BQ2084-V140_14 Datasheet, PDF (17/64 Pages) Texas Instruments – SBS v1.1-COMPLIANT GAS GAUGE FOR USE WITH THE bq29312

bq2084-V140
www.ti.com
SLUS664B – JULY 2005 – REVISED AUGUST 2006
voltage used for comparison to the thresholds is the highest cell voltage. These thresholds are set by three data
flash constants: Von Charge Voltage, Voff Charge Voltage, and Vmax Charge Voltage. Von is the lowest of
these, and is the threshold below which the charge FET is turned on, provided the minimum off time (Pulse
Minimum Off Time) has been met. When the voltage crosses the Voff threshold, the maximum on time (Pulse
Maximum On Time) begins to count down. When this time has expired, the charge FET is turned off. Any time
Vmax is exceeded, the charge FET is turned off immediately.
The rules are:
1. If charge FET is on:
a. If voltage above Vmax, turn off.
b. Else if voltage above Voff and max on time expired, turn off.
c. Else count down max on time.
2. If charge FET is off:
a. If voltage below Von and minimum off time, turn on.
b. Else increment off time.
Voltage is sampled every 125 ms; therefore, the minimum off time and maximum on time are in units of 125ms,
and pulse on and off times are integral multiples of 125 ms.
The voltage thresholds can be chosen in such a way that they alter the charge mechanism. If Voff is set equal to
Vmax, then every time this threshold is crossed, the charge FET turns off immediately. This effectively disables
the maximum charge time, so that the Voff threshold has no effect. In this case, the algorithm can be described
as having two voltage thresholds, rather than three. The charge FET simply turns on and off as it crosses the
two thresholds.
The minimum off time can be similarly disabled by setting it to zero or one. Due to the sample interval, the
charge FET always is off for at least one 125-ms cycle.
Thresholds must be chosen carefully to get the desired charging behavior. For example, if Von is set below
charge voltage minus taper voltage, the pack can never detect full charge. During pulse charging, the charge
FET remains off until the cells relax to below the Von voltage, which is below the qualification voltage for full
charge detection.
During cycle phase of pulse charging (charge FET ON), the voltage can exceed Voff value for a period of 125ms
until next sample is taken and FET is switched OFF. To prevent cell overvoltage termination, or pack
overvoltage termination during this period, values for cell OV should be set larger than voltage reached during
this period. Reasonable value of cell OV for given charger current I can be calculated, assuming cell impedance
of 0.08 Ω/cell, as V_(cell)OV > Von + 0.08 × I.
Correspondingly, the pack overvoltage margin should be set as V_(margin) > V_(cell)OV x n - V_(charge)
where: n = number of series cells.
When charging begins on a depleted battery pack, the voltage is below Voff, and may even stay below Von for
some time. This means the pack is under constant charge, with no pulsing, for some part of the charge cycle. As
the voltage on the cells rises, it crosses the Voff threshold (or the Vmax threshold if Voff is disabled), and the
charge FET turns off. Initially, the off time is short, because the cells are only barely over the threshold and is
quickly relaxed to below Von. As the cell voltages rises, the off times become longer and the on times shorter.
This effect, in combination with the reduced current drawn by the cells, results in a gradually declining charge
current. Eventually, this current falls below the taper current, and the pack detects the full charge condition and
stops charging.
The pulse-charging control operates during normal charging conditions and are overridden in case of a fault
condition. Charging is stopped for any fault conditions which may occur, such as overtemperature or
overcurrent, without regard to the voltage thresholds or time limits.
Primary Charge Termination
The bq2084-V140 terminates charge if it detects a charge-termination condition based on current taper. A
charge-termination condition includes the following:
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