BQ29312_15 Datasheet, PDF (24/39 Pages) Texas Instruments – FOUR CELL LITHIUM-ION

bq29312
SLUS546E – MARCH 2003 – REVISED MARCH 2005
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APPLICATION INFORMATION (continued)
In order to pass 0 V or precharge current an appropriate gate-source voltage V_(GS), for ZVCHG-FET must be
applied. Here, V_(PACK) can be expressed in terms of V_(GS) as follows:
V_(PACK) = V_(ZVCHG) + V_(GS)(ZVCHG-FET gate - source voltage)
ID
Point B
Point A
Precharge
Current
VGS
VDS
Figure 8. Drain Current vs Drain-Source Voltage Characteristics
In the bq29312, the initial state is for CHG-FET = OFF and ZVCHG-FET = ON with the V_(ZVCHG) clamped at 3.5 V
initially. Then the charger applies a constant current and raises V_(PACK) high enough to pass the precharge
current, point A. For example, if the V_(GS) is 2 V at this point, V_(PACK) is 3.5 V + 2 V = 5.5 V. Also, the
ZVCHG-FET is used in its MOS saturation region at this point so that V_(DS) is expressed as follows:
V_(PACK) = V_(BAT) + V_F + V_{DS(ZVCHG-FET)}
where V_(F) = 0.7 V is the forward voltage of a DSG-FET back diode and is typically 0.7 V.
This derives the following equation:
V_DS = 4.8 V - V_(BAT)
As the battery is charged V_(BAT) increases and the V_(DS) voltage decreases reaching its linear region. For
example: If the linear region is 0.2 V, this state continues until V_(BAT) = 4.6 V, (4.8 V - 0.2 V).
As V_(BAT) increases further, V_(PACK) and the V_(GS) voltage increase. But the V_DS remains at 0.2 V because the
ZVCHG-FET is driven in its MOS linear region, point B.
V_(PACK) = V_F + 0.2 V + _V(BAT)
where V_F = 0.7 V is the forward voltage of a DSG-FET back diode and is typically 0.7 V
The R_(ZVCHG) purpose is to split heat dissipation across the ZVCHG-FET and the resistor.
ZVCHG pin behavior is shown in Figure 9 where V_(ZVCHG) is set to 0 V at the beginning.
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