AN137 Silicon_Laboratories, AN137 Datasheet - Page 2

• Manufacturer Part Number: AN137
• Description: Lithium ION Battery Charger Using C8051f300
• Manufacturer: Silicon_Laboratories

AN137
Charging Basics
Batteries are exhaustively characterized to deter-
mine safe yet time-efficient charging profiles. The
optimum charging method for a battery is depen-
dent on the battery’s chemistry (Li-Ion, NiMH,
NiCd, SLA, etc.). However, most charging strate-
gies implement a 3-phase scheme:
1. Low-current conditioning phase
2. Constant-current phase
3. Constant-voltage phase/charge termination
All batteries are charged by transferring electrical
energy into them (refer to the references at the end
of this note for a battery primer). The maximum
charge current for a battery is dependent on the bat-
tery’s rated capacity (C). For example, a battery
with a cell capacity of 1000mAh is referred to as
being charged at 1C (1 times the battery capacity) if
the charge current is 1000mA. A battery can be
charged at 1/50C (20 mA) or lower if desired.
However, this is a common trickle-charge rate and
is not practical in fast charge schemes where short
charge-time is desired.
Most modern chargers utilize both trickle-charge
and rated charge (also referred to as bulk charge)
while charging a battery. The trickle-charge current
is usually used in the initial phases of charging to
minimize early self heating which can lead to pre-
mature charge termination. The bulk charge is usu-
ally used in the middle phase where the most of the
battery’s energy is restored.
During the final phase of battery charge, which
generally takes the majority of the charge time,
either the current or voltage or a combination of
both are monitored to determine when charging is
complete. Again, the termination scheme depends
on the battery’s chemistry. For instance, most Lith-
ium Ion battery chargers hold the battery voltage
constant, and monitor for minimum current. NiCd
2
Rev. 1.2
batteries use a rate of change in voltage or tempera-
ture to determine when to terminate.
Note that while charging a battery, most of the elec-
trical energy is stored in a chemical process, but not
all as no system is 100 percent efficient. Some of
the electrical energy is converter to thermal energy,
heating up the battery. This is fine until the battery
reaches full charge at which time all the electrical
energy is converted to thermal energy. In this case,
if charging isn’t terminated, the battery can be
damaged or destroyed. Fast chargers (chargers that
charge batteries fully in less than a couple hours)
compound this issue, as these chargers use a high
charge current to minimize charge time. As one can
see, monitoring a battery’s temperature is critical
(especially for Li-Ion as they explode if over-
charged). Therefore, the temperature is monitored
during all phases. Charge is terminated immedi-
ately if the temperature rises out of range.
Li-Ion Battery Charger -
Hardware
Currently, Li-Ion batteries are the battery chemistry
of choice for most applications due to their high
energy/space and energy/weight characteristics
when compared to other chemistries. Most modern
Li-Ion chargers use the tapered charge termination,
minimum current (see Figure 2), method to ensure
the battery is fully charged as does the example
code provided at the end of this note.
Buck Converter
The most economical way to create a tapered ter-
mination charger is to use a buck converter. A buck
converter is a switching regulator that uses an
inductor and/or a transformer (if isolation is
desired), as an energy storage element to transfer
energy from the input to the output in discrete
packets (for our example we use an inductor; the
capacitor in Figure 3 is used for ripple reduction).
Feedback circuitry regulates the energy transfer via
the transistor, also referred to as the pass switch, to
maintain a constant voltage or constant current