Datasheet

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APPLICATION INFORMATION
Precharge and 0-V Charging—Theory of Operation
0-V Charge FET Mode
PACK
ZVCHG
PMS
REG
CHG
DSG
BAT
ZVCHG-FET
R
(ZVCHG)
4.7 mF
Battery
DSG-FET CHG-FET
Pack+
I
ZVCHG
I
FASTCHG
CV
CC
Charger
DC Input
bq29312A
I
(ZVCHG)
= 0 V Precharge Current
I
(FASTCHG)
= Fast Current
OD
NC
bq29312A
SLUS629A – JANUARY 2005 – REVISED AUGUST 2005
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 three possible configurations for this function and the bq29312A can be easily
configured according to the application needs. The three 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.
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).
Figure 7. 0-V Charge FET Mode Circuit
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