LTC3555/LTC3555-X
18
3555fe
For more information www.linear.com/LTC3555
specification. If the load on V
OUT
exceeds the suspend
current limit, the additional current will come from the
battery via the ideal diode.
3.3V Always-On LDO Supply
The LTC3555 family includes a low quiescent current low
dropout regulator that is always powered. This LDO can be
used to provide power to a system pushbutton controller,
standby microcontroller or real time clock. Designed to
deliver up to 25mA, the always-on LDO requires at least
a 1µF low impedance ceramic bypass capacitor for com-
pensation. The LDO is powered from V
OUT
, and therefore
will enter dropout at loads less than 25mA as V
OUT
falls
near 3.3V. If the LDO3V3 output is not used, it should be
disabled by connecting it to V
OUT
.
V
BUS
Undervoltage Lockout (UVLO)
An internal undervoltage lockout circuit monitors V
BUS
and
keeps the PowerPath switching regulator off until V
BUS
rises above 4.30V and is about 200mV above the battery
voltage. Hysteresis on the UVLO turns off the regulator if
V
BUS
drops below 4.00V or to within 50mV of BAT. When
this happens, system power at V
OUT
will be drawn from
the battery via the ideal diode.
Battery Charger
The LTC3555 family includes a constant-current/
constant-voltage 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.
Battery Preconditioning
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 2.85V, 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 and indicates via the CHRG pin that the battery
was unresponsive.
Once the battery voltage is above 2.85V, the battery charger
begins charging in full power constant-current mode. The
current delivered to the battery will try to reach 1022V/
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 rate. The external
load will always be prioritized over the battery charge
current. The USB current limit programming will always
be observed and only additional power 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
voltage on the battery reaches the pre-programmed float
voltage, the battery charger will regulate the battery volt-
age and the charge current will decrease naturally. Once
the battery charger detects that the battery has reached
the float voltage, the four hour safety timer is started.
After the safety timer expires, charging of the battery will
discontinue and no more current will be delivered.
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 auto-
matically begin when the battery voltage falls below the
recharge threshold which is typically 100mV less than
the charger’s float voltage. In the event that the safety
timer is running when the battery voltage falls below the
recharge threshold, it will reset back to zero. To prevent
brief excursions below the recharge threshold from reset-
ting the safety timer, the battery voltage must be below
the recharge threshold for more than 1.3ms. 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), or if the battery charger is cycled on and off
by the I
2
C port.
Charge Current
The charge current is programmed using a single resis-
tor from PROG to ground. 1/1022th of the battery charge
current is sent to PROG which will attempt to servo to
1.000V. Thus, the battery charge current will try to reach
OPERATION
