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RBI Input

GPIO

Layout Considerations

Gas Gauge Operation

bq27000, bq27200

SLUS556D – SEPTEMBER 2004 – REVISED MARCH 2006

FUNCTIONAL DESCRIPTION (continued)

The RBI input pin is used with an external capacitor to provide backup potential to the internal registers when

M

Ω resistor can be added from the RBI pin to V

corruption by storing a redundant copy of the high byte of NAC and a checkbyte computed from LMD, CYCL,

CYCT, and other critical data. After a reset, the bqJUNIOR compares the redundant NAC and checkbyte values.

If the checks are correct, NAC, LMD, CYCL, and CYCT are retained; and the CI bit in FLAGS is left unchanged.

If these checks are not correct, NAC, CYCL, and CYCT are cleared; LMD is initialized from EEPROM and the CI

bit in FLAGS is set to 1. All other RAM is initialized on all resets.

The GPIO pin can be used as an input or an output. The initial state can be established by programming bit 7 in

the PKCFG EEPROM location. The input/output state can be changed at any time by changing the value in bit 7

of MODE.

The auto-calibrating DSCC approach effectively cancels the internal offset voltage within the bqJUNIOR, but any

external offset caused by PCB layout must be programmed in the EEPROM to be cancelled. The magnitude and

variability of the external offset makes it critical to pay special attention to the PCB layout. To obtain optimal

should tie into the high-current trace at a point directly next to the sense resistor. This should be a trace

carry any load current and no portion of the high-current PCB trace is included in the effective sense resistor (i.e.

Kelvin connection).

Figure 4 illustrates an operational overview of the gas gauge function.

The bqJUNIOR measures the capacity of the battery during actual use conditions and updates the Last

Measured Discharge (LMD) register with the latest measured value. The bqJUNIOR retains the learned LMD

value unless a full reset occurs. By measuring the capacity that the battery delivers as it is discharged from full to

the EDV1 threshold without any disqualifying events, the bqJUNIOR learns the capacity of the battery. The

bqJUNIOR does not need to learn a new capacity on each full discharge, and only a discharge during normal

use conditions should be used to learn a new capacity. In the event that some abnormal situation occurs that

could cause a significant reduction in learned capacity, the LMD value is restricted to a maximum LMD

learn-down during any single learning discharge of LMD/8. The Capacity Inaccurate (CI) bit in FLAGS is cleared

after a learning cycle. This bit remains cleared unless a full reset occurs or the cycle count since the last learning

cycle (CYCL) reaches a count of 32.

The full condition is defined as Nominal Available Capacity (NAC) = LMD. The Valid Discharge Flag (VDQ) in the

FLAGS register is set when this condition occurs and remains set until the learning discharge cycle completes or

an event occurs that disqualifies the learning cycle.

The learning discharge cycle completes when the battery is discharged to the condition where VOLT

≤ EDV1

threshold. The EDV1 threshold should be set at a voltage that ensures at least 6.25% of battery capacity below

that threshold. The EDVF threshold should be set at a voltage that the system sees as the zero-capacity battery

voltage. The bqJUNIOR EDV detection is designed to prevent premature detection of the EDV thresholds due to

dynamic load variations. EDV detection has a dynamically adjusted delay of up to 21.5 s with RSOC

≥ 6% and

down to 3 s when RSOC = 0%.

The bqJUNIOR does not learn the capacity between EDV1 and EDVF thresholds, but assumes that the capacity

is 6.25% of LMD; so, care should be taken to set EDV1 based on the characteristics of the battery. The

measured LMD value is determined by measuring the capacity delivered from the battery from NAC = LMD until

VOLT = EDV1, plus LMD/16 to account for the 6.25% capacity remaining below the EDV1 threshold.

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