LTC3455 Datasheet, PDF (19/24 Pages) Linear Technology – Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger

LTC3455
APPLICATIO S I FOR ATIO
Charge and Recharge Cycles
When external power is first applied, a new charge cycle is
always initiated. The battery will continue charging until
the programmed charge time is reached. If the battery
voltage is below 4.05V at the end of this cycle, the
LTC3455 will start a new charge cycle. This action will
continue until the battery voltage exceeds the 4.05V thresh-
old. This operation is typically seen only when charging
from USB power. Because the charge current can vary
dramatically when the LTC3455 is USB powered, it takes
considerably longer to charge a battery using the USB
supply (as compared to a wall adapter). If the timer
capacitor is chosen correctly, the battery should be fully
charged on one cycle when wall power is available.
If the battery is above the 4.05V threshold when a charge
cycle has expired, charging will stop. At this point, a
recharge cycle is initiated if any of the following occurs:
The battery voltage drops below 4.05V, external power is
removed and reapplied, the PROG pin is floated tempo-
rarily, or the SUSPEND pin is temporarily pulled high (if the
LTC3455 is under USB power).
Programming Charge Current
The maximum charge current is programmed using one
external resistor connected between the PROG pin and
GND (use the closest 1% resistor value):
R_PROG = 1000 • 1.23V / I_BAT
If only USB power is used (no wall adapter), select the
R_PROG value to be 2.49kΩ (or larger) to set the maximum
charge current at 500mA. If a wall adapter is also used,
I_CHARGE can be programmed up to 1A (with a 1.24kΩ
R_PROG value), and the USB power manager will automati-
cally throttle back the charge current to below 500mA
when under USB power.
Monitoring Charge Current
The voltage on the PROG pin is an accurate indication of
the battery charge current under all charging conditions.
I_BAT = 1000 • 1.23V / R_PROG
Capacitance on the PROG pin should be minimized to
ensure loop stability when in constant-current mode. Do
not place a capacitor directly from the PROG pin to ground.
Adding an external R-C network (see Figure 10) allows the
monitoring of average, rather than instantaneous, battery
charge current. Average charge current is typically of
more interest to the user, especially when the LTC3455 is
USB powered, as the battery charge current varies signifi-
cantly with normal load transients.
LTC3455
2
10k
PROG
GND
25
R_PROG C_FILTER
CHARGE
CURRENT
MONITOR
CIRCUITRY
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Figure 10. Monitoring Average Charge Current
Programming the Battery Charger Timer
An external capacitor on the TIMER pin sets the total
charge time. When this timer elapses the charge cycle
terminates and the CHRG pin assumes a high impedance
state. The total charge time is programmed as:
T_TIMER (hours) = C_TIMER • (3 hours) / (0.1µF)
Trickle Charge and Defective Battery Detection
If the battery voltage is below 2.85V at the beginning of the
charge cycle, the charger goes into trickle charge mode,
reducing the charge current to 10% of its programmed
full-scale value. If the low battery voltage remains for one
quarter of the programmed total charge time, the battery
is assumed to be defective, the charge cycle is terminated,
and the CHRG pin goes to a high impedance state. This
fault is cleared if any of the following occurs: The battery
voltage rises above 2.85V, external power is removed and
reapplied, the PROG pin is floated temporarily, or the
SUSPEND pin is temporarily pulled high (if the LTC3455 is
under USB power). The device will still operate normally
from USB or wall power even if the charger has turned off
due to a trickle-charge timeout.
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