Datasheet

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VDS
Precharge
Current
ID
VGS
Point A
Point B
bq29312A
SLUS629A – JANUARY 2005 – REVISED AUGUST 2005
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)
Figure 8. Drain Current vs Drain-Source Voltage Characteristics
In the bq29312A, 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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