LTC3557_15 Datasheet, PDF (15/28 Pages) Linear Technology – USB Power Manager with Li-Ion Charger and Three Step-Down Regulators

LTC3557/LTC3557-1
OPERATION
Battery Charger
The LTC3557/LTC3557-1 includes a constant current/con-
stant 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.
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 1000V/
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 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
LTC3557 or 4.1V for LTC3557-1), the charge current begins
to decrease as the LTC3557/LTC3557-1 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.
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 automati-
cally begin when the battery voltage falls below V_RECHRG
(typically 4.1V for the LTC3557 or 4V for LTC3557-1). 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 excursions below V_RECHRG from re-
setting the safety timer, the battery voltage must be below
V_RECHRG 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).
Charge Current
The charge current is programmed using a single resistor
from PROG to ground. 1/1000th 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
1000 times the current in the PROG pin. The program
resistor and the charge current are calculated using the
following equations:
R_PROG
= ^1000V ,
I_CHG
I_CHG
= ^1000V
R_PROG
In either the constant current or constant voltage charging
modes, the PROG pin voltage will be proportional to the
actual charge current delivered to the battery. Therefore,
the actual charge current can be determined at any
time by monitoring the PROG pin voltage and using the
following equation:
I_BAT
=
V_PROG
R_PROG
• 1000
In many cases, the actual battery charge current, I_BAT, will
be lower than I_CHG due to limited input current available and
prioritization with the system load drawn from V_OUT.
Thermal Regulation
To prevent thermal damage to the IC or surrounding
components, an internal thermal feedback loop will
automatically decrease the programmed charge current
if the die temperature rises to approximately 110°C.
Thermal regulation protects the LTC3557/LTC3557-1
from excessive temperature due to high power operation
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