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ROHM Develops High-Accuracy Coulomb Counter IC for Industrial Devices

OHM Co., Ltd. has developed the BD7220FV large-current measurement coulomb counter IC that can estimate, with high accuracy, the remaining battery level of robots and other industrial equipment, such as drones, storage devices, and backup power supplies.

This coulomb counter IC, which is used to count the inflow and outflow of electric charge, improves the conventional ±5 percent accuracy of electric current calculation to ±1 percent. With this development, ROHM has attained the first high-accuracy coulomb counter IC that is capable of measuring large current from ±10 to ± 2,000A.

BD7220FV is fabricated using a cutting-edge 0.13-micron bipolar-complementary metal-oxide-semiconductor (BiCDMOS) process and is supplied in 20-pin shrink small-outline package (SSOP) chips.

BD7220FV estimates the remaining battery level with high accuracy and meets the requirements for effective utilization of the battery capacity. In the initial stage, ROHM plans to supply this new IC for lithium ion batteries of industrial equipment. The company intends to expand its application to electric vehicles in the future.

BD7220FV coulomb counter IC
Photo 1: BD7220FV coulomb counter IC
In addition to this newly developed coulomb counter IC, ROHM offers a battery management system (BMS) reference board that is equipped with non-isolated field effect transistors (FET) gate drivers; battery monitoring LSIs N-channel power MOSFETs; built-in FET switching regulators; high-performance and ultralow power microcontrollers; and high-power, ultralow resistance shunt resistors. The company intends to propose this board as a BMS solution for industrial equipment.

Battery Level Measurement Process
To measure the remaining battery level, the newly developed BD7220FV coulomb counter IC applies an electric current measurement method that measures the inflow and outflow of electric charge. In this method, measurement is performed by multiplying electric current by hours, and thus the remaining battery level can be estimated with higher accuracy than the method that measures battery voltage. To improve the accuracy of current measurement even further, the coulomb counter IC is equipped with a delta-sigma analog-to-digital (A/D) converter, which measures the electric current read by the A/D converter assembled in a conventional microcontroller, a computer for adding up the obtained electric current, an OSC, and an operational amplifier. BD7220FV achieves high-accuracy calculation by means of ROHM’s proprietary real-time current calculation method.

BMS reference board
Photo 2: BMS reference board

This built-in delta-sigma A/D converter has 40 times higher time resolution (250msec, 4kHz) than that of a conventional microcontroller (10msec, 100Hz). This new A/D converter also enables 640 times higher electric current measurement performance in total, including increasing the A/D converter resolution from 12 bits (using a conventional microcontroller) to16 bits and increasing the resolution for measuring 200A from 97.7mA (using a conventional microcontroller) to 6.1mA, which is16 times higher.

Furthermore, each block circuit assembled in the coulomb counter IC optimizes the output. As a result, it has become possible to perform corrections in a batch process for the calibration, which checks and corrects the output of each device in order to obtain the measurement values accurately.

Because of these technologies, BD7220FV has improved the current calculation accuracy from ±5 percent to ±1 percent, and the development time has been reduced from the previous two months to one week at present.

The noise impact caused by motors and inverters has always been one of the issues for industrial equipment. To resolve this issue, BD7220FV is assembled with a digital noise filter. This noise filter can adjust noise impact in four levels. The noise filter lets BD7220FV measure electric current with a high-accuracy of ±1 percent and with fewer irregularities of the AC output values even in an environment impacted by noise.

Comparison of calibration function
Figure: Comparison of calibration function
In the case of a lithium ion battery that has a battery capacity of 50Ah, the actual battery capacity available for calculating the electric current with an accuracy of ±5 percent is 45Ah. On the other hand, the actual battery capacity available for calculating the electric current with an accuracy of ±1 percent is 49Ah. When 16 cells are connected, the actual battery capacity available for calculating the electric current with an accuracy of ±5 percent is 2,664Wh, whereas the actual battery capacity available for calculating the electric current with an accuracy of ±1 percent is 2,901Wh. This difference in the electric current calculation accuracy makes a significant difference to the actual battery capacity (that is 237Wh to be exact, in this case). It is also a significant difference for the effective utilization of battery capacity as well.

Battery Management System Reference Board
Along with this newly developed coulomb counter IC, ROHM also supplies a BMS reference board, which is assembled with this IC. The non-isolated FET gate driver assembled on this BMS reference board is a 2-channel high-side N-MOSFET gate driver. The BMS reference board can control charging and discharging of the battery. The built-in step-up circuit converts the signals from a microcontroller into signals that can drive the FET. The maximum rising and falling edges are 350µsec and 70µsec, respectively. Charge-discharge control of 1-channel small current is enabled. The withstand voltage is 80V and the operating voltage range is 6.5 to 64V. The operating temperature range is -40 to +105°C, and it is available in a 20-pin thin-shrink small-outline package (TSSOP).