Philips Semiconductors
Product data
2 to 4 cell redundant Lithium-ion overcharge monitor
NE57606
GENERAL DESCRIPTION
The NE57606 is a redundant overcharge detection IC for use within
2-4 cell Li-ion battery packs. It detects the voltage of each Li-ion cell
and issues an overcharge signal which then can be used to alert the
portable host or be used to turn-off a series charge MOSFET within
the battery pack. Its purpose is to act as a back-up protection circuit
to a primary Li-ion protection circuits such as the NE57605 and
NE57607. The overcharge signal is an open collector output which
can be wire-ORed with other safety functions.
FEATURES
•
Consumption current (V
CEL
= 3.8 V) 3.0
µA
typical
•
Consumption current (V
CEL
= 2.3 V) 0.3
µA
typical
•
Input current between cell pins (V
CEL
= 3.8 V)
±0.3 µA
max
•
Overcharge detection voltage = threshold voltage
±
50 mV
•
Overcharge detection delay time (CT = 0.22
µF)
1.5 s typical
•
Four voltage ranges available
SIMPLIFIED DEVICE DIAGRAM
APPLICATIONS
•
Li-ion Battery pack protection
V
C4
OV REF
V
CC
V
C3
OV REF
V
C2
OV REF
OUT
V
C1
OV REF
0.7 V
GND
C
T
SL01558
Figure 1. Simplified device diagram.
2003 Oct 29
2
Philips Semiconductors
Product data
2 to 4 cell redundant Lithium-ion overcharge monitor
NE57606
TECHNICAL DISCUSSION
The NE57606 is typically used in conjunction with a Li-ion protection
IC as redundant protection for a 2, 3, or 4-cell lithium-ion battery
pack. Lithium-ion cells can present a safety hazard if they become
overcharged, therefore careful monitoring of each cell’s operating
point is necessary. For very safety-sensitive applications, a back-up
protection circuit using a device such as the NE57606 is advisable.
The NE57606 monitors each cell within a 2-4 Li-ion cell battery
pack. If any cell within the battery pack exceeds the full-charge
threshold voltage, the overvoltage fault status output assumes a low
state. This output signal should be used to alert other parts of the
system that an overcharged state has been reached. This output
could also be used to open a MOSFET placed in series with the
positive battery terminal to interrupt the charging current from the
battery charger.
During normal operation of the Li-ion system, the battery charger
should be the circuit that terminates the charge. The Lithium-ion
protection circuit should never be used for routine termination of the
charging function. It should be viewed as a back-up protection
system in the event of a charger failure. The redundant overcharge
detection IC should disconnect the pack from the charger in the
event that both other systems have failed.
Setting the trip-point voltages are key to the system’s operation.
First, the trip point tolerances should not overlap, or the systems will
not become active in the proper order over large production. The trip
points should be typically set in the following fashion:
1. The battery charger should be set to terminate its charge at a
point just below the cell’s full charge voltage (–1%)
2. The Li-ion protection circuit is set to open the series charge
MOSFET switch at the rated full charge voltage of any of the
cell(s). (±1%)
3. The redundant overcharge detector is set to issue an alert and/or
disconnect a series charge MOSFET switch when any cell
voltage exceeds the rated full charge voltage (+1%)
With trip-points set as described above, the charger will taper its
charging current until the charging current falls below a certain
current level, after which the charger turns off. Only if the charger
does not or cannot terminate the charging, the protection IC will
open a series MOSFET switch, thus cutting off any charge current.
Lastly, if both the charger and the protection IC were to fail, the
NE57606 will open another series MOSFET.
Redundant protection of a Lithium-ion battery
pack
Within a typical Li-ion battery system, there are two or three major
circuits responsible for the monitoring and maintenance of the Li-ion
cells: the Li-ion battery charger, the Li-ion protection circuit, and
sometimes the redundant Li-ion overcharge detector. This type of
system is called a triple-redundant protected system. If any one of
the protection circuits fail, then there will be two other independent
systems to assume the protection function. If the product is
designed properly, that is, component de-rating, non-cascading
failure modes, ESD, packaging, etc., having two or more
simultaneous failures within the protection system is virtually
impossible.
2003 Oct 29
5
Talking
京公网安备 11010802033920号
EEWORLD
