LTC3550-1
10
35501f
PI FU CTIO S
UUU
USBIN (Pin 1): USB Input Supply Pin. Provides power to
the battery charger. The maximum supply current is 650mA.
This should be bypassed with a 1µF capacitor.
IUSB (Pin 2): USB Charge Current Program and Monitor
Pin. The charge current can be set by connecting a resis-
tor, R
IUSB
, to ground. When charging in constant-current
mode, this pin servos to 1V. The voltage on this pin can
be used to measure the charge current delivered from the
USB input using the following formula:
I
V
R
BAT
IUSB
IUSB
= • 1000
ITERM (Pin 3): Termination Current Threshold Program
Pin. The current termination threshold, I
TERMINATE
, can be
set by connecting a resistor, R
ITERM
, to ground. I
TERMINATE
is set by the following formula:
I
V
R
TERMINATE
ITERM
=
100
When the charge current, I
BAT
, falls below the termination
threshold, charging stops and the
⎯
C
⎯
H
⎯
R
⎯
G output becomes
high impedance.
This pin is internally clamped to approximately 1.5V. Driv-
ing this pin to voltages beyond the clamp voltage should
be avoided.
⎯
P
⎯
W
⎯
R (Pin 4): Open-Drain Power Supply Status Output.
When the DCIN or USBIN pin voltage is suffi cient to
begin charging (i.e., when the supply is greater than
the undervoltage lockout threshold and at least 180mV
above the battery terminal), the
⎯
P
⎯
W
⎯
R pin is pulled low by
an internal N-channel MOSFET. Otherwise,
⎯
P
⎯
W
⎯
R is high
impedance. The output is capable of sinking up to 10mA,
making it suitable for driving an LED.
⎯
C
⎯
H
⎯
R
⎯
G (Pin 5): Open-Drain Charge Status Output. When
the LTC3550-1 is charging, the
⎯
C
⎯
H
⎯
R
⎯
G pin is pulled low by
an internal N-channel MOSFET. When the charge cycle is
completed,
⎯
C
⎯
H
⎯
R
⎯
G becomes high impedance. This output
is capable of sinking up to 10mA, making it suitable for
driving an LED.
V
OUT
(Pin 6): Output Voltage Feedback Pin. Receives the
feedback voltage from the buck regulator output.
V
CC
(Pin 7): Buck Regulator Input Supply Pin. Must be
closely decoupled to GND (Pins 8, 9) with a 2.2µF or
greater ceramic capacitor.
GND (Pins 8, 9): Ground.
SW (Pin 10): Switch Node Connection to Inductor. This
pin connects to the drains of the internal main (top) and
synchronous (bottom) power MOSFET switches.
RUN (Pin 11): Buck Regulator Run Control Input. Forcing
this pin above 1.5V enables the regulator. Forcing this pin
below 0.3V shuts it down. In shutdown, all buck regulator
functions are disabled drawing <1µA supply current from
V
CC
. Do not leave RUN fl oating.
ENABLE (Pin 12): Charger Enable Input. When the
LTC3550-1 is charging from the DCIN source, a logic low
on this pin enables the charger. When the LTC3550-1 is
charging from the USBIN source, a logic high on this pin
enables the charger. If this input is left fl oating, an internal
2MΩ pulldown resistor defaults the LTC3550-1 to charge
when a wall adapter is applied and to shut down if only
the USB source is applied.
USBPWR (Pin 13): Open-Drain USB Power Status Output.
When the voltage on the USBIN pin is suffi cient to begin
charging and there is insuffi cient power at DCIN, the
USBPWR pin is high impedance. In all other cases, this pin
is pulled low by an internal N-channel MOSFET, provided
that there is power present at DCIN, USBIN, or BAT inputs.
This output is capable of sinking up to 1mA, making it
suitable for driving high impedance logic inputs.
IDC (Pin 14): Wall Adapter Charge Current Program and
Monitor Pin. The charge current is set by connecting a
resistor, R
IDC
, to ground. When charging in constant-
current mode, this pin servos to 1V. The voltage on this
pin can be used to measure the charge current using the
following formula:
I
V
R
BAT
IDC
IDC
= •1000
BAT (Pin 15): Charger Output. This pin provides charge
current to the battery and regulates the fi nal fl oat voltage
to 4.2V.
