SI9730 Datasheet, PDF (13/14 Pages) Vishay Siliconix – Dual-Cell Lithium Ion Battery Control IC

Si9730
Vishay Siliconix
Four Cell Application
Figure 9 shows a method for using the Si9730 in a 4-cell
application. Basically, this is two complete 2-cell circuits
stacked in series. Each half of the complete circuit monitors its
own 2-cell portion of the battery, and opens its own MOSFET
switch under any of the appropriate conditions. Observe that
the total percent power loss in this circuit is identical to that in
the 2-cell application; although there are now two sets of
MOSFETs in series, there is also double the battery voltage,
and so total efficiency is the same.
One novel feature of this 4-cell circuit is the increase in the size
of the bypass capacitors. Each half of the circuit retains its own
output cap, to reduce noise seen by the circuit. Since the two
halves interact with each other (when one opens its switch, the
other one is also opened), there can be additional noise, which
must be rejected for proper operation. The capacitors have
been increased to 100 mF for this reason; remember that they
must be rated to take the full maximum voltage rating of the
charger, not half of it, since if one switch is closed and the other
open, the charger (minus two cells’ voltage drop, which might
be zero) is applied across the other capacitor.
A second addition on this circuit is the (optional) two zeners,
one each for each Si9730, placed from V_DD to V_M. These are
necessary only if the charger voltage is higher than the 15-V
absolute maximum of the IC plus two cells’ voltage drop. Just
as with the capacitor, if one switch is open and the other
closed, the IC will see this charger voltage, and must be
protected. The power rating of the zener can be inferred by
observing that the current through it is limited by the 100-W
resistor. A tradeoff can be made here between the power rating
of the zener, which can be decreased by increasing the
resistor value, and the accuracy of the voltage measurement
by the Si9730, which can be increased by decreasing the
resistor value.
Reset from Shutdown
There are two specialized conditions that can place the Si9730
in shutdown mode. The first condition can occur when the
circuit is first attached to a battery in the factory. When the IC
comes up, it will be in the undervoltage shutdown mode. The
Si9730 may also enter this mode when the ambient
temperature drops and the battery is nearly in UV. When the
temperature drops, the battery pack voltage will drop and the
IC may enter the shutdown mode. In either case, the Si9730
must be reset by raising the V_SS pin higher than the V_M pin by
V_CPHD. Figure 10 shows a circuit that resets the circuit once
it has entered the shutdown mode.
The circuit works by initially connecting the 0.1-mF capacitor to
the battery’s center tap and placing the switch in position #1.
Although the MOSFETs are open, the 1-mW resistor is
sufficient to allow the capacitor to charge up in about
300-400 msec. Once the capacitor is charged, the switch is
placed in position #2, momentarily making V_SS higher than V_M,
thus placing the Si9730 in the normal operating mode. The
entire circuit provides a leakage of only a few microamps,
which is much lower than the self discharge current of the LiIon
battery.
Si9730
1 ^VM ^DCO 8
2 ^NC
V_{SS 7}
3 ^VDD
I_{S 6}
4 ^CD
V_{C 5}
#2 #1
SPDT
15 MW
Document Number: 70658
S-40135—Rev. F, 16-Feb-04
1 MW
0.1 mF
FIGURE 10. Factory Startup Circuit
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