LTC3554 Datasheet, PDF (16/36 Pages) Linear Technology – Micropower USB Power Manager with Li-Ion Charger and Two Step-Down Regulators

LTC3554
OPERATION
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). In the event that the safety timer is run-
ning when the battery voltage falls below V_RECHRG, the
timer will reset back to zero. To prevent brief excursions
below V_RECHRG from resetting 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 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
In either the constant-current or constant-voltage charg-
ing modes, the PROG pin voltage will be proportional to
the actual charge current delivered to the battery. There-
fore, 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
•
750
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 au-
tomatically decrease the programmed charge current if
the die temperature rises to approximately 110°C. Ther-
mal regulation protects the LTC3554 from excessive
temperature due to high power operation or high ambient
thermal conditions and allows the user to push the limits
of the power handling capability with a given circuit board
design without risk of damaging the LTC3554 or external
components. The benefit of the LTC3554 thermal regula-
tion loop is that charge current can be set according to the
desired charge rate rather than worst-case conditions with
the assurance that the battery charger will automatically
reduce the current in worst-case conditions.
Charge Status Indication
The CHRG pin indicates the status of the battery charger. An
open-drain output, the CHRG pin can drive an indicator LED
through a current limiting resistor for human interfacing or
simply a pull-up resistor for microprocessor interfacing.
When charging begins, CHRG is pulled low and remains
low for the duration of a normal charge cycle. When charg-
ing is complete, i.e., the charger enters constant-voltage
mode and the charge current has dropped to one-tenth
of the programmed value, the CHRG pin is released (high
impedance). The CHRG pin does not respond to the C/10
threshold if the LTC3554 reduces the charge current due
to excess load on the V_OUT pin. This prevents false end
of charge indications due to insufficient power available
to the battery charger. Even though charging is stopped
during an NTC fault the CHRG pin will stay low indicating
that charging is not complete.
3554p
16