MIC79050 Datasheet, PDF (11/20 Pages) Micrel Semiconductor – Simple Lithium-Ion Battery Charger Preliminary Information

MIC79050
Applications Information
Protected Constant-Current Charger
Another form of charging is using a simple wall adapter that
offers a fixed voltage at a controlled, maximum current rating.
The output of a typical charger will source a fixed voltage at
a maximum current unless that maximum current is ex-
ceeded. In the event that the maximum current is exceeded,
the voltage will drop while maintaining that maximum current.
Using an MIC79050 after this type of charger is ideal for
lithium-ion battery charging. The only obstacle is end of
charger termination. Using a simple differential amplifier and
a similar comparator and reference circuit, similar to Figure 1,
completes a single cell lithium-ion battery charger solution.
Figure 2 shows this solution in completion. The source is a
fixed 5V source capable of a maximum of 400mA of current.
When the battery demands full current (fast charge), the
source will provide only 400mA and the input will be pulled
down. The output of the MIC79050 will follow the input minus
a small voltage drop. When the battery approaches full
charge, the current will taper off. As the current across R_S
approaches 50mA, the output of the differential amplifier
(MIC7300) will approach 1.225V, the reference voltage set by
the LM4041. When it drops below the reference voltage, the
output of the comparator (MIC6270) will allow the base of Q1
to be pulled high through R2.
Zero-Output Impedance Source Charging
Input voltage sources that have very low output impedances
can be a challenge due to the nature of the source. Using the
circuit in Figure 3 will provide a constant-current and constant
voltage charging algorithm with the appropriate end-of-charge
termination. The main loop consists of an op-amp controlling
the feedback pin through the schottky diode, D1. The charge
current through R_S is held constant by the op-amp circuit until
the output draws less than the set charge-current. At this
point, the output goes constant-voltage. When the current
through R_S gets to less than 50mA, the difference amp output
becomes less than the reference voltage of the MIC834 and
the output pulls low. This sets the output of the MIC79050 less
than nominal, stopping current flow and terminating charge.
Lithium-Ion Battery Charging
Single lithium-ion cells are typically charged by providing a
constant current and terminating the charge with constant
voltage. The charge cycle must be initiated by ensuring that
the battery is not in deep discharge. If the battery voltage is
below 2.5V, it is commonly recommended to trickle charge
the battery with 5mA to 10mA of current until the output is
above 2.5V. At this point the battery can be charged with
constant current until it reaches its top off voltage (4.2V for a
typical single lithium-ion cell) or a time out occurs.
For the constant-voltage portion of the charging circuit, an
extremely accurate termination voltage is highly recom-
mended. The higher the accuracy of the termination circuit,
the more energy the battery will store. Since lithium-ion cells
do not exhibit a memory effect, less accurate termination
Micrel
does not harm the cell but simply stores less usable energy
in the battery. The charge cycle is completed by disabling the
charge circuit after the termination current drops below a
minimum recommended level, typically 50mA or less, de-
pending on the manufacturer’s recommendation, or if the
circuit times out.
Time Out
The time-out aspect of lithium-ion battery charging can be
added as a safety feature of the circuit. Often times this
function is incorporated in the software portion of an applica-
tion using a real-time clock to count out the maximum amount
of time allowed in the charging cycle. When the maximum
recommended charge time for the specific cell has been
exceeded, the enable pin of the MIC79050 can be pulled low,
and the output will float to the battery voltage, no longer
providing current to the output.
As a second option, the feedback pin of the MIC79050 can be
modulated as in Figure 4. Figure 4. shows a simple circuit
where the MIC834, an integrated comparator and reference,
monitors the battery voltage and disables the MIC79050
output after the voltage on the battery exceeds a set vaue.
When the voltage decays below this set threshold, the
MIC834 drives Q1 low allowing the MIC79050 to turn on
again and provide current to the battery until it is fully charged.
This form of pulse charging is an acceptable way of maintain-
ing the full charge on a cell until it is ready to be used.
MIC79050-4.2BMM
IN BAT
VIN _{EN FB}
GND
4.7µF
100k
MIC834
VDD OUT
R1
INP GND
Li-Ion
Cell
R2
GND
V_BAT(low) = V_REF ^{}1+ _RR21^{}
V_REF=1.240V
Figure 4. Pulse Charging For
Top-off Voltage
Charging Rate
Lithium-ion cells are typically charged at rates that are
fractional multiples of their rated capacity. The maximum
varies between 1C – 1.3C (1× to 1.3× the capacity of the cell).
The MIC79050 can be used for any cell size. The size of the
cell and the current capability of the input source will deter-
mine the overall circuit charge rate. For example, a 1200mAh
battery charged with the MIC79050 can be charged at a
maximum of 0.5C. There is no adverse effects to charging at
lower charge rates; that charging will just take longer. Charg-
ing at rates greater than 1C are not recommended, or do they
decrease the charge time linearly.
The MIC79050 is capable of providing 500mA of current at its
nominal rated output voltage of 4.2V. If the input is brought
June 2000
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MIC79050