Datasheet

LM3420

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SNVS116D –MAY 1998–REVISED MAY 2013

As the battery charges, its voltage begins to rise, and is sensed at the IN pin of the LM3420. Once the battery

voltage reaches 8.4V, the LM3420 begins to regulate and starts sourcing current to the base of Q2. Transistor

Q2 begins controlling the ADJ. pin of the LM317 which begins to regulate 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 regulated 8.4V across the battery and the charging current is dependent on the state of

charge of the battery. As the cells approach a fully charged condition, the charge current falls to a very low value.

Figure 32 shows a Li-Ion battery charger that features a dropout 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 feedback loops. The first loop controls the constant charge current 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 emitter 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 reference to develop

the constant charge current through the current sense resistor R1.

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 Q3's current reaches 1A, 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 0.05Ω current sensing resistor,

thus maintaining a constant 1A 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 8.4V, 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 8.4V, the LM3420 will begin 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 will

disconnect the battery from the charger circuit when the input supply voltage is removed to prevent the battery

from discharging.

Product Folder Links: LM3420