UCC39571, UCC39572, UCC39573, UCC39574

SLUS236B – JANUARY 1999 – REVISED SEPTEMBER 2002

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APPLICATION INFORMATION

undervoltage protection

When any cell is found to be overdischarged (below the normal-to-undercharge threshold), the state machine

turns off both high-side FETs and enters the sleep mode, where current consumption drops to about 3.5

µA.

It remains in sleep mode until the application of a charger is sensed by the wakeup pin (WU) being raised above

VDD.

charging

Once a charger has been applied, the charge FET is turned on as long as the charge-enable input pin (CHGEN)

is pulled up to the DVDD pin. If the CHGEN input is left open (or connected to AN4), the charge FET remains

off.

During charge, the charge and discharge FETs cycle on and off while the device is in the sleep state

(undercharge mode), until the cell voltages are all above the undercharge-to-normal threshold. Once the cell

voltages are above this threshold, the device enters the normal state and the FETs remain on continuously.

While the device is charging and in undercharge mode, there is an approximate on time of 8 ms corresponding

to one sampling period, with a very short off time corresponding to undercharge-voltage detect and sleep-mode;

once WU is pulled back up to PACK(+), wake-up detect again occurs, and a new sampling period/charge cycle

is initiated.

open wire protection

The UCC3957 provides protection against broken-cell sense connections within the pack. If the sense

connection to one of the cells (pins AN1, AN2, or AN3) should become disconnected, weak internal-current

sources make the cells that are connected to that wire appear to be in overcharge and charging of the pack is

prevented.

overvoltage protection and the smart discharge feature

If any cell is charged to a voltage exceeding the normal-to-overcharge threshold, the charge FET is turned off,

preventing further charge current. Hysteresis keeps the charge FET off until the cell voltages have dropped

below the overcharge-to-normal threshold. In most protector designs, the charge FET is held off completely

within this voltage band. During this time, discharge current must be conducted through the body diode of the

charge FET. This forward voltage drop can be as high as 1 V, causing significant power dissipation in the charge

FET and wasting precious battery power.

The UCC3957 has a unique smart discharge feature that allows the charge FET to return to on mode (for

discharge only) while still in the overcharge hysteresis band. This greatly reduces power dissipation in the

charge FET. This is accomplished by sensing the voltage drop across the current-sense resistor. If this drop

exceeds 15 mV (corresponding to 0.6 A of discharge current using a .025

Ω sense resistor), the charge FET

is turned back on. This threshold assures that only discharge current is conducted. In an example using a

20-mW FET with a 1-V body diode drop and a 1-A load, the power dissipation in Q1 would be reduced from 1

W to 0.02 W.

NOTE: A similar technique is not used during charge (when the discharge MOSFET is off due to

cells being in undervoltage) because the charge current should be low while the cells are in

undervoltage.

Not Recommended for New Designs