UM1481 datasheet(33/55 Pages) STMICROELECTRONICS

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UM1481

Hardware design description

Doc ID 022389 Rev 1

33/55

7.2

Battery monitor IC (STC3105)

The STC3105 monitors the critical parameters of a single-cell Li-Ion battery (voltage, and

current, SOC, etc.) and includes hardware functions to implement a gas-gauge for battery

charge monitoring, based on a programmable 12- to 14-bit A-D converter. With a typical

20 m

Ω external sense resistor, the gas-gauge system provides a capacity of up to ±10950

mAh with a resolution of 0.2 mAh, as described below.

●

Calculation of the maximum battery capacity being supported by the STC3105:

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 LSB value is set by the internal gain and internal reference and is 11.77

uV at maximum resolutions. The conversion cycle for n bit resolution is 2 n clock cycles.

Using the 32,768 Hz internal clock, the conversion cycle time is 125 to 500 ms for a 12- to

14-bit resolution. The AD converter output is in 2's complement format. When a conversion

cycle is completed, the result is added to the charge accumulator.

The LSB value of the charge accumulator (in mAH) = LSB value of ADC * conversion cycle

time* 10-3 sense resistor *3600.

(LSB value of ADC= 11.77 µV, conversion time 500 mS for 14-bit resolution; sense resistor =

10 m

Ω to 50 mΩ.)

Maximum capacity of battery supported (in ± mAH) = LSB value of charge accumulator

* 228 2.

Resolution of charge register = maximum capacity of battery supported 216.

The device is programmable through the I2C interface. This device also has 16-RAM bytes,

1-byte unique device ID, and an ALM pin. RAM contents are retained even up to a minimum

battery voltage of 2.0 V. For more details refer to the STC3105 datasheet.

Table 3.

Maximum capacity and charge register LSB resolution for various values

of Rsense

Sense resistor value

Maximum capacity of battery

supported (in ± mAH)

Resolution of charge register (in

mAH)

10 m

21900

0.6

20 m

10950

0.3

30 m

7300

0.2

40 m

5450

0.16

50 m

4350

0.13

Table 4.

STC3105 details

Feature

Description

Order code

STC3105IQT

Package

TDFN 3.0 X 2.0 X 0.75

Operating voltage

2.7 to 5.5 V

Operating current consumption

50 µA

UM1481 Datasheet(PDF) 33 Page - STMicroelectronics

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