LTC4055 Linear Technology, LTC4055 Datasheet - Page 15

• Manufacturer Part Number: LTC4055
• Description: USB Power Controller and Li-Ion Linear Charger
• Manufacturer: Linear Technology

OPERATIO
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 film
resistors are recommended.
Battery Charger
The battery charger circuits of the LTC4055 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 final float 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 at all times. An NTC input provides the
option of charge qualification 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 from excessive temperature, and allows the
user to push the limits of the power handling capability of
a given circuit board without risk of damaging the LTC4055.
Another benefit of the LTC4055 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
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 flows until the input voltage is greater than the
V
battery voltage is below 2.8V, the charger goes into trickle
UVCL
level. At the beginning of the charge cycle, if the
U
IN
or V
OUT
at least 4.4V.
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
voltage, the charge current begins to decrease as the
LTC4055 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 imped-
ance state. While charging in constant-current mode, if the
charge current is decreased due to load current, under-
voltage 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 final float potential,
the internal bandgap reference, voltage amplifier 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 the battery charge current, when not being
limited, is:
I
CHG
PROG
. When the battery approaches the final float
I
PROG
•
50 000
,
R
V
PROG
PROG
•
50 000
,
LTC4055
15
4055p