[Introduction]In recent years, with the increasing demand for battery power from consumer Electronic terminals such as smartphones and tablet computers, consumers’ demands for the charging speed and charging experience of electronic products are also increasing. For high-efficiency and high-power charging applications of large-capacity lithium batteries, TI has launched a fast-charging chip with a switched capacitor architecture represented by the BQ25980, which can achieve a fast-charging solution with an efficiency of up to 98.6%, bringing consumers fast, stable, efficient and easy-to-use fast charging. used charging experience.
In the practical application of BQ25980, as shown in Figure 1, it is usually necessary to use traditional inductive charging chips (such as BQ2579x series) to form a fast charging system with master and slave. Among them, the BQ2579x as the main charging chip is responsible for completing the basic detection of the charging protocol, battery pre-charging, trickle charging and cut-off charging, and provides stable power supply for the system load combined with the power path management function; The BQ25980 is responsible for realizing high-efficiency and high-power fast charging in the constant current charging and constant voltage charging stages of the battery.
Figure 1. Typical Application Scenario of BQ25980
In order to realize the status monitoring and reliable protection of the fast charging system and chip operation, the BQ25980 chip integrates an analog-to-digital conversion (ADC) circuit of up to 16 bits, which can be used to read the working voltage, current, and chip junction temperature of the charging chip in real time. and cell temperature and other important parameters. Among them, regarding the sampling measurement of the working current, due to the different sampling circuit principles, the BQ25980 has a certain difference in the measurement accuracy of the charging output current (IBAT) and the measurement accuracy of the bus input current (IBUS); the details are shown in the following table:
It can be seen that in terms of the current detection function of BQ25980, IBAT_ADC has higher sampling measurement accuracy than IBUS_ADC. Further, considering that in the actual charging system, the accurate measurement and monitoring protection of the battery charging current is usually undertaken by a high-performance fuel gauge chip (such as BQ28Z610), and does not need to rely on the IBAT_ADC of the charging chip; therefore, in the In a specific application, we can skillfully use IBAT_ADC and related resources of its sampling circuit to sample and measure the bus input current IBUS of BQ25980, thereby significantly improving the detection accuracy of BQ25980 input current; at the same time, due to the stable operation of the switched capacitor topology When the current of the input terminal is only about half of that of the output terminal, transferring the current sampling resistor to the input terminal can also reduce the heat loss of the resistor to a certain extent. The specific application method is shown in Figure 2.
Figure 2. High Precision BQ25980 Input Current Sensing Scheme
As shown in the figure, in order to use the IBAT sampling circuit (low-side sampling) to measure the bus input current of the BQ25980, it is necessary to connect the high-precision current sampling resistor Rsense in series (2mΩ or 5mΩ) to the low-side side of the input end of the BQ25980, and connect the Rsense resistors to two The terminals are respectively connected to the SRP and SRN pins of the chip (the positive and negative input pins of the current sensing sampling respectively). Among them, when the sampling resistor Rsense is selected as 2mΩ, the resolution of reading the current using IBAT_ADC can reach LSB=1mA.
After the hardware connection is completed, the registers of the BQ25980 can be read and written through the I2C command; after the ADC function of the chip is correctly configured and enabled, the value of IBAT_ADC can be obtained by reading the 0x31h register, and the bus input current of the BQ25980 can be obtained.
Further, a verification experiment can be performed on the current detection method based on the EVM board of the BQ25980. The specific test scheme is shown in Figure 3.
Figure 3. BQ25980 input current detection scheme verification experiment
Based on the EVM board of BQ25980, the current sampling resistor and its sensing circuit are replaced to the input side; the DC programmable power supply is used as the input power supply, and a high-precision multimeter is connected in series in the input loop to measure the input current. Connect the EVM board of the BQ25980 to the computer through the EV2400 interface board, and then use the host computer software bqStudio to read, write and monitor the registers of the BQ25980.
After the system is powered on, configure the BQ25980 in the switched-capacitor charging mode by referring to the instructions in the document “BQ25980EVM (BMS040) Evaluation Module User Guide”, gradually adjust the DC voltage of the program-controlled power supply, and observe and record the input current reading measured on the multimeter; , enable the ADC function of BQ25980, observe the value of the 0x25h register (IBUS_ADC) and the 0x31h register (IBAT_ADC) and the change of the corresponding current value reading; the experiment is repeated three times in total, and the average value is taken and recorded as shown in the following table. Among them, I_IN is the current reading measured by the multimeter, I_Read1 is the current reading measured by IBUS_ADC, and I_Read2 is the current reading measured by IBAT_ADC; Error1 and Error2 are the relative errors of I_Read1 and I_Read2 deviating from the I_IN value, respectively.
It can be seen that although both IBUS_ADC and IBAT_ADC can be used to collect and measure the input current of the BQ25980, there is a certain degree of difference in the measurement accuracy of the two methods. The figure below provides a further comparison of the measurement errors of the two methods.
Figure 4. BQ25980 input current detection error comparison
Observing the above figure, it can be seen that for the input current detection of BQ25980, in the test range of 0.5A~4A, the error performance of the IBAT_ADC detection method proposed in this paper is better than that of the IBUS_ADC measurement method. Specifically, as far as this experiment is concerned, the measurement error using the IBAT_ADC method can be basically controlled within 1.5%; while the error of directly measuring the input current with IBUS_ADC can only be controlled within 3.5%.
To sum up, the detection method of BQ25980 input current based on IBAT_ADC proposed in this paper can significantly improve the detection accuracy of chip input current, and provide convenience for system design and practical application.
Texas Instruments, BQ25980 Datasheet
Texas Instruments, BQ25980EVM (BMS040) Evaluation Module User Guide
Texas Instruments, Introduction of Charging System with High Efficiency Charge Pump Charger
Texas Instruments, How to use the BQ25970 for Flash Charging
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Link to this article：A High-precision BQ25980 Input Current Detection Method