Datasheet

Common FET
I-PC:PrechargeCurrent
I-QC:Quick-ChargeCurrent
CV
Charger
CHG
PACK
GPOD
PMS
REG
DSG
bq77PL900
Battery
BAT
DSG-FET CHG-FET
S0353-01
PACK+
I-PC
I-QC
CC
Open
bq77PL900
www.ti.com ...................................................................................................................................................... SLUS844B – JUNE 2008 – REVISED JANUARY 2009
In this mode, the PMS pin is connected to PACK+. In this configuration, the charger must have a 0-V/precharging
function which is typically controlled as follows:
• The cell voltage is lower than a certain constant voltage (normally about 3 V/cell).
– Apply 0-V/precharging current.
• The cell voltage is higher than a certain constant voltage (normally about 3 V/cell).
– Apply fast-charging current.
When the charger is connected and VPMS is greater than or equal to 0.7 V, the CHG FET is turned ON. The
charging current flows through the CHG FET and the back diode of the DSG.
V
PACK+
= V
BAT
+ 0.7 V (VF: forward voltage of a DSG-FET back diode) + V
DS
(CHG-FET)
Figure 27. Common FET Circuit Diagram
When the PACK pin voltage is maintained at higher than 0.7 V and the precharging current is maintained, the
PACK voltage and BAT voltage are under the minimum bq77PL900 supply voltage, so the regulator is inactive.
When the BAT voltage rises and the PACK pin voltage reaches the bq77PL900 minimum supply voltage, an
internal 3.3-V regulator is turned ON. Then, the CHG FET state is controlled by UVP and OVP functions. When
the all the cell voltages reach fast-charge voltage (about 3 V per cell), the charger starts the fast-charging mode.
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