Typical Application

The low dropout linear charger shown in

Figure 2 provides

constant current and constant voltage charging of 1-cell

lithium-ion battery packs. J1 and J2 are used for selecting

the operation of the low-voltage detection. The LM3622 ini-

tializes the charge cycle based on the battery voltage and

the enable status of the low-voltage detection.

When the low-voltage detection is disabled, the LM3622

starts the charge cycle constant current mode if the battery

voltage is below the controller’s regulation level. In constant

current mode, the LM3622 modulates the base drive of Q2 to

regulate a constant 100mV across the current sense resistor

R1, thus generating charge current of

I-charge = 0.1V/R1

which is equal to 0.5A in this case.

Once the battery voltage reaches the target regulation level

set by the LM3622, Q2 is controlled to regulate the voltage

across the battery, and the constant voltage mode of the

charging cycle starts. Once the charger is in the constant

voltage mode, the charger maintains a regulated voltage

across the battery and the charging current is dependent on

the state of the charge of the battery. As the cell approaches

a fully charged condition, the charge current falls to a very

low value.

When the low-voltage detection is enabled and the initial bat-

tery voltage is below the low-voltage threshold, the LM3622

turns Q2 off and forces the LV pin low to drive Q1 on to start

a wake up charge phase. Q1 in conjunction with R2 provides

a low current source to recondition the battery. During the

wake up charge mode, Q1 is driven into saturation and the

wake up charge current is programmed by R2,

I-charge (wake) = (V

where V

IN

is the input supply voltage, V

collector-emitter on state voltage of Q1, V

ward voltage of D1, and LVth is the low-voltage threshold

level set by switch J2.

Once the battery voltage reaches the low-voltage threshold,

the LV pin transitions to a high-impedance state to end the

wake up charge phase, and the EXT pin resumes the base

drive of Q2 to start the constant current mode. The charging

cycle is completed in constant voltage mode when the bat-

tery is fully charged.

Figure 3 shows the timing diagram of

the charge cycle with the low-voltage detection enabled.

D1 is a general-purpose silicon diode used for isolating the

battery from the charger circuitry that could discharge the

battery when the input source is removed. Changing D1 to a

Schottky diode will reduce the overall dropout voltage of the

circuit, but the penalty is higher leakage current associated

with Schottky diodes.

DS100974-13

FIGURE 2. Low Dropout, Constant Current/Constant Voltage Li-ion Battery Charger

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