Operation description

STBCFG01

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DocID026474 Rev 3

5.5

Fuel gauge

The voltage mode fuel gauge provides the accurate information about the state of the

Lithium-Ion battery. Battery voltage is constantly monitored to evaluate the state of charge

of battery and open circuit voltage.

At power-up, the fuel gauge algorithm uses the voltage reading to provide a first evaluation

of SOC based on battery modeling data. The evolution of voltage is then used to track the

changes of SOC of battery while cycling. The external software driver performs the

temperature compensation.

Initial accuracy depends on the state of the battery. When the fuel gauge is active the

battery is supposed to be in fully relaxed state. If the battery is not fully relaxed the initial

error in the evaluation of the state of charge is high but converges to lower values while the

battery is being used.

The fuel gauge block can be adapted to different batteries. Programmable parameters are

used to tailor the algorithm to each battery model.

In order to keep the optimal performance and avoid losing information learned during

battery cycling, the user is supposed to save data contained in the device's volatile memory

when power is removed. The same data has to be restored at power-up.

The STBCFG01 also provides programmable alarms to notify low battery voltage and low

SOC conditions.

5.5.1

Operating mode

At start-up and when the battery voltage is below UVLO threshold, the fuel gauge is in

standby mode. This block is active when the battery voltage is above UVLO and GG_RUN

bit has been set through I²C. When the fuel gauge is in standby mode, all register values

are frozen and the algorithm does not run. In active mode, a voltage reading is acquired

5.5.2

Battery connection

When a battery is connected, the device wakes up and attempts reading the battery

voltage before high current loads can change the open circuit voltage. The charging

process is inhibited until the battery voltage reading is completed. If the battery voltage is

too low, the charger is not inhibited to allow the battery to be charged up to a stable voltage

and avoid oscillations.

5.5.3

Low battery alarm

The voltage mode fuel gauge provides low SOC and low battery voltage alarms, which are

notified to the host through the active low open drain interrupt register (IRQn). Both of

alarms can be disabled clearing ALRM_en bit in FG_Mode register.

Low battery voltage threshold and low SOC threshold can be changed writing

ALARM_Voltage and ALARM_SOC registers. The default thresholds are 3.00 V battery

voltage and 1% SOC.

When a low battery voltage condition or a low SOC condition is detected and alarms are

enabled, the corresponding bit in FG_CTRL register is set (ALRM_VBAT or ALRM_SOC)

and IRQn output is pulled low.

ALRM_VBAT and ALRM_SOC bits have to be cleared by the host to release the interrupt

pin (if no other interrupt is active in the device).