LTC3586-1_15 Datasheet, PDF (17/36 Pages) Linear Technology – High Efficiency USB Power Manager with Boost, Buck-Boost and Dual Bucks

LTC3586/LTC3586-1
OPERATION
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).
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
1022 times the current in the PROG pin. The program
resistor and the charge current are calculated using the
following equations:
R_PROG
=
1022V
I_CHG
,
I_CHG
=
1022V
R_PROG
In either the constant-current or constant-voltage charging
modes, the voltage at the PROG pin 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
• 1022
In many cases, the actual battery charge current, I_BAT, will
be lower than I_CHG due to limited input power available and
prioritization with the system load drawn from V_OUT.
Charge Status Indication
The CHRG pin indicates the status of the battery charger.
Four possible states are represented by CHRG which in-
clude charging, not charging, unresponsive battery, and
battery temperature out of range.
The signal at the CHRG pin can be easily recognized as
one of the above four states by either a human or a mi-
croprocessor. 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.
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