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UCC3957-3 Datasheet(PDF) 4 Page - List of Unclassifed Manufacturers

Part # UCC3957-3
Description  Three - Four Cell Lithium-Ion Protector Circuit
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Manufacturer  ETC1 [List of Unclassifed Manufacturers]
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UCC3957 -1/-2/-3/-4
APPLICATION INFORMATION
Overview
The UCC3957 provides complete protection against
over-discharge, over-charge and overcurrent for a three
or four cell Lithium-Ion battery pack. It uses a “flying ca-
pacitor” technique to sample the voltage across each
battery cell and compare it to a precision reference. If
any cell is in over or under-voltage, the internal state ma-
chine takes the appropriate action to prevent further
charge or discharge. High-side P-MOSFETs are used to
independently control charge and discharge current. Typ-
ical application circuits are shown in Figures 1 and 2.
Connecting the Cell Stack
When connecting the cell stack to the circuit, it is impor-
tant to do it in the proper order. First, the bottom of the
stack should be connected to AN4 . Next, the top of the
stack should be connected to VDD. The cell taps can
then be connected to AN1-AN3 in any order.
Choosing Three or Four Cells
For three cell packs, the cell count pin (CLCNT) should
be connected to the DSPLY pin, and the AN3 pin should
be tied to the AN4 pin. For four cell applications, the
CLCNT pin should be grounded (to AN4) and the AN3
pin will be connected to the positive terminal of the bot-
tom cell in the stack.
Under-voltage Protection
When any cell is found to be over-discharged (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 3.5uA. It re-
mains in sleep mode until the application of a charger is
sensed by the Wake Up (WU) pin being raised above
VDD.
Charging
Once a charger has been applied, the Charge FET will
be turned on as long as the Charge Enable input
(CHGEN) is pulled up to the DSPLY pin. If the CHGEN
input is left open (or connected to AN4), the Charge FET
will remain off.
During charge, the Discharge FET will be off (current will
be conducted through its body diode) until the cell volt-
ages are all above the Undercharge to Normal threshold.
Once the cell voltages are above this threshold, the Dis-
charge FET will be turned on, minimizing power dissipa-
tion.
Open Wire Protection
The UCC3957 provides protection against broken cell
sense connections within the pack. If the sense connec-
tion to one of the cells (pins AN1, 2 or 3) should become
disconnected, weak internal current sources will make
the cells connected to that wire appear to be in over-
charge and charging of the pack will be prevented.
Over-voltage Protection and the “Smart Discharge”
Feature
If any cell is charged to a voltage exceeding the Normal
to Overcharge threshold, the Charge FET will be 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 pro-
tector designs, the Charge FET is held off completely
within this voltage band. During this time, discharge cur-
rent must be conducted through the body diode of the
Charge FET. This forward voltage drop can be as high as
1V, 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 come back on (for dis-
charge 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
15mV (corresponding to 0.6 amps of discharge current
using a .025 sense resistor), the Charge FET is turned
back on. This threshold assures that only discharge cur-
rent will be conducted. In an example using a 20m
FET
with a 1V body diode drop and a 1 amp load, the power
dissipation in Q1 would be reduced from 1 watt to 0.02
watts. Note that a similar technique is not used during
charge (when the Discharge FET is off due to cells being
in undervoltage) because the charge current should be
low while the cells are in undervoltage.
Protection Against a Runaway Charger
The use of a small N-channel level shifter (Q3 in the ap-
plication diagrams) allows the IC to interface with the
high-side Charge FET (Q1), even in the presence of a
runaway charger. Only the drain-source voltage rating of
the charge FET limits the charge voltage that the protec-
tion circuit can withstand. The Wakeup (WU) pin is de-
signed to handle input voltages greater than VDD, as
long as the current is limited. In the examples shown, the
Charge FET’s gate-source resistor (R1) provides this
current limiting. Note that in Figure 2, a resistor and
zener (R2 and VR1) have been added to protect Q1
against any possibility of a voltage transient exceeding
its maximum gate-source rating.
Overcurrent Protection
The UCC3957 protects the battery pack from an over-
load or a hard short circuit using a two-tier overcurrent
protection scheme. The overcurrent protection is de-


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