LTC3554-1_15 Datasheet, PDF (15/36 Pages) Linear Technology – Micropower USB Power Manager with Li-Ion Charger and Two Step-Down Regulators

LTC3554/LTC3554-1/
LTC3554-2/LTC3554-3
OPeration
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.
3554123ff
15