STw4102 STMicroelectronics, STw4102 Datasheet - Page 22

• Manufacturer Part Number: STw4102
• Description: Dual source USB Li-ion charger with gas gauge
• Manufacturer: STMicroelectronics

Battery monitoring
8
8.1
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Battery monitoring
Gas gauge
The gas gauge is designed to monitor the battery capacity. A current sensing resistor
(typically of 30 mΩ) is needed between the negative terminal of the battery and the ground.
The sensing resistor voltage drop is input to a 13-bit integrating AD converter. When a
conversion cycle is completed, depending on the output sign, the result is forwarded to the
charge or discharge 24-bit accumulator, and the number of conversions is incremented in a
12-bit counter.
One accumulator accumulates current flowing into the battery during charging and the other
accumulates discharge current during system operation. The remaining battery charge is
given by the difference between the charge and discharge accumulators.
Figure 23. Gas gauge block diagram
The Digital Base Band (DBB) can control, enable and read gas gauge data through I
control registers. The DBB can read the value of the most recent conversion in two’s
complement format by reading the CONVDATA registers.
The RD_REQ bit enables the transfer of the charge/discharge accumulators and conversion
counter register. The transfer can take up to eight 32 kHz cycles, therefore a delay of
ICG+
ICG-
12 bits + sign bit
AD converter
cg_calibration
EOC
cg_enable
cg_clock
From 2’s
to binary
sign
32kHz
24 bit max
Σ
12 bits Counter
Control logic
accumulator
Charge
registers
Control
accumulator
Discharge
13
24
24
Conv_number
registers
Conv_data
Discharge
registers
registers
registers
Charge
STw4102
2
C