STC3105IQT Datasheet, PDF (10/23 Pages) STMicroelectronics – Battery monitor IC with alarm output for gas gauge applications

Functional description
6
Functional description
STC3105
6.1
Battery monitoring functions and operating modes
The monitoring functions include the measurement of the battery voltage and battery
current. A coulomb counter is used to track the state-of-charge when the battery is charging
or is discharging at a high rate. A 14-bit sigma-delta A/D converter is used to measure both
the voltage and current.
The STC3105 can operate in two modes with different power consumption. The selection is
made by the PWR_SAVE bit in the register 0 (Refer to Table 11 on page 17 for register 0
definition).
In power-saving mode, the current is measured only half of the time. This saves about half of
the power consumption.
In active mode, the current is continuously measured (except for one conversion cycle used
every 4 seconds for the voltage measurement). This provides the highest accuracy from the
coulomb counter.
When the battery voltage falls below the undervoltage lockout threshold of 2.7 V, the
monitoring functions are stopped and the STC3105 stays in power-down mode with
minimum quiescent current. All registers are maintained down to 2.0 V. Below 2.0 V, the
registers are reset to their default power-on values.
By writing 0 to the GG_RUN bit in register 0, the STC3105 measurement is automatically
stopped and the register values are frozen and unchanged. After the GG_RUN bit is again
set to 1, the STC3105 continues to measure from the point it was stopped.
6.2
Battery voltage monitoring
The battery voltage is measured by using one conversion cycle of the A/D converter every 4
seconds.
The conversion cycle takes 2¹³ = 8192 clock cycles. Using the 32768 Hz internal clock, the
conversion cycle time is 250 ms.
The voltage range is 0 to 5 V, and the resolution is 2.44 mV. The accuracy of the voltage
measurement is ±0.5% over the temperature range. This allows accurate state-of-charge
information from the battery open-circuit voltage.
6.3
Current sensing
The voltage drop across the sense resistor is integrated during a conversion period and
input to the 14-bit sigma-delta A/D converter.
Using the 32768 Hz internal clock, the conversion cycle time is 500 ms for a 14-bit
resolution. The LSB value is 11.77 µV. The A/D converter output is in two’s complement
format.
When a conversion cycle is completed, the result is added to the coulomb counter
accumulator and the number of conversions is incremented in a 16-bit counter.
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