Datasheet

LTC4020
19
4020fd
For more information www.linear.com/LTC4020
operaTion
Use of the timer function also enables bad battery detection
during CC/CV or lead-acid charging. This fault condition is
achieved if the battery does not respond to preconditioning
(V
FB
< 1.75V), such that the charger remains in (or enters)
precondition mode after one-eighth of the programmed
T
EOC
time. A bad battery fault halts the charging cycle, and
the fault condition is reported on the status pins.
CC/CV Charging Overview (MODE = 0V)
To program the LTC4020 for CC/CV charging, connect the
MODE pin to ground. This mode is commonly used for
Li-Ion, Li-Polymer, and LiFePO
4
battery charging.
If the voltage on the VFB pin is below 1.75V, the LTC4020 en-
gages precondition mode, which provides low level charge
currents to gently increase voltage on heavily discharged
batteries.
During preconditioning, the maximum charge
current is reduced to one-fifteenth of the programmed
value as set by R
CS
, the battery charge current program-
ming resistor. Full charge current capability is restored
once the voltage on VFB rises above 1.75V.
Full charge
current capability remains until the VFB pin approaches
the 2.5V float voltage. This is the constant-current (CC)
portion of the charge cycle.
When the voltage on the VFB pin approaches the 2.5V
float voltage, the charger transitions into constant-voltage
(CV) mode, and charge current is reduced from the maxi
-
mum programmed value. If timer termination is used, the
safety timer
period
starts when CV mode is initiated, and
the charge cycle will terminate when the timer achieves
end-of-cycle (T
EOC
). This timer is typically programmed
to achieve T
EOC
in three hours, but can be configured
for any amount of time by setting an appropriate timing
capacitor value (C
TIMER
).
During CV mode, the required charge current is steadily
reduced as the battery voltage is maintained such that the
voltage on the VFB pin remains close to 2.5V. If the char
-
ger is configured for C/10 termination, when the battery
charge current falls below one-tenth of the programmed
maximum current (<C/10),
the charge cycle will terminate
and the charger indicates not charging on the status pins.
When timer termination is used, the charger continues to
operate with charging current less than one-tenth of the
programmed maximum current. The STAT1 status pin,
however, responds to the <C/10 current level regardless
of termination scheme, so the IC will indicate a not charg
-
ing status when the charging current is below the C/10
current level.
The charge cycle will continue, however,
and the charger will source <C/10 current into the bat
-
tery. Programmed float voltage is maintained while the
charger tops-off the battery with low currents until the
programmed T
EOC
time has elapsed, at which time the
charge cycle will terminate, charge current flow into the
battery will be disabled, and the battery will be discon
-
nected from the converter output.
After termination, if the battery discharges such that
the voltage on the VFB pin drops to 2.4375V, or 97.5%
of the programmed float voltage, a new charge cycle is
automatically initiated.
TYPICAL CC/CV CHARGE CYCLE VOLTAGES
(PER CELL)
Li-Ion LiFePO
4
Precondition 2.94V 2.52V
Float 4.2V 3.6V
Recharge 4.095V 3.51V
Lead-Acid Charging Overview (MODE = INT_VCC)
To program the LTC4020 for lead-acid charging, connect
the MODE pin to the INTV
CC
pin. The LTC4020 supports
a 4-step, 3-stage lead-acid charging profile.
The first step of the charging profile provides low level
charge current to gently increase voltage on heavily dis
-
charged batteries. If the voltage on VFB is below 1.75V,
which corresponds to just over 10
V for a 6-cell (12V)
battery, the maximum charge current is reduced to one-
fifteenth of the programmed value as set by RCS. Once
the VFB voltage rises above 1.75V, full charge current
capability is restored, and the bulk charging stage begins.
The bulk charging stage of the charge profile, which is
the first stage of 3-stage battery charging, is a constant-
current charging stage, with the maximum programmed
charge current forced into the battery. This continues until
the battery voltage rises such that the VFB pin approaches
the 2.5V absorption reference voltage.
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