. However, a 1 ohm resistor will also bring a greater voltage drop. When the charging current of the earphone is reduced to about 5mA, the voltage drop on the resistor is only 5mV, which will not have any effect. When the charging current of the earphone increases, the charging current of the two earphones may reach 300mA or more, and the voltage drop on the resistor will be 300mV, which cannot be ignored. The table below calculates the error of a boost chip, including a feedback error of 2% and a voltage divider resistor error of 1%. From the calculation, it can be seen that the maximum deviation of the output voltage is +/-0.18V. In order to obtain a stable 5V voltage at the output end, the 300mV voltage drop on the resistor needs to be considered. The output voltage needs to be added with 300mV, and the maximum will reach 5.48V. When lightly loaded, this voltage will bring considerable risks to the back-end system.

2) Use current detection chip. The following figure 2 shows the block diagram of TI's INA series current detection chip, which has the following advantages:

The external current sampling resistor can use milliohm-level resistance, which greatly reduces the voltage drop on the resistor. While reducing losses, the accuracy of the output voltage is guaranteed.

The minimum current accuracy can be about 1% of the full range, which makes it easy to achieve low current shutdown. At the same time, it can charge the earphone battery more fully and improve battery life.

There are small package options such as SC70, SOT563, DSBGA and UQFN, with simple periphery, flexible and convenient.

If you want to monitor two headphones separately, you can also use the dual-channel INA2180A2 chip.

Figure 2

The following four materials can be selected according to needs. Among them, INA2180 is dual-channel and can detect the current of two headphones at the same time.