Application Information

Battery Charger Operation Information

Figure 4 shows a general battery charger configuration using

LM3621. The LM3621 provides all the necessary control

functions for charging Lithium-Ion batteries precisely and

safely. It features constant voltage and constant current

mode operation. The on chip timer provides all critical timing

functions to properly initialize the charging cycle, avoid over

charging the battery above its maximum capacity level, and

cut off charging to a defective battery. The LM3621 also pro-

vides the high accuracy of output voltage regulation without

requiring a trim pot or precision resistors. A built-in power

down switch prevents the battery from discharging through

the LM3621.

Input Supply Voltage

The voltage supply to LM3621 must never exceed 6.0V or

catastrophic damage may occur to the IC. When power is

first applied to the battery charger, the LM3621 will remain in

a reset state until the voltage on the V

for at least one second. This ensures that the power source

is stable before a charge cycle is initiated.

Figure 2 shows

the state diagram for the charge cycle operation. Once the

LM3621 emerges from the reset state after power up, it will

not go into the reset state again until the voltage on the V

DD

pin drops below approximately 2.2V. When the LM3621 is in

the reset state, the PFD (Pass FET Drive) pin is pulled low

so that current is not allowed to flow into the load (e.g., Li+

battery). The LM3621 transitions from the reset state to

standby mode when the conditions for a stable power source

are met.

Digital Control Inputs

The LM3621 has four digital input control pins: Enable,

Boost, V

allow unused digital input pins to be left open circuited. En-

able is used to initiate a charge cycle, Boost is used to in-

crease the maximum regulation current output, V

trols the maximum output voltage, and ChgInt is used to

instantaneously interrupt (and resume) a charge cycle in

progress.

Current Boost Mode

This mode is activated when the “Boost” pin is pulled high.

The charger increases the maximum output current equal to

aV

a charge interrupt signal.

Charge Interrupt Control

Charging is interrupted with the digital input “ChgInt” pulled

low. The PFD pin is pulled to ground turning off the series

low pass FET. This is useful to prevent charging a battery if,

for example, the ambient temperature is excessively high.

DS100107-4

FIGURE 4. General Application Circuit

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