Programmable Charge Termination Voltage

The charging status for battery of graphite or coke anode is

programmed with the digital input “V

charge termination voltage of 4.1V, a float voltage of 3.84V

and over voltage detection threshold (V

V

age of 4.09V and V

Defective Pack Detection

The LM3621 automatically checks for defective battery.

Whenever a defective pack is detected, the charging se-

quence is suspended and LED1 driver is pulsed at a rate of

0.8 Hz. A defective battery is indicated under three condi-

tions: (1) when a battery fails to charge up to 2.55V within

one hour, (2) when the battery ESR plus the contact resis-

tance is excessive, and (3) when the pack is an open circuit.

Step by Step Description of a Typical Charging

Sequence. (Refer to

Figure 3)

1. Standby Mode

The LM3621 will remain in standby while the digital inputs to

the Enable and Boost pins are low. Boost overrides Enable,

and Boost mode may be activated at any time with two ex-

ceptions: (1) when a deeply discharged battery (i.e., V

BAT-

TERY

≤ 2.5V) is attached to the charger or (2) when ChgInt

has been activated during a normal charge cycle. Both LED

drivers pull low during Boost mode. During standby, the

charger’s output voltage V

4.1V depending on the state of the digital input to the V

SET

pin (V

V

LED driver pins are high impedance (open drain).

Table 1

shows the state of the LED drivers during various modes of

operation.

2. Charging Sequence Initialization

The LM3621 determines the presence or absence of a bat-

tery pack by the state of the “Enable” pin. Enable should be

pulled low when the pack is not present. When a pack is in-

serted into the charger, Enable should go high to initiate the

charging sequence.

After Enable is asserted with a battery connected to the

charger, LED1 goes low and there is a one-second delay be-

fore current is allowed to flow into the battery. During this de-

lay, the LM3621 checks the charge condition of the battery to

determine if it is grossly overcharged or if it is deeply dis-

charged. If the battery is grossly overcharged, PFD stays low

keeping the low side pass FET off, and both LED drivers

pulse at a frequency of 0.8 Hz to indicate a fault condition. If

the battery is less than 2.55V, the LM3621 goes to the Hi-Z

( battery conditioning) charge mode. During the Hi-Z charge

mode, V

across the current sense resistor (R

mV. The LM3621 is capable of charging a battery that has

been discharged down to as low as 0V. This is accomplished

by the fact that the “ON” resistance of the low side N-channel

pass FET is modulated by the PFD output. This ensures that

V

voltage is, and it prevents the charger’s regulation loop from

collapsing in the event that the charger’s output is shorted.

The LM3621 will remain in the Hi-Z Charge mode for a maxi-

mum of one hour. If the battery voltage does not increase to

at least 2.55V by the end of the one-hour interval, then PFD

goes low to turn off the low side pass FET and LED1 will

pulse at 0.8 Hz to indicate a defective battery.

3. Fast Charge Mode

The LM3621 will transition to Fast Charge mode whenever

the battery voltage is above 2.55V. During Fast Charge

mode LED1 stays low and LED2 stays open drain, and the

LM3621 controls the charger to behave as a current limited,

constant output voltage source to the battery. If the battery

voltage is less than 4.2V (for V

trols the charger to behave as a constant current source.

V

livered to the battery is equal to 0.5V / R

voltage reaches 4.2V, the LM3621 controls the charger to

behave as a constant voltage source, and V

tained at 4.2V. As the battery continues to charge up, the

charge current will decrease. Correspondingly, V

crease as well.

The battery will continue to charge until V

to 40 mV, at which time PFD pulls low to interrupt the charge

current for one second. At the end of the one second interval,

the LM3621 checks to see if the battery’s Resistance Free

Voltage (RFV) is greater than 3.65V or less than 3.65V. If the

battery’s RFV is less than 3.65V, then the ESR of the battery

is excessively high, PFD stays low to keep the charging se-

quence suspended and LED1 pulses at 0.8 Hz to indicate a

defective battery. If the battery’s RFV is greater than 3.65V,

then the LM3621 transitions to Top-Off mode. At this transi-

tion, LED1 becomes open drain and LED2 pulls low indicat-

ing a fully charged battery.

4. Top-Off Mode

During Top-Off mode, the battery is trickle charged for one

hour. At the end of the one hour interval, PFD goes low to

terminate charging (i.e., the charge current is interrupted),

and the LM3621 transitions to Battery Monitor mode.

5. Monitor Mode

During Monitor mode, V

or 3.84V (if V

LM3621 typically draws less than 1 µA from the battery dur-

ing Monitor mode. If the battery self discharges so that its

voltage decreases to 4.09V (if V

is low), then the LM3621 will transition back to Fast Charge

mode to refresh the battery and charge it back up to its full

capacity (

Figure 2 & Figure 5). This process will continue un-

til Enable goes low.

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