LM3420 Datasheet, PDF (14/26 Pages) Texas Instruments – 8.4-V Li-Ion Battery Charge Controller

LM3420
SNVS116E – MAY 1998 – REVISED DECEMBER 2014
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The constant current feedback loop operates as follows. Initially, the emitter and collector current of Q2 are both
approximately 1 mA, thus providing gate drive to the MOSFET Q3, turning it on. The output of the LM301A op-
amp is low. As the Q3 current reaches 1 A, the voltage across R1 approaches 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 0.05-Ω current sensing resistor,
thus maintaining a constant 1 A of charge current.
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 compensation 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 prevents the op-amp from pulling more current through the emitter of Q2. This is important when the
battery voltage reaches 8.4 V, and the 1A charge current is no longer needed. Resistor 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
driving the LM3420. When the battery voltage reaches 8.4 V, the LM3420 begins regulating by sourcing current
into R8, which controls the collector current of Q2, which in turn reduces the gate voltage of Q3 and becomes a
constant voltage 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 during the transition from the
constant-current to the constant-voltage mode. D2 and the PNP transistor input stage of the LM10C disconnects
the battery from the charger circuit when the input supply voltage is removed to prevent the battery from
discharging.
Figure 16. Low Dropout Constant Current/Constant Voltage 2-Cell Charger
9.2.4.2 High-Efficiency Switching Regulator Constant Current/Constant Voltage 2-Cell Charger
A switching regulator, constant-current, constant-voltage two-cell Li-Ion battery charging circuit is shown in
Figure 17. This circuit provides much better efficiency, especially over a wide input voltage range than the linear
topologies. For a 1-A charger an LM2575-ADJ. switching regulator IC is used in a standard buck topology. For
other currents, or other packages, 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 1.23 V. Initially the LM358 output is low causing the output of the LM2575-ADJ to rise thus
causing some charging current to flow into the battery. When the current reaches 1 A, 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 1.23V, which is
applied to the feedback pin of the LM2575-ADJ to satisfy the feedback loop.
Once the battery voltage reaches 8.4 V, the LM3420 takes over and begins to control the feedback pin of the
LM2575-ADJ. The LM3420 now regulates the voltage across the battery, and the charger becomes a constant-
voltage charger. Loop compensation network R6 and C3 ensure stable operation of the charger circuit under
both constant-current and constant-voltage conditions. If the input supply voltage is removed, diode D2 and the
PNP input stage of the LM358 become reversed biased and disconnects the battery to ensure that the battery is
not discharged. Diode D3 reverse biases to prevent the op-amp from sinking current when the charger changes
to constant voltage mode.
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