LTC4055/LTC4055-1
15
4055fb
OPERATION
exceeding 500mA due to LTC4055/LTC4055-1 tolerances
and quiescent currents. This will give a typical current limit
of approximately 467mA in high power mode (HPWR = 1)
or 92mA in low power mode (HPWR = 0).
For best stability over temperature and time, 1% metal
fi lm resistors are recommended.
Battery Charger
The battery charger circuits of the LTC4055/LTC4055-1
are designed for charging single-cell lithium-ion batteries.
Featuring an internal P-channel power MOSFET, the charger
uses a constant-current/constant-voltage charge algorithm
with programmable current and a programmable timer for
charge termination. Charge current can be programmed up
to 1A. The fi nal fl oat voltage accuracy is ±0.8% typical. No
blocking diode or sense resistor is required when charging
through IN1/IN2. The CHRG open-drain status output
provides information regarding the charging status of the
LTC4055/LTC4055-1 at all times. An NTC input provides the
option of charge qualifi cation using battery temperature.
An internal thermal limit reduces the programmed charge
current if the die temperature attempts to rise above a
preset value of approximately 105°C. This feature protects
the LTC4055/LTC4055-1 from excessive temperature, and
allows the user to push the limits of the power handling
capability of a given circuit board without risk of dam-
aging the LTC4055/LTC4055-1. Another benefi t of the
LTC4055/LTC4055-1 thermal limit is that charge current
can be set according to typical, not worst-case, ambient
temperatures for a given application with the assurance
that the charger will automatically reduce the current in
worst-case conditions.
An internal voltage regulation circuit, called undervoltage
current limit, UVCL, reduces the programmed charge
current to keep the voltage on V
IN
or V
OUT
at least 4.4V.
This feature prevents the charger from cycling in and out
of undervoltage lockout due to resistive drops in the USB
or wall adapter cabling.
The charge cycle begins when the voltage at the input
(IN1/IN2) rises above the input UVLO level and the battery
voltage is below the recharge threshold. No charge current
actually fl ows until the input voltage is greater than the
V
UVCL
level. At the beginning of the charge cycle, if the
battery voltage is below 2.8V, the charger goes into
trickle-charge mode to bring the cell voltage up to a safe
level for charging. The charger goes into the fast charge
constant-current mode once the voltage on the BAT pin rises
above 2.8V. In constant current mode, the charge current
is set by R
PROG
. When the battery approaches the fi nal
fl oat voltage, the charge current begins to decrease as the
LTC4055/LTC4055-1 switches to constant-voltage mode.
An external capacitor on the TIMER pin sets the total
minimum charge time. When this time elapses the
charge cycle terminates and the CHRG pin assumes a
high impedance state. While charging in constant-cur-
rent mode, if the charge current is decreased due to load
current, undervoltage charge current limiting or thermal
regulation the charging time is automatically increased.
In other words, the charge time is extended inversely
proportional to charge current delivered to the battery.
For lithium-ion and similar batteries that require accurate
fi nal fl oat potential, the internal bandgap reference, voltage
amplifi er and the resistor divider provide regulation with
±1% maximum accuracy.
TRICKLE CHARGE AND DEFECTIVE BATTERY
DETECTION
At the beginning of a charge cycle, if the battery voltage
is low (below 2.8V) the charger goes into trickle-charge
reducing the charge current to 10% of the full-scale current.
If the low battery voltage persists for one quarter of the
total charge time, the battery is assumed to be defective,
the charge cycle is terminated and the CHRG pin output
assumes a high impedance state. If for any reason the
battery voltage rises above ~2.8V, the charge cycle will
be restarted. To restart the charge cycle (i.e., when the
dead battery is replaced with a discharged battery), simply
remove the input voltage and reapply it, cycle the TIMER
pin to 0V or cycle the SHDN pin to 0V.
PROGRAMMING CHARGE CURRENT
The formula for programming the battery charge current,
when not being limited, is:
II
V
R
CHG PROG
PROG
PROG
==•, •,48 500 48 500
