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LTC4020

4020fd

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operaTion

As the bulk charging stage completes and the voltage on

the VFB pin rises to approach 2.5V, the charger transi-

tions into the absorption stage, which is the 2nd stage of

3-stage battery charging. During the absorption stage,

the required charge current is steadily reduced as the

battery voltage approaches the absorption voltage. This is

a constant-voltage charging stage, as the battery voltage

is maintained such that the VFB pin remains close to the

2.5V absorption reference voltage. It is during this stage

that the battery stored charge increases to 100% capac-

ity. The 2.5V absorption reference typically corresponds

to 14.4V for a 6-cell battery.

When the absorption stage charge current is reduced

to one-tenth of the programmed maximum current, the

charger will initiate the third stage in the charge profile,

the float charging stage. The safety timer can be used with

a lead-acid charger to limit the duration of the absorption

stage of the charging profile. The timer is initiated at the

start of the absorption stage, and forces the charger into

float if the charge current does not fall to the required

one-tenth of the programmed maximum current during

the absorption stage before the timer reaches TEOC. A

0.47µF capacitor on the TIMER pin is typically used, which

generates a 6.8 hour absorption stage safety timeout.

Once the float charging stage is initiated, the battery

reference voltage is reduced to 92.5% of the absorption

voltage, or 2.3125V. The battery voltage is maintained at

a voltage corresponding to this reference voltage, and

maximum charge current is reduced to one-fifteenth of

the programmed maximum. This level corresponds to

13.3V for a 6-cell battery.

Once float charging is achieved, the LTC4020 charger re-

mains active and will attempt to maintain the float voltage

on the battery indefinitely. During float charging, if a load

on the battery exceeds the maintenance charge current

of one-fifteenth of the programmed maximum, the bat-

tery voltage will begin to discharge. If a load discharges

the battery such that the voltage on VFB falls to 2.1875V,

corresponding to 12.6V for a 6-cell battery, the LTC4020

restarts the full 3-stage charging cycle by reinitiating the

bulk charging stage. Bulk charging is engaged by resetting

the internal VFB reference to the 2.5V absorption voltage

reference and increasing the charge current capability to

the programmed maximum.

TYPICAL LEAD-ACID CHARGE CYCLE

VOLTAGES (12V System)

Precondition

10.1V

Absorption

14.4V

Float

13.3V

Bulk Restart

12.6V

CC Charging Overview (MODE = NC)

To program the LTC4020 for CC charging, leave the MODE

pin unconnected. This mode can be used for charging

NiCd and NiMH batteries, supercap charging, or in any

other application where a timed current source is desired.

CC mode can also be used when the voltage dependent

precondition mode is not desired.

In CC mode, the LTC4020 will maintain full programmed

charge current capability for the duration of the timer

period. The trickle charge function is disabled, although

maximumchargecurrentwillbereducedduringlowerdeck

operation if there is excessive voltage (>0.3V) imposed

across the PowerPath FET. The charger will terminate the

charge cycle and the PowerPath FET will become high-

impedance once timer EOC is reached.

While the charge cycle is designed to be voltage inde-

pendent, a maximum VBAT voltage can be programmed

corresponding to VFB = 2.5V, allowing constant-voltage

functionality at that level if desired.

Once the timer reaches TEOC and the charge cycle ter-

minates, input power or SHDN must be cycled to initiate

another charge cycle. If the timer function is disabled

(TIMER = 0V), the current source function remains active

indefinitely.

Note: For nickel-chemistry batteries (e.g. NiCd or NiMH),

the possibility of overcharging must be considered. A

typical method is to charge with low currents for a long

period of time. NiCd and NiMH batteries can absorb a

C/300 charge rate indefinitely. Shorter duration charging

is possible using a timed current source charge algorithm.

It is recommended to ensure a depleted battery before

charging,thensubsequentlychargethebatterytonomore

than 125% capacity. For example, a depleted 2000mAh

NiMH battery is charged with 2.5A for one hour.

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