SI9730 Datasheet, PDF (10/14 Pages) Vishay Siliconix – Dual-Cell Lithium Ion Battery Control IC

Si9730
Vishay Siliconix
APPLICATION CIRCUIT
C_D
Cell 1
V_C1
100 W
V_DD 3
C₁
0.1 mF
100 W
V_C 5
Cell 2
V_C2
100 W
V_SS 7
Current
Sense
14 MW
Si9936DY
C₂
0.47 mF
Filter
I_S 6
R_IS
47 kW
DCO
Si9730
+
10 mF
C
4
Cell Balancing
Network
A/D
Converter ^OUT
Undervoltage
Lockout
Control
Logic
Timer
C_DELAY
Time Out
CLK
Load
Oscillator
1.2 VREF
Comparator
I_LIMIT
8
S_OUT
V_SS
GS Gen.
V_M
1
V_M
Chgr.
8.4 V
1A
FIGURE 8. Typical 2 -Cell Circuit
General Considerations
Figure 8 shows a typical application of the Si9730, controlling
a 2-cell lithium-ion battery (carbon or coke chemistry).
Specifics of the selection of MOSFETs, current sensing
resistor, and output capacitor are detailed below. In addition,
there are several typical features of this circuit to be observed.
First, each connection from a cell to the IC has a 100-W resistor
in series with it. The purpose of the resistor is to ensure that in
the unlikely event of the IC shorting, the cells themselves will
not see a short. The maximum size of this resistance is set by
the current drain of the IC; for example, the V_DD pin draws a
maximum of 60 mA, which will drop V = 60 mA * 100 W = 6 mV
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across the resistor. This drop constitutes an error in the
measured cell voltage, and so the resistor must be small
enough that the error voltage is acceptable.
A second typical feature demonstrated in Figure 8 is the
current sense filter formed by RIS and C2. This provides a
noise filter, to prevent the Si9730 from opening the connection
to the battery if there is noise on its current sense pin. It also
causes a delay in the response of the IC to a genuine
overcurrent, the amount of the delay being inversely
proportional to the amount of overcurrent, since the Is pin
senses a voltage. Increasing this filter’s time constant could be
used to allow short-time surges of current out of the battery
without compromising its ability to protect the battery.
Document Number: 70658
S-40135—Rev. F, 16-Feb-04