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BQ29312_15 Datasheet, PDF (23/39 Pages) Texas Instruments – FOUR CELL LITHIUM-ION
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bq29312
SLUS546E – MARCH 2003 – REVISED MARCH 2005
APPLICATION INFORMATION
Precharge and 0-V Charging—Theory of Operation
In order to charge, the charge FET (CHG-FET) must be turned on to create a current path. When the V(BAT) is 0
V and CHG-FET = ON, the V(PACK) is as low as the battery voltage. In this case, the supply voltage for the device
is too low to operate. There are 3 possible configurations for this function and the bq29312 can be easily
configured according to the application needs. The 3 modes are 0-V Charge FET Mode, Common FET Mode
and Precharge FET Mode.
1. 0-V Charge FET Mode – Dedicates a precharge current path using an additional FET (ZVCHG-FET) to
sustain the PACK+ voltage level. The host charger is expected to provide a precharge function.
2. Common FET Mode – Does not use a dedicated precharge FET. The charge FET (CHG-FET) is assured to
be set to ON state as default. The charger is expected to provide a precharge function.
3. Precharge FET Mode – Dedicates a precharge current path using an additional open drain (OD) pin drive
FET (PCHG-FET) FET to sustain the PACK+ voltage level. The charger does not provide any precharge
function.
0-V Charge FET Mode
In this mode, a dedicated precharge current path using an additional FET (ZVCHG-FET) is required to sustain a
suitable PACK+ voltage level. The charger is expected to provide the precharge function in this mode where the
precharge current level is suitable to charge cells below a set level, typically below 3 V per cell. When the lowest
cell voltage rises above this level, then a fast charging current is applied by the charger.
The circuit diagram for this method is shown in Figure 7, showing how the additional FET is added in parallel
with the charge FET (CHG-FET).
R(ZVCHG)
ZVCHG-FET
DSG-FET
CHG-FET
Pack+
Charger
CC
IZVCHG
DC Input
Battery
DSG
BAT
bq29312
CHG
PACK
ZVCHG
REG
OD PMS
IFASTCHG
CV
4.7 µF
NC
I(ZVCHG) = 0 V Percharge Current
I(FASTCHG) = Fast Current
Figure 7. 0-V Charge FET Mode Circuit
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