Datasheet

LTC3554/LTC3554-1/
LTC3554-2/LTC3554-3
15
3554123fd
Ideal Diode From BAT to V
OUT
The LTC3554 has an internal ideal diode from BAT to V
OUT
designed to respond quickly whenever V
OUT
drops below
BAT. If the load increases beyond the input current limit,
additional current will be pulled from the battery via the ideal
diode. Furthermore, if power to V
BUS
(USB) is removed,
then all of the application power will be provided by the
battery via the ideal diode. The ideal diode is fast enough
to keep V
OUT
from dropping significantly with just the
recommended output capacitor. The ideal diode consists
of a precision amplifier that enables an on-chip P-channel
MOSFET whenever the voltage at V
OUT
is approximately
15mV (V
FWD
) below the voltage at BAT. The resistance of
the internal ideal diode is approximately 240mΩ.
Suspend Mode
When the SUSP pin is pulled high the LTC3554 enters
suspend mode to comply with the USB specification. In
this mode, the power path between V
BUS
and V
OUT
is put
in a high impedance state to reduce the V
BUS
input current
to 15μA. The system load connected to V
OUT
is supplied
through the ideal diode connected to BAT.
V
BUS
Undervoltage Lockout (UVLO) and Undervoltage
Current Limit (UVCL)
An internal undervoltage lockout circuit monitors V
BUS
and keeps the input current limit circuitry off until V
BUS
rises above the rising UVLO threshold (3.8V) and at least
200mV above V
BAT
. Hysteresis on the UVLO turns off the
input current limit circuitry if V
BUS
drops below 3.6V or
within 50mV of V
BAT
. When this happens, system power at
V
OUT
will be drawn from the battery via the ideal diode. To
minimize the possibility of oscillation in and out of UVLO
when using resistive input supplies, the input current limit
is reduced as V
BUS
falls below 4.45V typical.
Battery Charger
The LTC3554 includes a constant-current/constant-volt-
age battery charger with automatic recharge, automatic
termination by safety timer, low voltage trickle charging,
bad cell detection and thermistor sensor input for out of
temperature charge pausing. When a battery charge cycle
begins, the battery charger first determines if the battery
is deeply discharged. If the battery voltage is below V
TRKL
,
typically 2.9V, an automatic trickle charge feature sets the
battery charge current to 10% of the programmed value. If
the low voltage persists for more than 1/2 hour, the battery
charger automatically terminates. Once the battery voltage
is above 2.9V, the battery charger begins charging in full
power constant current mode. The current delivered to
the battery will try to reach 750V/R
PROG
. Depending on
available input power and external load conditions, the
battery charger may or may not be able to charge at the
full programmed current. The external load will always be
prioritized over the battery charge current. The USB cur-
rent limit programming will always be observed and only
additional current will be available to charge the battery.
When system loads are light, battery charge current will
be maximized.
Charge Termination
The battery charger has a built-in safety timer. When the
battery voltage approaches the float voltage (4.2V for
LTC3554/LTC3554-2/LTC3554-3 or 4.1V for LTC3554-1),
the charge current begins to decrease as the LTC3554
enters constant-voltage mode. Once the battery charger
detects that it has entered constant-voltage mode, the four
hour safety timer is started. After the safety timer expires,
charging of the battery will terminate and no more current
will be delivered to the battery.
OPERATION