BQ24085_17 Datasheet, PDF (20/42 Pages) Texas Instruments – 750-mA Single-Chip Li-Ion and Li-Pol Charge Management IC With Thermal Regulation

bq24085, bq24086, bq24087, bq24088
SLUS784E – DECEMBER 2007 – REVISED DECEMBER 2015
5
www.ti.com
4.50
4
3.50
3
2.50
2
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
t - Time - s
Figure 15. Battery-Absent Detection Waveforms
The battery absent detection function is disabled if the voltage at the BAT pin is held above the battery recharge
threshold, V_(RCH), after termination detection. When the voltage at the BAT pin falls to the recharge threshold,
either by connection of a load to the battery or due to battery removal, the bq24085/6/7/8 begins a battery absent
detection test. This test involves enabling a detection current, I_DET(DOWN), for a period of t_(DETECT) and checking to
see if the battery voltage is below the pre-charge threshold, V_(LOWV). Following this, the precharge current,
I_DET(UP) is applied for a period of t_(DETECT) and the battery voltage checked again to be above the recharge
threshold.
Passing both of the discharge and charging tests (battery terminal voltage being below the pre-charge and above
the recharge thresholds on the battery detection test) indicates a battery absent fault at the STAT1 and STAT2
pins. Failure of either test starts a new charge cycle. For the absent battery condition, the voltage on the BAT pin
rises and falls between the V_(LOWV) and V_O(REG) thresholds indefinitely. See the Operational Flow Chart,
Figure 17, for more details on this algorithm. If it is desired to power a system load without a battery, TI
recommends to float the TMR pin which puts the charger in LDO mode (disables termination).
The battery absent detection function is disabled when the termination is disabled.
After termination, the bq24085/6/7/8 provides a small battery leakage current, I_BAT(DONE) (1 μA typical), to pull
down the BAT pin voltage in the event of battery removal. If the leakage on the OUT pin is higher than this
pulldown current, then the voltage at the pin remains above termination and a battery-absent state will not be
detected. This problem is fixed with the addition of a pulldown resistor of 2 MΩ to 4 MΩ from the OUT pin to
VSS. A resistor too small (< 2 MΩ) can cause the OUT pin voltage to drop below the V_(LOWV) threshold before the
recharge deglitch (typical 25 ms) expires, causing a fault condition. In this case, the bq24085/6/7/8 provides a
fault current (typical 750 μA) to pull the pin above the termination threshold.
8.3.14 Charge Safety Timer
As a protection mechanism, the bq24085/6/7/8 has a user-programmable timer that monitors the total fast charge
time. This timer (charge safety timer) is started at the beginning of the fast charge period. The safety charge
timeout value is set by the value of an external resistor connected to the TMR pin (R_TMR); if pin TMR is left open
(floating) the charge safety timer is disabled.
Use Equation 7 to calculate the charge safety timer time-out value:
t_(CHG) = [K_(CHG) × R_(TMR)]
(7)
The safety timer operation modes are shown in Table 7
20
Submit Documentation Feedback
Copyright © 2007–2015, Texas Instruments Incorporated
Product Folder Links: bq24085 bq24086 bq24087 bq24088