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t-Time-s
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O
CHARGE SAFETY TIMER
bq2406x
SLUS689A – JUNE 2006 – REVISED OCTOBER 2006
Figure 23. 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 bq2406x 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 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 LDD mode (disables termination).
The battery absent detection function is disabled when the termination is disabled.
The bq2406x provides a small battery leakage current, IBAT(DONE) (1 µ A 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 M Ω to 4 M Ω from the OUT pin to VSS. A
resistor too large (< 2 M Ω ) 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 bq2406x provides a fault
current (typical 750 µ A) to pull the pin above the termination threshold.
As a safety mechanism the bq2406x 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.
The charge safety timer time-out value is calculated as follows:
T
(CHG)
= [K
(CHG)
× R
(TMR)
]
The safety timer operation modes are shown in Table 7
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