LTC3555 Linear Technology, LTC3555 Datasheet - Page 17

• Manufacturer Part Number: LTC3555
• Description: High Efficiency USB Power Manager + Triple Step-Down DC/DC
• Manufacturer: Linear Technology

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Battery Charger
The LTC3555 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.
Battery Preconditioning
When a battery charge cycle begins, the battery charger
fi rst determines if the battery is deeply discharged. If the
battery voltage is below V
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 ⎯ C ⎯ H ⎯ R ⎯ G 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
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
fl oat voltage of 4.200V, the battery charger will regulate
the battery voltage and the charge current will decrease
naturally. Once the battery charger detects that the battery
has reached 4.200V, 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
OPERATION
PROG
. Depending on available input power and external
TRKL
, typically 2.85V, an automatic
the battery is always topped off, a charge cycle will auto-
matically begin when the battery voltage falls below 4.1V.
In the event that the safety timer is running when the
battery voltage falls below 4.1V, it will reset back to zero.
To prevent brief excursions below 4.1V from resetting the
safety timer, the battery voltage must be below 4.1V for
more than 1.3ms. The charge cycle and safety timer will
also restart if the V
(e.g., V
charger is cycled on and off by the I
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:
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:
In many cases, the actual battery charge current, I
be lower than I
prioritization with the system load drawn from V
Charge Status Indication
The ⎯ C ⎯ H ⎯ R ⎯ G pin indicates the status of the battery charger.
Four possible states are represented by ⎯ C ⎯ H ⎯ R ⎯ G which in-
clude charging, not charging, unresponsive battery, and
battery temperature out of range.
The signal at the ⎯ C ⎯ H ⎯ R ⎯ G 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 ⎯ C ⎯ H ⎯ R ⎯ G pin can
I
R
BAT
PROG
BUS
=
R
V
=
is removed and then replaced), or if the battery
PROG
PROG
1022
I
CHRG
CHG
• 1022
V
due to limited input power available and
BUS
,
I
CHRG
UVLO cycles low and then high
=
1022
R
PROG
V
2
C port.
LTC3555
OUT
17
BAT
.
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