LTC4121/LTC4121-4.2
9
4121fc
For more information www.linear.com/LTC4121
PIN FUNCTIONS
BAT (Pin 9): Battery Output Pin. Battery charge current
is delivered from this pin through the internal charge
current sense resistor. In low battery conditions a small
linear charge current, I
LOWBAT
, is sourced from this pin
to precondition the battery. Decouple the BAT pin with a
low ESR 22µF ceramic capacitor to GND.
BATSNS (Pin 10, LTC4121-4.2 Only): Battery Voltage
Sense Pin. For proper operation, this pin must always be
connected physically close to the positive battery terminal.
FB (Pin 10, LTC4121 Only): Battery Voltage Feedback
Reference Pin. The charge function operates to achieve a
final float voltage of 2.4V at this pin. Battery float voltage
is programmed using a resistive divider from BAT to FB
to FBG, and can be programmed from 3.5V up to 18V.
The feedback pin input bias current, I
FB
, is 25nA. Using a
resistive divider with a Thevenin equivalent resistance of
588k compensates for input bias current error.
FBG (Pin 11, LTC4121 Only): Feedback Ground Pin. This
pin disconnects the external FB divider load from the battery
when it is not needed. When sensing the battery voltage
this pin presents a low resistance, R
FBG
, to GND. When in
disabled or shutdown modes this pin is high impedance.
NTC (Pin 12): Input to the Negative Temperature Coefficient
Thermistor Monitoring Circuit. The NTC pin connects to
a negative temperature coefficient thermistor which is
typically co-packaged with the battery to determine if the
battery is too hot or too cold to charge. If the battery’s
temperature is out of range, the LTC4121 enters STANDBY
mode and charging is paused until the battery tempera-
ture re-enters the valid range. A low drift bias resistor is
required from INTV
CC
to NTC and a thermistor is required
from NTC to GND. Tie the NTC pin to GND, and omit the
NTC resistive divider to disable NTC qualified charging if
NTC functionality is not required.
PROG (Pin 13): Charge Current Program and Charge
Current Monitor Pin. Connect a 1% resistor between
3.01k (400mA) and 24.3k (50mA) from PROG to ground
to program the charge current. While in constant-current
mode, this pin regulates to 1.227V. The voltage at this pin
represents the average charge current using the following
formula:
I
CHG
= h
PROG
•
PROG
R
PROG
where h
PROG
is typically 988. Keep parasitic capacitance
on the PROG pin to a minimum. If monitoring charge cur-
rent via the voltage at the PROG pin add a series resistor
of at least 2k to isolate stray capacitance from this node.
CHRG (Pin 14): Open-drain Charge Status Output Pin.
Typically pulled up through a resistor to a reference
voltage, the CHRG pin indicates the status of the battery
charger. The pin can be pulled up to voltages as high as
IN when disabled, and can sink currents up to 5mA when
enabled. When the battery is being charged, the CHRG
pin is pulled low. When the termination timer expires or
the charge current drops below 10% of the programmed
value, the CHRG pin is forced to a high impedance state.
FAULT (Pin 15): Open-drain Fault Status Output Pin. Typi-
cally pulled up through a resistor to a reference voltage,
this status pin indicates fault conditions during a charge
cycle. The pin can be pulled up to voltages as high as IN
when disabled, and can sink currents up to 5mA when
enabled. An NTC temperature fault causes this pin to be
pulled low. A bad battery fault also causes this pin to
be pulled low. If no fault conditions exist, the FAULT pin
remains high impedance.
RUN (Pin 16): Run Pin. When RUN is pulled below V
EN
and its hysteresis, the device is disabled. In disabled
mode, battery charge current is zero and the CHRG and
FAULT pins assume high impedance states. If the voltage
at RUN is pulled below V
SD
, the device is in SHUTDOWN
mode. When the voltage at the RUN pin rises above V
EN
,
the INTV
CC
LDO turns on. When the INTV
CC
LDO rises
above its UVLO threshold the charger is enabled. The
RUN pin should be tied to a resistive divider from V
IN
to
program the input voltage at which charging is enabled.
Do not float the RUN pin.
