Datasheet

LTC4081
9
4081f
PI FU CTIO S
UUU
BAT (Pin 1): Charge Current Output and Buck Regulator
Input. Provides charge current to the battery and regulates
the fi nal fl oat voltage to 4.2V. An internal precision resistor
divider from this pin sets the fl oat voltage and is disconnected
in charger shutdown mode. This pin must be decoupled
with a low ESR capacitor for low-noise buck operation.
V
CC
(Pin 2): Positive Input Supply Voltage. This pin provides
power to the battery charger. V
CC
can range from 3.75V
to 5.5V. This pin should be bypassed with at least a 1μF
capacitor. When V
CC
is less than 32mV above the BAT pin
voltage, the battery charger enters shutdown mode.
⎯
E
⎯
N
⎯
_
⎯
C
⎯
H
⎯
R
⎯
G (Pin 3): Enable Input Pin for the Battery Charger.
Pulling this pin above the manual shutdown threshold
(V
IH
) puts the LTC4081 charger in shutdown mode, thus
stopping the charge cycle. In battery charger shutdown
mode, the LTC4081 has less than 10μA supply current and
less than 5μA battery drain current provided the regula-
tor is not running. Enable is the default state, but the pin
should be tied to GND if unused.
PROG (Pin 4): Charge Current Program and Charge Cur-
rent Monitor Pin. Connecting a 1% resistor, R
PROG
, to
ground programs the charge current. When charging in
constant-current mode, this pin servos to 1V. In all modes,
the voltage on this pin can be used to measure the charge
current using the following formula:
I
V
R
BAT
PROG
PROG
= •400
NTC (Pin 5): Input to the NTC (negative temperature coef-
fi cient) Thermistor Temperature Monitoring Circuit. For
normal operation, connect a thermistor from the NTC pin
to ground and a resistor of equal value from the NTC pin
to V
CC
. When the voltage at this pin drops below 0.35 •
V
CC
at hot temperatures or rises above 0.76 • V
CC
at cold,
charging is suspended, the internal timer is frozen and the
⎯
C
⎯
H
⎯
R
⎯
G pin output will start to pulse at 2Hz. Pulling this
pin below 0.016 • V
CC
disables the NTC feature. There is
approximately 3°C of temperature hysteresis associated
with each of the input comparator’s thresholds.
⎯
C
⎯
H
⎯
R
⎯
G (Pin 6): Open-Drain Charge Status Output. The
charge status indicator pin has three states: pulldown,
high impedance state, and pulsing at 2Hz. This output can
be used as a logic interface or as an LED driver. When the
battery is being charged, the
⎯
C
⎯
H
⎯
R
⎯
G pin is pulled low by
an internal N-channel MOSFET. When the charge current
drops to 10% of the full-scale current, the
⎯
C
⎯
H
⎯
R
⎯
G pin is
forced to a high impedance state. When the battery volt-
age remains below 2.9V for one quarter of the full charge
time, the battery is considered defective, and the
⎯
C
⎯
H
⎯
R
⎯
G
pin pulses at a frequency of 2Hz with 75% duty cycle.
When the NTC pin voltage rises above 0.76 • V
CC
or drops
below 0.35 • V
CC
, the
⎯
C
⎯
H
⎯
R
⎯
G pin pulses at a frequency of
2Hz (25% duty cycle).
FB (Pin 7): Feedback Pin for the Buck Regulator. A resistor
divider from the regulator’s output to the FB pin programs
the output voltage. Servo value for this pin is 0.8V.
MODE (Pin 8): Burst Mode Enable Pin. Tie this pin high
to force the LTC4081 regulator into Burst Mode operation
for all load conditions. Tie this pin low to force constant-
frequency mode operation for all load conditions. Do not
fl oat this pin.
EN_BUCK (Pin 9): Enable Input Pin for the Buck Regulator.
Pull this pin high to enable the regulator, pull low to shut
down. Do not fl oat this pin.
SW (Pin 10): Switch Pin for the Buck Regulator. Minimize
the length of the metal trace connected to this pin. Place
the inductor as close to this pin as possible.
GND (Pin 11): Ground. This pin is the back of the Exposed
Pad package and must be soldered to the PCB for electrical
connection and rated thermal performance.