LM3420-42 Datasheet, PDF (13/17 Pages) National Semiconductor (TI) – Lithium-Ion Battery Charge Controller

Application Circuits (Continued)
controlling the ADJ. pin of the LM317 which begins to regu-
late the voltage across the battery and the constant voltage
portion of the charging cycle starts. Once the charger is in
the constant voltage mode, the charger maintains a regu-
lated 8.4V across the battery and the charging current is de-
pendent on the state of charge of the battery. As the cells ap-
proach a fully charged condition, the charge current falls to a
very low value.
Figure 6 shows a Li-Ion battery charger that features a drop-
out voltage of less than one volt. This charger is a
constant-current, constant-voltage charger (it operates in
constant-current mode at the beginning of the charge cycle
and switches over to a constant-voltage mode near the end
of the charging cycle). The circuit consists of two basic feed-
back loops. The first loop controls the constant charge cur-
rent delivered to the battery, and the second determines the
final voltage across the battery.
With a discharged battery connected to the charger, (battery
voltage is less than 8.4V) the circuit begins the charge cycle
with a constant charge current. The value of this current is
set by using the reference section of the LM10C to force 200
mV across R7 thus causing approximately 100 µA of emitter
current to flow through Q1, and approximately 1 mA of emit-
ter current to flow through Q2. The collector current of Q1 is
also approximately 100 µA, and this current flows through
R2 developing 50 mV across it. This 50 mV is used as a ref-
erence to develop the constant charge current through the
current sense resistor R1.
The constant current feedback loop operates as follows. Ini-
tially, the emitter and collector current of Q2 are both ap-
proximately 1 mA, thus providing gate drive to the MOSFET
Q3, turning it on. The output of the LM301A op-amp is low.
As Q3’s current reaches 1A, the voltage across R1 ap-
proaches 50 mV, thus canceling the 50 mV drop across R2,
and causing the op-amp’s output to start going positive, and
begin sourcing current into R8. As more current is forced into
R8 from the op-amp, the collector current of Q2 is reduced
by the same amount, which decreases the gate drive to Q3,
to maintain a constant 50 mV across the 0.05Ω current sens-
ing resistor, thus maintaining a constant 1A of charge cur-
rent.
The current limit loop is stabilized by compensating the
LM301A with C1 (the standard frequency compensation
used with this op-amp) and C2, which is additional compen-
sation needed when D3 is forward biased. This helps speed
up the response time during the reverse bias of D3. When
the LM301A output is low, diode D3 reverse biases and pre-
vents the op-amp from pulling more current through the emit-
ter of Q2. This is important when the battery voltage reaches
8.4V, and the 1A charge current is no longer needed. Resis-
tor R5 isolates the LM301A feedback node at the emitter of
Q2.
The battery voltage is sensed and buffered by the op-amp
section of the LM10C, connected as a voltage follower driv-
ing the LM3420. When the battery voltage reaches 8.4V, the
LM3420 will begin regulating by sourcing current into R8,
which controls the collector current of Q2, which in turn re-
duces the gate voltage of Q3 and becomes a constant volt-
age regulator for charging the battery. Resistor R6 isolates
the LM3420 from the common feedback node at the emitter
of Q2. If R5 and R6 are omitted, oscillations could occur dur-
ing the transition from the constant-current to the
constant-voltage mode. D2 and the PNP transistor input
stage of the LM10C will disconnect the battery from the
charger circuit when the input supply voltage is removed to
prevent the battery from discharging.
DS012359-12
FIGURE 7. High Efficiency Switching Regulator
Constant Current/Constant Voltage 2-Cell Charger
DS012359-13
FIGURE 8. Low Dropout Constant Current/Constant
Voltage Li-Ion Battery Charger
A switching regulator, constant-current, constant-voltage
two-cell Li-Ion battery charging circuit is shown in Figure 7.
This circuit provides much better efficiency, especially over a
wide input voltage range than the linear topologies. For a 1A
charger an LM2575-ADJ. switching regulator IC is used in a
standard buck topology. For other currents, or other pack-
ages, other members of the SIMPLE SWITCHER™ buck
regulator family may be used.
Circuit operation is as follows. With a discharged battery
connected to the charger, the circuit operates as a constant
current source. The constant-current portion of the charger is
formed by the loop consisting of one half of the LM358 op
amp along with gain setting resistors R3 and R4, current
sensing resistor R5, and the feedback reference voltage of
1.23V. Initially the LM358’s output is low causing the output
of the LM2575-ADJ. to rise thus causing some charging cur-
rent to flow into the battery. When the current reaches 1A, it
is sensed by resistor R5 (50 mΩ), and produces 50 mV. This
50 mV is amplified by the op-amps gain of 25 to produce
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