Datasheet

LM3420

SNVS116D –MAY 1998–REVISED MAY 2013

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Figure 36. (Fast) Pulsed Constant Current 2-Cell Charger

A rapid charge Lithium-Ion battery charging circuit is shown in Figure 36. 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 1A. As the battery voltage reaches 8.4V, this charger changes from a constant

continuous current of 1A to a 5 second pulsed 1A. 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 8.4V. After charging the battery with 1A for 5 seconds,

the charge stops, and the battery voltage begins to drop. When it drops below 8.4V, the LM555 timer again starts

the timing cycle and charges the battery with 1A for another 5 seconds. This cycling continues with a constant 5

second charge time, and a variable off time. In this manner, the battery will be charged with 1A for 5 seconds,

followed by an off period (determined by the battery's state of charge), setting up a periodic 1A charge current.

The off time is determined by how long it takes the battery voltage to decrease back down to 8.4V. When the

battery first reaches 8.4V, the off time will be very short (1 ms or less), but when the battery approaches full

charge, the off time will begin increasing to tens of seconds, then minutes, and eventually hours.

The constant-current loop for this charger and the method used for programming the 1A 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 8.4V, the output of IC3 will be held low, and the LM555 one-shot will never

fire (the output of the LM555 will be held high) and the one-shot will have no effect on the charger. Once the

battery voltage exceeds the 8.4V regulation voltage of IC3, the trigger pin of the LM555 is pulled high, enabling

the one shot to begin timing. The charge current will now be pulsed into the battery at a 5 second rate, with the

off time determined by the battery's state of charge. The LM555 output will go high for 5 seconds (pulling down

the collector of Q1) which allows the 1A constant-current loop to control the circuit.

Product Folder Links: LM3420