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Detection of Loss of Cell Sense
Connections
DESCRIPTION
The UCC3957 is a BiCMOS three or four cell lithium-ion battery pack
protector designed to operate with external P-channel MOSFETs. Uti-
lizing external P-channel MOSFETs provides the benefits of no loss of
system ground in an overdischarge state, and protects the IC as well as
battery cells from damage during an overcharge state. An internal state
machine runs continuously to protect each lithium-ion cell from over-
charge and overdischarge. A separate overcurrent protection block pro-
tects the battery pack from excessive discharge currents.
If any cell voltage exceeds the overvoltage threshold, the appropriate ex-
ternal P-channel MOSFET is turned off, preventing further charge cur-
rent. An external N-channel MOSFET is required to level shift to this high
side P-channel MOSFET. Discharge current can still flow through the
second PFET. Likewise, if any cell voltage falls below the undervoltage
limit, the second P-channel MOSFET is turned off and only charge cur-
rent is allowed. Such a cell voltage condition will cause the chip to go
into low power sleep mode. Attempting to charge the battery pack will
wake up the chip. A cell count pin (CLCNT) is provided to program the IC
for three or four cell operations.
A two tiered overcurrent controller and external current shunt protect the
battery pack from excessive discharge currents. If the first overcurrent
threshold level is exceeded, an internal timing circuit charges an external
capacitor to provide a user programmable blanking time.
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BLOCK DIAGRAM
UDG-97060
SLUS236 - JANUARY 1999
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