SA57608 Datasheet, PDF (6/10 Pages) NXP Semiconductors – One-cell Lithium-ion battery protection with over/undercharge and overcurrent protection

Philips Semiconductors
One-cell Lithium-ion battery protection with
over/undercharge and overcurrent protection
Product data
SA57608
Charging Lithium cells
The lithium cells must be charged with a dedicated charging IC such
as the NE57600. These dedicated charging ICs perform a
current-limited, constant-voltage charge, as shown in Figure 4.
The charger IC begins charging with a current that is typically the
rating of the cell (1C) or the milliampere rating of the cell. As the cell
approaches its full-charge voltage rating (V_OV), the current entering
the cell decreases, and the charger IC provides a constant voltage.
When the charge current falls below a preset amount, 50 mA for
example, the charge is discontinued.
If charging is begun below the overdischarged voltage rating of the
cell, it is important to slowly raise the cell voltage up to this
overdischarged voltage level. This is done by a reconditioning
charge. A small amount of current is provided to the cell (50 mA for
example), and the cell voltage is allowed a period of time to rise to
the overdischarged voltage. If the cell voltage recovers, then a
normal charging sequence can begin. If the cell does not reach the
overdischarged voltage level, then the cell is too damaged to charge
and the charge is discontinued.
To take advantage of the larger energy density of lithium cells it is
important to allow enough time to completely charge the cell . When
the charger switches from constant current to constant voltage
charge (Point B, Figure 4) the cell only contains about 80 percent of
its full capacity. When the cell is 100 mV less than its full rated
charge voltage the capacity contained within the cell is 95 percent.
Hence, allowing the cell to slowly complete its charge takes
advantage of the larger capacity of the lithium cells.
1.0
0.5
CONSTANT
CURRENT
CONSTANT
VOLTAGE
Vov
4.0
3.0
1.0
2.0
TIME (HOURS)
Point B
SA57608 OPERATION
The SA57608 continuously monitors the terminal voltage and battery
pack current of a single Li-ion battery pack. Li-ion cells must be
maintained within a set of a very defined operating conditions to
operate safely and with with a long life. If the cell voltage exceeds
the cell’s full-charge voltage, the charge current is interrupted. If the
cell voltage falls below the overdischarge rating of the cell, the
discharge current is interrupted. Also, whenever the discharge
current exceeds the threshold voltage across the RDS(on) of the
two MOSFETs, the short-circuit current is interrupted.
V_CC
OV
V_CC
OV
DELAY
CF
CONTROL
GND
V–
UV
V_REF
CHARGER
DETECTOR
OC REF
C_DLY
V_CC
UV
DELAY
DF
CONTROL
Figure 5. SA57608 block diagram.
SL01579
Overvoltage condition
When the cell’s terminal voltage exceeds the value of V_OV1,
measured from V_CC (pin 5) to GND (pin 6), the overvoltage time
delay is initiated. After this time has elapsed, the gate of the charge
MOSFET (CF, pin 3) is driven LOW and the charge current is
interrupted. The terminal voltage of the cell may immediately fall due
to the amount of the charge current times the series resistance of
the Li-ion cell (I_chg × R_ESR). The charge MOSFET will not turn on
again until the cell voltage has fallen below V_OV(rel), or when a load
is detected across the battery pack terminals. A load is detected
when the VM pin (pin 2) is drawn 0.7 V above the cell’s negative
terminal (GND, pin 6).
The timing capacitor C_DLY (pin 4) provides a time period between
the overvoltage threshold (V_OV1) being exceeded and when the
charge MOSFET is turned off. Its timing period is approximately:
t_DLY = C_DLY (V_CC – 0.7 V) / 0.43 µA
(Equation 1)
The variation in timing is approximately ±16 percent.
1.0
2.0
TIME (HOURS)
Figure 4. Lithium Cell charging Curves
SL01554
2003 Oct 29
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