A rapid charge Lithium-Ion battery charging circuit is shown

in

Figure 10. 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 an-

other 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 deter-

mined 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 sec-

ond rate, with the off time determined by the battery’s state

of charge. The LM555 output will go high for 5 seconds (pull-

ing down the collector of Q1) which allows the 1A

constant-current loop to control the circuit.

Figure 11 shows a low dropout constant voltage charger us-

ing a MOSFET as the pass element, but this circuit does not

include current limiting. This circuit uses Q3 and a Schottky

diode to isolate the battery from the charging circuitry when

the input voltage is removed, to prevent the battery from dis-

charging. Q2 should be a high current (0.2

Ω) FET, while Q3

can be a low current (2

Ω) device.

Note: Although the application circuits shown here have

been built and tested, they should be thoroughly evalu-

ated with the same type of battery the charger will even-

tually be used with.

Different battery manufacturers may use a slightly dif-

ferent battery chemistry which may require different

charging characteristics. Always consult the battery

manufacturer for information on charging specifications

and battery details, and always observe the manufactur-

ers precautions when using their batteries. Avoid over-

charging or shorting Lithium-Ion batteries.

DS012359-16

FIGURE 11. MOSFET Low Dropout Charger

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