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IRC APPLICATION NOTES
Resistor Applications in Lithium-Ion Battery Chargers
Contact: Jerry Seams, IRC/AFD
Phone: (361)985-3132
Email:afdeng@irctt.com
Background:
Technological advances in portable electronics and electric
vehicles are driving battery technology to new limits of
performance. The superior battery performance demanded by today's
higher power microprocessors, bigger and brighter notebook
computer displays, electric vehicles and demands for longer
operating periods between recharges are driving systems designers
to new, lithium-ion battery technology.
There are 4 popular rechargeable battery systems in use today.
Lead/acid, nickel-metal-hydride (NiMH), nickel-cadmium (NiCd),
and lithium-ion (Li-ion). Measured by galvanometric energy
density (watt-hours/kg) and volumetric energy density
(watt-hours/liter),lithium based batteries
provide superior performance to traditional battery technologies.
Li-ion batteries provide energy densities of about 120 Wh/kg and
about 280 Wh/liter. Compare this to NiMH and
NiCd batteries which have galvanometric energy densities of only
70 Wh/kg and 50Wh/kg respectively, and the attraction to Li-ion
technology becomes apparent. Li-ion batteries can also tolerate
more charge cycles in their lifetime. Li-ion batteries can have
lifetimes as long as 1,000 cycles, compared with about 800 for
NiMH and 500 for NiCd.
While lithium-ion battery technology has some attractive
attributes, it is not without its problems. The liquid
electrolyte used in today's Li-ion batteries is extremely
flammable. Combine this unfortunate characteristic with the
production of carbon dioxide gas when overcharged, and explosion
or venting of flammable electrolyte can result. One manufacturer
of notebook computers actually delayed delivery of a new model
because the Li-ion batteries used in the systems sparked and
caught fire in the lab just prior to shipment.
Overcharging of Li-ion cells must be avoided at all cost. Not
only is the safety concern present, but Li-ion batteries are
damaged by any degree of overcharge. Overcharging a Li-ion cell
by as little as 1% can degrade its cycle life. Conversely,
undercharging a Li-ion cell by the same amount doesn't take
advantage of the full storage capacity of the cell. Cell voltages
are generally 3.0 to 4.2 volts. Thus, the charge voltage must be
controlled to within ±30 to ±40 millivolts.
| Application: IC manufacturers
have responded by developing integrated circuits
specifically designed for charging Li-ion batteries.
These chargers prevent overcharging while still allowing
full energy storage utilization. The  precision current and voltage control
required by Li-ion batteries is provided by the internal
IC reference voltage and the precision resistors used in
the voltage divider and current sense networks. In
typical charger circuits using ICs such as the LT1510 and
LT1511 manufactured by Linear Technology Corporation
(Milpitas, CA), thin film resistors are used to create a
precision voltage divider to return an accurate feedback
voltage to the charger IC.
A simplified schematic of the divider function is
shown in figure 1. Although only two resistors are needed
to form the divider, Linear Technology uses a divider
consisting of four resistors which can be switched in or
out of the circuit as needed to provide a one, two or
three cell charger.
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 A Linear Technology
demonstration board is shown in figure 2. This 3 amp
charger occupies approximately 3 square inches of the
total pc board area. The precision chip
resistors used in the circuit are 1206 size, PFC series,
precision, thin film chips manufactured by IRC Advanced
Film Division in Corpus Christi, TX. The chips are ±0.1%
tolerance, have a temperature coefficient of ±25 ppm/°C
and a 1/4 watt power rating. The current sense
resistors shown in figure 2 are used to sense input
current and current to the battery. The voltages
generated by these sense resistors are fed into the
charger IC where they are monitored to prevent overloads
at the power input or to the battery load. IRC
Advanced Film Division's 2010 size LRC series chips are
used on the sample board. The sense resistors are .033
ohms with a ±1.0% tolerance and a ½ watt power rating.
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Other Options: Other solutions for
the precision voltage divider in a one cell charger
include the   PFC
divider and SOT 23 resistor divider packages. These
devices are shown in figures 3 and 4, respectively.
Advantages of the PFC divider and SOT 23 include smaller
footprint, lower component costs and lower assembly costs
since only one device is placed on the board versus two
devices for the discrete solution.
If a multi-cell charger is required, the designer
should consider a small, custom network package such as
the 8 pin SOIC or 16 pin QSOP. Again, multiple resistors
could be manufactured in a single package reducing
assembly and placement costs. The disadvantages of the
custom design over the discrete chip solution are
slightly longer lead times, NRE charges for the custom
masks and some loss of design flexibility.
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| Summary: In response to the
demand for the lighter, more energy dense batteries that
lithium-ion provides, battery chargers are becoming more
sophisticated and require more input and output
monitoring than did designs for older battery
technologies such as NiCd, NiMH and lead/acid. Continuous
monitoring of input and output currents require low
value, current sense resistors for reliable feedback to
the charger IC. The compelling need for safety when
charging Li-ion batteries motivates designers to search
for precision resistors with which to accurately monitor
the battery voltage while connected to the charger. IRC's
line of thick film current sense resistors afford
reliable current sensing while IRC's line of precision,
thin film chips and networks provide the accuracy and
stability required for the accurate feedback of battery
voltage necessary for safe, complete charging cycles of
Lithium-ion batteries.
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