SI9730 Datasheet, PDF (7/14 Pages) Vishay Siliconix – Dual-Cell Lithium Ion Battery Control IC

Si9730
Vishay Siliconix
DETAILED DESCRIPTION
Overview
The purpose of the Si9730 is to safely and reliably control the
charging and discharging of a two-cell lithium-ion battery
(carbon or coke chemistry). It provides protection against all
possible fault conditions, including:
D external short circuits
D reversed charger
D overcharged cell or cells
D undervoltage
D battery open center-tap
General Concepts
The Si9730 operates by connecting or disconnecting the
negative terminal of the battery to the negative side of the load
and/or charger (see Figure 8); that is, it does ground side
switching. It is important to bear the distinction between these
two “grounds” in mind in order to understand the operation of
the Si9730. The switching is accomplished by controlling two
“back-to-back” MOSFETs: having the two MOSFETs in this
arrangement is mandatory to ensure that current cannot flow
in either direction when the MOSFETs are off. To turn the
switch on, the Si9730 applies a gate-source voltage to both
MOSFETs (from the DCO pin) that is high with respect to the
sources. The Si9730 DCO signal is referenced to the V_M pin
while the battery is being charged, and to the Vss pin while the
battery is being discharged. The Si9730 causes the DCO to be
referenced to the lower of the two voltages. This prevents the
switch from turning on or off unintentionally.
The Si9730 is designed to operate only with a current-limited
lithium-ion battery charger. Specifically, the battery charger
must have an open-circuit voltage that does not exceed the
absolute maximum IC voltage, and it must have a limited
short-circuit current that does not exceed the allowed charging
current of the battery.
The following descriptions cover all the common operational
scenarios; additional information on unusual battery
conditions can be found in the state transition table.
Normal Charging
The cells are in normal charging conditions if a) both cells are
above the Over-Discharge Detect Threshold (V_ODC ~ 2.2 V);
b) both cells are under the Over-Charge Detect Threshold
Document Number: 70658
S-40135—Rev. F, 16-Feb-04
(V_OC ~ 4.2 V); and c) the center tap is connected to the V_C pin.
When a charger is present in these conditions, the switch will
be on, charging the cells at the current limit of the charger.
Normal Discharging
The cells are in normal discharging conditions if a, b, and c
above are satisfied, and if in addition d) the load current is less
than the discharge current limit. With no charger present, the
switch will be on, discharging the cells and powering the load.
Overcharged Cell(s) Charging
The most destructive condition that a LiI+ cell can experience
is overcharging. If the cell becomes overcharged beyond its
recommended limits, it can become permanently disabled.
If one or both cells rise above the over-charge detect threshold
(V_OC1 and V_OC2), and a charger is present, the Si9730 will
open the switch (to prevent further charging) and begin
bleeding off charge (15-mA typical) from the overcharged cell
or cells.
The details of this operation depend on the fact that the voltage
level of lithium-ion batteries drops for a short time after
charging ceases (due to momentary changes in battery
chemistry, ESR, etc.). Because of this recovery, the Si9730
allows the battery to continue charging for a short time (the
overcharge time, t_DL2OC). This additional charge time only
occurs if the overcharge condition persists for more than
8 msec (two periods of an internal 4msec oscillator). T_DL2OC
is determined by the capacitor attached to the C_D pin, see
Figure 8.
Once the overcharge time has ended, the switch is opened,
preventing the battery from further overcharging. Now, the
Si9730 begins bleeding current off the overcharged cell or
cells (I_BAL1 and I_BAL2), as long as a charger is present.
Eventually, the cell(s) will return into their normal range, and
charging will begin, starting the whole cycle over again.
Overcharged Cell(s) Discharging
If one or more cells is overcharged, and a load is connected,
the switch is turned on, permitting the battery to power the
load.
Over-Discharged Cell(s) Discharging
Repeated over-discharging of LiI+ cells can cause irreversible
reactions in the cells which lead to decreased cycle life.
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