LTC3554 LINER [Linear Technology], LTC3554 Datasheet - Page 16

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LTC3554

Manufacturer Part Number
LTC3554
Description
Micropower USB Power Manager with Li-Ion Charger and Two Step-Down Regulators
Manufacturer
LINER [Linear Technology]
Datasheet

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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 au-
tomatically begin when the battery voltage falls below
V
is running when the battery voltage falls below V
the timer will reset back to zero. To prevent brief excur-
sions below V
battery voltage must be below V
2ms. The charge cycle and safety timer will also restart
if the V
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:
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:
16
RECHRG
R
I
BAT
PROG
BUS
=
(typically 4.1V). In the event that the safety timer
R
V
=
UVLO cycles low and then high (e.g., V
PROG
PROG
750V
I
CHG
RECHRG
• 750
,I
CHG
from resetting the safety timer, the
=
R
750V
PROG
RECHRG
for approximately
RECHRG
BUS
, is
,
In many cases, the actual battery charge current, I
be lower than I
prioritization with the system load drawn from V
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 benefi t 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
of charge indications due to insuffi cient 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.
CHG
due to limited input current available and
OUT
pin. This prevents false end
OUT
BAT
, will
.
3554f

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