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Three - Four Cell Lithium-Ion Protector Circuit (Rev. B)
Texas Instruments

The UCC3957 is a BiCMOS three- or four-cell lithium-ion battery pack protector designed to operate with external P-channel MOSFETs. Utilizing 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 overcharge and overdischarge. A separate overcurrent-protection block protects the battery pack from excessive discharge currents. If any cell voltage exceeds the overvoltage threshold, the appropriate external P-channel MOSFET is turned off, preventing further charge current. 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 P-channel MOSFET. Likewise, if any cell voltage falls below the undervoltage limit, the second P-channel MOSFET is turned off and only charge current is allowed. Such a cell-voltage condition causes the chip to go into low-power sleep mode. Attempting to charge the battery pack wakes 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. If at the end of the blanking time the overcurrent condition still exists, the external discharge FET is turned off for a period 17 times longer than the first blanking period, and then the discharge FET is turned back on. If at any time a second higher overcurrent threshold is exceeded for more than a user programmable time, the discharge FET is turned off, and remains off for the same period as the first tier off time. This two tiered overcurrent-protection scheme allows for charging capacitive loads while retaining effective short-circuit protection.
Three- or Four-Cell Operation Two-Tier Overcurrent Limiting 30-uA Typical Supply-Current Consumption 3.5-uA Typical Supply Current in Sleep Mode Smart Discharge Minimizes Losses in Overcharge Mode 6.5-V to 20-V VDD Supply Range Highly Accurate Internal Voltage Reference Externally Adjustable Delays in Overcurrent Controller Detection of Loss-of-Cell Sense Connections

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