Datasheet

LM3622

SNVS043B –FEBRUARY 2000–REVISED APRIL 2013

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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 battery 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

– V

1 – V

1 – LVth)/R2

where V

is the input supply voltage, V

1 is the collector-emitter on state voltage of Q1, V

1 is the diode

forward 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 battery is fully charged. Figure 10

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.

TIMING DIAGRAM

Figure 10. Typical Charge Cycle with Low-Voltage Detection Enabled.

Product Folder Links: LM3622