August 2005 9 MIC79050
MIC79050 Micrel, Inc.
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 exceeded. 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 bat
-
tery 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 mi-
nus 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-volt-
age. 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 ex-
tremely accurate termination voltage is highly recommended.
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 does not
harm the cell but simply stores less usable energy in the bat-
tery. The charge cycle is completed by disabling the charge
circuit after the termination current drops below a minimum
recommended level, typically 50mA or less, depending 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 func-
tion is incorporated in the software portion of an application
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 maintaining the full
charge on a cell until it is ready to be used.
MIC79050-4.2BM
MIC834
SD101
1
/
2
MIC7122
1
/
2
MIC7122
IN BAT
FB
GND
EN
8.06M
4.7µF
R
2
=124k
R
3
=1k
R
4
=124k
0.01µF
VDD OUT
GNDINP
R
1
=1k
D1
221k
16.2k
16k
10k
5V
R
S
=0.200Ω
I
CC
=
80mV
R
S
I
EOC
=
1.24V × R
1
R
2
× R
S
Figure 3.
