LTC3553-2_15 Datasheet, PDF (16/36 Pages) Linear Technology – Micropower USB Power Manager with Li-Ion Charger, Always-On LDO and Buck Regulator

LTC3553-2
OPERATION
P-channel MOSFET whenever the voltage at V_OUT is ap-
proximately 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 LTC3553-2 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 LTC3553-2 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
16
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, the charge
current begins to decrease as the LTC3553-2 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.
Automatic Recharge
After the battery charger terminates, it will remain off
drawing only microamperes of current from the battery.
If the portable product remains in this state long enough,
the battery will eventually self discharge. To ensure that
the battery is always topped off, a charge cycle will au-
tomatically begin when the battery voltage falls below
V_RECHRG (typically 4.1V). In the event that the safety timer
is running when the battery voltage falls below V_RECHRG,
the timer will reset back to zero. To prevent brief excur-
sions below V_RECHRG from resetting the safety timer, the
battery voltage must be below V_RECHRG for approximately
2ms. The charge cycle and safety timer will also restart
if the V_BUS UVLO cycles low and then high (e.g., V_BUS, is
removed and then replaced).
Charge Current
The charge current is programmed using a single resis-
tor from PROG to ground. 1/750th of the battery charge
current is delivered to PROG which will attempt to servo
to 1.000V. Thus, the battery charge current will try to
reach 750 times the current in the PROG pin. The program
resistor and the charge current are calculated using the
following equations:
R _PROG
=
750V
I_CHG
,I_CHG
=
750V
R _PROG
35532f