STC3100

Functional description

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7

Functional description

7.1

Gas gauge

The gas gauge is used to monitor the available battery capacity. The voltage drop across the

external sense resistor is integrated during a conversion period and input to a 12- to 14-bit

AD converter. The output conversion is accumulated into a 28-bit accumulator. The system

controller can control the gas gauge and read the data (upper 16 bits of the accumulator)

through the I2C control registers.

The AD converter output is in two

’s complement format. When a conversion cycle is

completed, the result is added to the charge accumulator and the number of conversions is

incremented in a 16-bit counter.

Figure 7.

Gas gauge block diagram

The controller can read the value of the most recent conversion in two

’s complement format

by reading the REG_CURRENT registers. These registers are updated at the end of each

conversion.

The differential inputs are scaled to the full range of the AD converter, introducing a small

offset error. A high value written to the CG_CAL bit of the control register connects the

inputs of the AD converter together, allowing the controller to measure the digital offset

error. Using this measurement, one can calibrate the gas gauge and reduce errors due to

the internal offset error.

AD converter

12/14-bit

CG

GND

28-bit

accumulator

Charge

register

Current

register

register

Counter

16-bit counter

EOC

16

16

16

Control logic

cg_enable

cg_calibration

cg_clock

Control

registers

32768 Hz

16

28

28

cg_res

2

rd_req

cg_rst

AM00832