ne57610 NXP Semiconductors, ne57610 Datasheet - Page 9

• Manufacturer Part Number: ne57610
• Description: Li-ion Battery Charger Control With Adjustable Thresholds
• Manufacturer: NXP Semiconductors

Philips Semiconductors
TECHNICAL DISCUSSION
Lithium-ion cells: general information
Lithium-ion and polymer cells have higher voltage than nickel
cadmium (NiCd) or nickel metal hydride (NiMH) rechargeable cells.
The average operating voltage of a lithium-ion or polymer cell is
3.6 V compared to the 1.2 V of NiCd and NiMH cells. The internal
resistances of the various types of lithium cells are 50 m to
300 m , compared to the 5 m to 50 m of the nickel chemistries.
This makes Lithium-ion and polymer cells better for lower battery
current applications, less than 1 ampere, such as cellular and
wireless telephones, palmtop and laptop computers, etc.
Lithium-ion and polymer cells are safe as long as the cell is
maintained within a particular set of operating boundaries. The cells
have a porous carbon, or graphite anode where individual lithium
ions can lodge themselves within the pores. This keeps the lithium
ions separated, and any hazardous condition is avoided, if the cell is
kept within the safe operating boundaries.
A lithium cell protection circuit is placed within the battery pack. It
monitors the level of voltage across each cell for overcharge and
overdischarge conditions, and the discharge current in the event of
an overcurrent or short-circuit condition. If the lithium cell is
overcharged, pure metallic lithium plates out onto the surface of the
anode. Also volatile gas is generated within the cell. This creates a
hazard. Conversely, if the cell were allowed to over-discharge (V
less than typically 2.3 V), the chemistry of the cell changes and the
copper metal used in its construction enters the electrolyte solution.
This severely shortens the cycle life of the cell, but presents no
future safety hazard. When the cell experiences high charge or
discharge currents, then the internal series resistance of the cell
creates heating and generation of the volatile gas which could again
present a hazard.
Charging lithium cells
An integral part of any Li-ion battery system is a battery charger
specifically designed for the lithium cell being used, with its
particular over and undercharge limits, capacity, etc. The battery
charger should be viewed as a part of the entire lithium battery
system so that safe cell operation can be ensured.
Lithium cells must be charged with a dedicated charging controller
such as the NE57610. The charging ICs, in general, can be
described as performing: a current-limited, constant-voltage charge
process. When the cell is very discharged, the charger IC outputs a
constant current into the battery, which limits the internal heating of
the cells. The maximum charge rate is typically the capacity rating of
the cell. That is, the maximum charge current is the mAHr rating of
the cell(s), that is, a 1000 mAHr cell will be charge with a maximum
of 1000 mA. When the cell voltage approaches its full-charged
voltage rating (V
decrease, and the charger IC provides a constant voltage-mode of
charge. The charge current begins to exponentially decrease over a
long period of time (approximately 1.5 – 2.0 hours). When the
charge current falls below a preset amount, the charge current is
discontinued.
2002 Nov 05
Li-ion battery charger control
with adjustable thresholds
OV
), the current entering the cell begins to
cell
9
If charging begins with the cell voltage below the overdischarged
voltage rating of the cell (V
cell voltage up to this overdischarged voltage level. This is done with
a reconditioning charge. A small amount of current is allowed into
the cell, and the cell voltage is allowed, for a pre-set period of time,
to rise to the overdischarged voltage (V
recovers, a normal charging sequence can begin as described
above. If the cell does not reach the overdischarged voltage level,
then the cell is considered too damaged to charge and the charge is
discontinued.
It is important to allow enough time to charge the cell to take
advantage of the higher energy density of the lithium cells. When the
charger switches from constant current charge to constant voltage
charge (Point B, Figure 16) the cell only contains about 80 percent
of its full-rated capacity. When the cell is 100 mV less than its full
rated charge voltage, the capacity contained within the cell is about
95 percent. Allowing the cell to slowly complete its charge takes
advantage of the larger capacity of the lithium cells. The complete
charging curve can be seen in Figure 16.
1.0
0.5
4.0
3.0
Vov
Figure 16. Lithium-ion charging curves.
CONSTANT
CURRENT
UV
TIME (HOURS)
TIME (HOURS)
1.0
), it is very important to slowly raise the
1.0
Point B
UV
). If the cell voltage
CONSTANT
VOLTAGE
NE57610
2.0
2.0
Product data
SL01554