Functional description

STC3105

10/23

Doc ID 022198 Rev 1

6

Functional description

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.

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.