Datasheet
LTC3557/LTC3557-1
10
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PIN FUNCTIONS
V
IN2
(Pin 16): Power Input for Step-Down Switching Regu-
lators 2 and 3. This pin will generally be connected to V
OUT
.
SW3 (Pin 17): Power Transmission (Switch) Pin for
Step-Down Switching Regulator 3.
V
NTC
(Pin 18): Output Bias Voltage for NTC. A resistor from
this pin to the NTC pin will bias the NTC thermistor.
NTC (Pin 19): The NTC pin connects to a battery’s thermistor
to determine if the battery is too hot or too cold to charge. If
the battery’s temperature is out of range, charging is paused
until it drops back into range. A low drift bias resistor is
required from V
NTC
to NTC and a thermistor is required from
NTC to ground. If the NTC function is not desired, the NTC
pin should be grounded.
PROG (Pin 20): Charge Current Program and Charge
Current Monitor Pin. Connecting a resistor from PROG to
ground programs the charge current:
I
CHG
(A)=
1000V
R
PROG
If suffi cient input power is available in constant current
mode, this pin servos to 1V. The voltage on this pin always
represents the actual charge current.
GATE (Pin 21): Ideal Diode Gate Connection. This pin
controls the gate of an optional external P-channel MOSFET
transistor used to supplement the internal ideal diode. The
source of the P-channel MOSFET should be connected
to V
OUT
and the drain should be connected to BAT. It is
important to maintain high impedance on this pin and
minimize all leakage paths.
BAT (Pin 22): Single Cell Li-Ion Battery Pin. Depending on
available power and load, a Li-Ion battery on BAT will either
deliver system power to V
OUT
through the ideal diode or be
charged from the battery charger.
V
OUT
(Pin 23): Output Voltage of the PowerPath Controller
and Input Voltage of the Battery Charger. The majority of
the portable product should be powered from V
OUT
. The
LTC3557/LTC3557-1 will partition the available power
between the external load on V
OUT
and the internal battery
charger. Priority is given to the external load and any extra
power is used to charge the battery. An ideal diode from
BAT to V
OUT
ensures that V
OUT
is powered even if the load
exceeds the allotted input current from V
BUS
or if the V
BUS
power source is removed. V
OUT
should be bypassed with
a low impedance multilayer ceramic capacitor. The total
capacitance on V
OUT
should maintain a minimum of 5µF
over operating voltage and temperature.
V
BUS
(Pin 24): USB Input Voltage. V
BUS
will usually be
connected to the USB port of a computer or a DC output wall
adapter. V
BUS
should be bypassed with a low impedance
multilayer ceramic capacitor.
ACPR (Pin 25): Wall Adapter Present Output (Active
Low). A low on this pin indicates that the wall adapter
input comparator has had its input pulled above its input
threshold (typically 4.3V). This pin can be used to drive the
gate of an external P-channel MOSFET to provide power to
V
OUT
from a power source other than a USB port.
V
C
(Pin 26): High Voltage Buck Regulator Control Pin. This
pin can be used to drive the V
C
pin of an approved external
high voltage buck switching regulator. An external P-channel
MOSFET is required to provide power to V
OUT
with its gate
tied to the ACPR pin. The V
C
pin is designed to work with the
LT
®
3480, LT3481 and LT3505.
CLPROG (Pin 27): Input Current Program and Input Current
Monitor Pin. A resistor from CLPROG to ground determines
the upper limit of the current drawn from the V
BUS
pin
(i.e., the input current limit). A precise fraction of the input
current, h
CLPROG
, is sent to the CLPROG pin. The input
PowerPath delivers current until the CLPROG pin reaches
2.0V (10x Mode), 1.0V (5x Mode) or 0.2V (1x Mode).
Therefore, the current drawn from V
BUS
will be limited to an
amount given by h
CLPROG
and R
CLPROG
. In USB applications
the resistor R
CLPROG
should be set to no less than 2.1k.
CHRG (Pin 28): Open-Drain Charge Status Output. The
CHRG pin indicates the status of the battery charger.
Four possible states are represented by CHRG: charging,
not charging (i.e., fl oat charge current less than 1/10th
programmed charge current), unresponsive battery (i.e., its
voltage remains below 2.8V after 1/2 hour of charging) and
battery temperature out of range. CHRG requires a pull-up
resistor and/or LED to provide indication.
Exposed Pad (Pin 29): Ground. The exposed package pad
is ground and must be soldered to the PC board for proper
functionality and for maximum heat transfer.