UCC3952A-1, UCC3952A-2, UCC3952A-3, UCC3952A-4

SINGLECELL LITHIUMION BATTERY PROTECTION IC

SLUS463C − AUGUST 2000 − REVISED MARCH 2001

7

POST OFFICE BOX 655303

POST OFFICE BOX 1443

APPLICATION INFORMATION

battery voltage monitoring

The battery cell voltage is sampled by connecting a resistor divider across it and comparing the resulting voltage

to a precision internal reference voltage. Under normal conditions (cell voltage is below the overvoltage

threshold and above the undervoltage threshold), the UCC3952A consumes less than 8

µA of current and the

internal MOSFET is fully turned on with the aid of a charge pump.

When the cell voltage falls below the undervoltage threshold for two consecutive samples, the IC disconnects

the load from the battery pack and enters a super-low-power mode. The pack remains in this state until it detects

the application of a charger, at which point charging is enabled. The requirement of two consecutive readings

below the undervoltage threshold filters out momentary drops in cell voltage due to load transients, preventing

nuisance trips.

If the cell voltage exceeds the overvoltage threshold for the overcharge delay time, charging is disabled;

however, discharge current is still allowed. This feature of the IC is explained further in the controlled

charge/discharge mode section of this document.

overcurrent monitoring and protection

Discharge current is continuously monitored via an internal sense resistor. In the event of excessive current,

system bypass capacitors without tripping the overcurrent protection. A 0.1-

µF capacitor on the CBPS pin

provides momentary holdup for the IC to assure proper operation in the event that a hard short suddenly pulls

the cell voltage below the minimum operating voltage.

Once the overcurrent condition has been declared, the internal MOSFET turns off. To return the device to normal

operation, the UCC3952A needs to have a load impedance greater than 7.5 M

Ω placed across PACK+ to

PACK−. This typically can be achieved by removing the battery pack from the system. At this point, the pack

returns to its normal state of operation.

controlled charge/discharge mode

When the chip senses an overvoltage condition, it prevents any additional charging, but allows discharge. This

is accomplished by activating a linear control loop, which controls the gate of the MOSFET based on the

differential voltage across its drain-to-source terminals. The linear loop attempts to regulate the differential

voltage across the MOSFET to 100 mV. When a light load is applied to the part, the loop adjusts the impedance

of the MOSFET to maintain 100 mV across it. As the load increases, the impedance of the MOSFET is

decreased to maintain the 100-mV control. At heavy loads (still below the overcurrent limit), the loop does not

maintain regulation and drives the gate of the MOSFET to the battery voltage (not the charge-pump output

voltage drop (and associated power loss) across the internal MOSFET in this mode of operation is still

significantly lower than the typical solution of two external back-to-back MOSFETs, where the body diode is

conducting.

When the chip senses an undervoltage condition, it disconnects the load from the battery pack and shuts itself

down to minimize current drain from the battery. Several circuits remain powered and detect placement of the

battery pack into a charger. Once the charger presence is detected, the linear loop is activated and the chip

allows charging current into the battery. This linear control mode of operation is in effect until the battery voltage