BQ24085 Datasheet, PDF(22/34 Page) Texas Instruments – 750 mA SINGLE-CHIP Li-Ion/Li-Pol CHARGE MANAGEMENT IC WITH THERMAL REGULATION

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bq24085, bq24086

bq24087, bq24088

SLUS784B â DECEMBER 2007 â REVISED MAY 2008 .................................................................................................................................................... www.ti.com

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, IDET(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,

IDET(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 VO(REG) thresholds indefinitely. See the operation flowchart for more

details on this algorithm. If it is desired to power a system load without a battery, it is recommended 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.

The bq24085/6/7/8 provides a small battery leakage current, IBAT(DONE) (1 ÂµA typical), after termination 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 Mâ¦ to 4 Mâ¦ from the OUT pin to

VSS. A resistor too large (< 2 Mâ¦) 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 ÂµA) to pull the pin above the termination threshold.

CHARGE SAFETY TIMER

As a safety 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 (RTMR); if pin TMR is left open

(floating) the charge safety timer is disabled.

The charge safety timer time-out value is calculated as follows:

t(CHG) = [K(CHG) Ã R(TMR)]

The safety timer operation modes are shown in Table 7

Table 7. Charge Safety Timer Operational Modes

bq24085/6/7/8

STANDBY

CHARGING

SUSPEND

CHARGING, TMR PIN NOT OPEN

CHARGING, TMR PIN OPEN

V(OUT) > V(LOWV)

Yes

CHARGE SAFETY TIMER MODE

RESET

SUSPEND

COUNTING

RESET

In SUSPEND mode, the charge safety timer is put on hold (i.e., charge safety timer is not reset), normal

operation resumes when the TS fault is removed and the timer returns to the normal operating mode

(COUNTING). If charge termination is not reached within the timer period, a fault condition is detected. Under

those circumstances, the LED status is updated to indicate a fault condition and the charger is turned off.

When the charge safety timer fault latch is set and the charger is turned off, a small current IFAULT is applied to

the OUT pin, as long as input power (IN) is detected AND V(OUT) < V(RCHG), as part of a timer fault recovery

protocol. This current allows the output voltage to rise above the recharge threshold V(RCHG) if the pack is

removed, and assures that the charge safety timer fault latch is reset if the pack is removed and re-inserted.

Table 8 further details the charge safety timer fault latch operation.

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Product Folder Link(s): bq24085, bq24086 bq24087, bq24088

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