Micrel Semiconductor 1849 FORTUNE DRIVE SAN JOSE, CA 95131 U.S.A. 7
mode. In trickle mode, the output of U2 (MIC6211) is on, biasing
Q2 on, allowing the output of the MIC2179 to source 121mA of
charging current to the battery. The intelligent system or
microprocessor senses the low logic level at the output of U4 and
places the charger into the bulk-mode charging state (1.2A).
Sequencing from trickle mode to bulk mode is accomplished by
applying a logic high on Q2’s gate. In bulk-mode, current ramps
up to 1.20A (for a 1-cell lithium application) through sense-resistor
R7 (100mΩ) creating a 120mV drop.
Initially, the inverting input of U2 is lower than the non-
inverting input causing the output to go higher , increasing drive to
Q3. This increases the current through the parallel combination of
R3 and R4 until 1.24V is developed across them. The MIC2179
feedback pin senses the 1.24V and compares it to the internal 1.24V
bandgap reference and reduces output duty cycle until 120mV is
maintained across resistor R7. Finally , there is 120mV at both inputs
of the MIC621 1 (op amp), completing the negative feedback loop.
Trickle charge and bulk charge is calculated using:
Trickle Charge Current 1.24V
R3 R5
R7 121mA=
=
Bulk Charge Current 1.24V
R3|| R4 R5
R7 1.20A=
=
Constant-Voltage Source
Once the battery’s terminal reaches the 4.2V (1-cell lithium-
ion battery overcharge threshold), the constant-current source circuit
is biased off. As the output voltage of the charger tries to go higher
than 4.2V, the output of U3 (open-emitter configuration) biases D3
on. The feedback pin of the MIC2179 is now pulled-up toward the
4.2V V
OUT
rail, reducing duty cycle and maintaining output voltage
regulation. The constant voltage feedback loop consists of U3
(LM3420A), D3, U1 (MIC2179), L1, R7, D2, and Q4.
Because overvoltage conditions greatly reduce the life span
of a lithium battery, an LM3420A was chosen for the voltage
feedback loop to help maintain terminal voltage to 0.5%.
In this charger design when the input supply is disconnected,
diode D2 is reverse biased to prevent battery discharge. Q4 is
bifunctional: it prevents the battery from being discharged when
the input supply is removed; and it alleviates a race condition
between the LM3420 and the start-up of the MIC2179 switching
regulator. The LM3420 must be on before the output of the
MIC2179 comes up.
End-of-Charge Circuit
The end-of-charge circuit is used to signal the microprocessor
or another subsystem when the lithium-ion battery pack has reached
the overcharge threshold. Again, in this 1-cell lithium-ion
application, the overcharge threshold is 4.2V.
Starting with a discharged battery and the charger in bulk-
charge mode, the constant-current is decreasing and the voltage
across the battery is increasing over time. The change in current
versus voltage is a function of the changing internal impedance of
the lithium-ion battery under charge.
U5 is being used as a differential amplifier to monitor the output
current by sensing the I·R drop across sense resistor R7. The gain
of U5 is set to 102.5. The gain of U5 is set to maintain greater than
1.24V at its output down to a I
OUT
of greater than 120mA. U4
compares the output of U5 against the 1.24V bandgap reference on
its non-inverting pin generated by the internal bandgap reference
of MIC2179. In bulk-charge mode, the output of U5 is always higher
than U4’s reference voltage, thus maintaining a logic level low at
the end-of-charge pin.
Once R7 has less than a 12mV drop across it, the output of U5
can no longer sustain greater than 1.24V at its output. Now U4 has
1.24V on both its inputs.
Next, the I
OUT
drawn by the battery reduces even further due
to charging. Now the voltage at U4’ s inverting pin is lower than the
1.24V reference producing a logic level high at the output of U4,
signaling an end-of-charge.
The present industry-standard variable used to determine
lithium-ion battery end-of-charge is the current draw at about 90%
charge (see graph below). The end-of-charge current is typically
about C/10. A simple countdown timer circuit (not shown) is usually
started upon reaching the end-of-charge state. Based on the
individual manufacture specification, this completes the charge
cycle.
End-of-charge output current is calculated using a 1.24V reference:
IR7
R9
R6 1.24V
OUT
≤×
()
=
End-of-Charge Theshold
1.24V is the reference for U4 pin 3 which is the end-of-charge
comparator.
Note: This circuit uses 120mA as an end-of-charge threshold. At
the end of charge, the charging circuit should be shut down. It is
not recommended to float charge li-ion cells for long periods of
time.
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0 0.5 1.0 1.5 2.0 2.5 3.0
CELL VOLTAGE (V)
CHARGE CURRENT (A)
TIME (HRS)
Current
Cell Voltage
Typical lithium cell voltage vs. charging current