SLUA707
3 to 6 Cells Battery-Management System Based On bq76925 + MSP430G2xx2 3
1 Features
To implement a high-accuracy digital battery-management solution, bq76925 + MSP430G2xx2
can support a complete pack monitoring, balancing, protection, and gas gauging system for 3 to
6 series cell Lithium-Ion / Polymer battery. This solution is designed to focus on power tool
projects, while it can also be a reference design for other similar applications.
2 Introduction
The bq76925 is a dedicated analog front end (AFE) for 3 to 6 series cell application that
provides 3 analog outputs that allow a microcontroller to easily monitor cell voltage, current, and
temperature. Cell voltages are level-shifted, scaled, and multiplexed to the VCOUT pin. Cell
current is monitored through a sense resistor placed in series with the cell stack. The voltage
across the sense resistor is amplified and driven to the VIOUT pin. The VTB pin supplies a
switched bias to stimulate a thermistor network for temperature measurement. The bq76925
supplies a 3.3-V regulated output to power the MSP430G2xx2 and an accurate 3.3-V reference
voltage for the MSP430G2xx2 analog-to-digital converter (ADC). The AFE also includes
integrated cell balancing FETs that are under control of the MSP430G2xx2. Finally, the on-board
comparator of the AFE signals an overcurrent condition to the MSP430G2xx2 for fast fault
response.
The MSP430G2xx2 series is an ultra-low-power microcontroller with one built-in 16-bit timer, a
fast 10-bit ADC with integrated reference, built-in communication capability using the universal
serial communication interface, and up to 24 I/O pins. The architecture, combined with five low-
power modes, is optimized to achieve extended battery life in portable measurement
applications. The MSP430G2xx2 application circuit is provided to read the analog signals from
the bq76925 and let the user create custom firmware based on the application code.
With the AFE of bq76925, the MSP430G2xx2 is evaluated as the host to control bq76925 by I
2
C
of USI module, the MSP430G2xx2 can read the data of analog signals from
bq76925 by ADC10.Then the MSP430G2xx2 can easily monitor individual cell voltages, pack
current, and temperature to achieve battery charge and discharge management. Although it is
only a 10-bit ADC result for the MSP430G2xx2, bq76925 can calibrate it into a high-accuracy
value with the particular compensated algorithmic of correction factors. For example, the ADC10
value deviation of cell voltage does not exceed 5 mV, which can strongly make this solution
outstanding in this field requirement. To make this solution more competitive, the MSP430G2xx2
is selected for its advantages of low power and low cost.
The user can choose the part number of the MSP430G2xx2 from MSP430G2x32/G2x52, as
there are eight part numbers for the different configurations, like flash size, Comparator_A+.
More software functions mean more flash size is need. The user can choose the right part
number based on the necessary functions of the MSP430G2xx2.
