SI9730 Datasheet, PDF (8/14 Pages) Vishay Siliconix – Dual-Cell Lithium Ion Battery Control IC

Si9730
Vishay Siliconix
To avoid this, if one or both cells becomes over-discharged
(V_CELL < V_ODC) and no charger is present, the Si9730 opens
the switch to prevent further discharging, and goes into a
shutdown mode in which it draws minute power from the
battery (I_{DD_UVL} < 1 mA).
Over-Discharged Cell(s) Charging
If one or both cells is over-discharged, and a charger is
present, charging can begin, and so the Si9730 closes the
switch. However, removal of the charger in this condition could
potentially damage the battery if the removal is not recognized
and the cells are discharged. Since the voltage drop across
the switch is small, the Si9730 actually cycles the switch at a
7/8 duty cycle; during the 1/8 time when the switch is open, the
IC checks that the charger is still present.
Once both cells are back into the normal operating range,
normal charging resumes.
Undervoltage Charging
If for some reason the battery drops below about 3.7 V (V_UVL),
there is insufficient voltage for the Si9730 to properly monitor
fault conditions. Of course, the switch is already open, since
V_UVL < V_ODC x 2. However, when a charger is detected, the
Si9730 recovers and goes into an undervoltage mode. (A
charger is detected if the V_S pin is higher than the V_M pin by
at least V_CHPD = 1.1 V, see Figure 6). In this undervoltage
mode, the switch is on at a 1/8 duty cycle, to limit the power
dissipation across the switch, and, again, to detect the
continuing presence of the charger.
Once the battery voltage is above V_UVL, the charging
continues in the over-discharged state.
I_VMSHORT current causes the voltage on the V_M pin to equal the
voltage on the V_DD pin while the short is present, or the voltage
on the V_M pin to equal the voltage on the V_SS pin if the short
is removed. If the short is not removed, I_VMSHORT current will
continue to flow until the battery voltage becomes
overdischarged. Once the short is removed, the IC is allowed
to turn the switch back on.
The current limit threshold has a temperature coefficient of
0.18%/_C. This can partially compensate for a copper circuit
board trace being used as the sense resistor.
Open Center Tap
An open center tap is a mechanical failure of the battery pack
such that the Si9730’s V_C pin is disconnected from the center
point of the two-cell battery. If this connection is open, the IC
opens the switch, as it cannot measure the cell voltages in this
condition. The switch is left open until connection is
re-established. If the battery is under-voltaged and the
charger is present in this case, the battery is allowed to charge
even with the center tap open. In this state, batteries are
almost impossible to damage by 1/8 duty cycle charging.
Once the battery voltage reaches the over-discharged
voltage, the switch is turned off.
State Transition Table
The number of different states of the Si9730 can seem
overwhelming at first. This state transition table will help to
organize thinking about the different operational conditions of
the IC, by listing each possible transition from one condition to
another.
Output Short
If too much current is drawn from the battery due to a load
short, the switch must be opened quickly to prevent damage
to the battery. The Si9730 monitors the load current by looking
at the voltage across an external sense resistor (see Figure 8).
If the voltage across the sense resistor exceeds V_ILIMIT ~ 28
mV, the switch is opened. The Si9730 leaves the switch open
until the load is completely removed.
Of course, the IC must have some way of detecting that the
load has been removed. For this purpose, a small current
(I_VMSHORT) passes through the Si9730, from pin V_M to pin V_SS
once the short is detected and the switch is turned off. The
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8
Reading the table is straightforward. There are two cells
constituting the battery, one with its positive terminal
connected to V_DD and its negative terminal connected to V_C,
referred to as the high cell (see Figure 8); and one cell with its
positive terminal connected to V_C and its negative terminal
connected to V_SS, referred to as the low cell. Each cell can be
in one of three voltages:
D Over-discharge (ODC), where V_CELL < V_ODC;
D Normal Operation (NO), where
V_ODC < V_CELL < V_OC;
or
D Overcharge (OC), where V_OC < V_CELL.
Document Number: 70658
S-40135—Rev. F, 16-Feb-04