LM3420 Datasheet, PDF (16/26 Pages) Texas Instruments – 8.4-V Li-Ion Battery Charge Controller

LM3420
SNVS116E – MAY 1998 – REVISED DECEMBER 2014
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9.2.4.4 High-Efficiency Switching Charger With High Side Current Sensing
Another variation of a constant current/constant voltage switch mode charger is shown in Figure 19. The basic
feedback loops for current and voltage are similar to the previous circuits. This circuit has the current sensing
resistor, for the constant current part of the feedback loop, on the positive side of the battery, thus allowing a
common ground between the input supply and the battery. Also, the LMC7101 op-amp is available in a very
small SOT-23-5 package thus allowing a very compact PC board design. Diode D4 prevents the battery from
discharging through the charger circuitry if the input voltage is removed, although the quiescent current of the
LM3420 is still present (approximately 85 μA).
Figure 19. High Efficiency Switching Charger
With High Side Current Sensing
9.2.4.5 Fast-Pulsed Constant Current 2-Cell Charger
A rapid charge Lithium-Ion battery charging circuit is shown in Figure 20. This configuration uses a switching
regulator to deliver the charging current in a series of constant current pulses. At the beginning of the charge
cycle (constant-current mode), this circuit performs identically to the previous LM2575 charger by charging the
battery at a constant current of 1 A. As the battery voltage reaches 8.4 V, this charger changes from a constant
continuous current of 1 A to a 5-second pulsed 1 A. This allows the total battery charge time to be reduced
considerably. This is different from the other charging circuits that switch from a constant current charge to a
constant voltage charge once the battery voltage reaches 8.4 V. After charging the battery with 1 A for 5
seconds, the charge stops, and the battery voltage begins to drop. When it drops below 8.4 V, the LM555 timer
again starts the timing cycle and charges the battery with 1 A for another 5 seconds. This cycling continues with
a constant 5-second charge time, and a variable off time. In this manner, the battery is charged with 1 A for 5
seconds, followed by an off period (determined by the battery's state of charge), setting up a periodic 1-A charge
current. The off time is determined by how long it takes the battery voltage to decrease back down to 8.4 V.
When the battery first reaches 8.4 V, the off time is very short (1 ms or less), but when the battery approaches
full charge, the off time begins increasing to tens of seconds, then minutes, and eventually hours.
The constant-current loop for this charger and the method used for programming the 1-A constant current is
identical to the previous LM2575-ADJ charger. In this circuit, a second LM3420-8.4 has its V_REG increased by
approximately 400 mV (via R2), and is used to limit the output voltage of the charger to 8.8V in the event of a
bad battery connection, or the battery is removed or possibly damaged.
The LM555 timer is connected as a one-shot, and is used to provide the 5-second charging pulses. As long as
the battery voltage is less than the 8.4 V, the output of IC3 is held low, and the LM555 one-shot never fires (the
output of the LM555 is held high) and the one-shot has no effect on the charger. Once the battery voltage
exceeds the 8.4-V regulation voltage of IC3, the trigger pin of the LM555 is pulled high, enabling the one shot to
begin timing. The charge current is now pulsed into the battery at a 5-second rate, with the off time determined
by the battery's state of charge. The LM555 output goes high for 5 seconds (pulling down the collector of Q1)
which allows the 1-A constant-current loop to control the circuit.
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