Datasheet

LTC4020
20
4020fd
For more information www.linear.com/LTC4020
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 batter
y 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 batter
y.
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 T
EOC
. 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 batter
y 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
maximum charge current will be reduced during lower deck
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 V
BAT
voltage can be programmed
corresponding to VFB = 2.5V, allowing constant-voltage
functionality at that level if desired.
Once the timer reaches T
EOC
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, then subsequently charge the battery to no more
than 125% capacity. For example, a depleted 2000mAh
NiMH battery is charged with 2.5A for one hour.
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