LT3651-8.2/LT3651-8.4
7
36518284fa
For more information www.linear.com/LT3651-8.2
PIN FUNCTIONS
NTC (Pin 1): Battery Temperature Monitor Pin. This
pin is used to monitor battery temperature. Typically a
10kΩ NTC (negative temperature coefficient) thermistor
(B = 3380) is embedded with the battery and connected
from the NTC pin to ground. The pin sources 50µA into
the resistor and monitors the voltage across the therm
-
istor, regulating charging based on the voltage. If this
function is not desired, leave the NTC pin unconnected.
ACPR (Pin 2): Open-Collector AC Present Status Pin.
This pin sinks current to indicate that V
IN
is valid and the
charger is on. Typically a resistor pull-up is used on this
pin. This pin can be pulled up to voltages as high as V
IN
when disabled, and can sink currents up to 10mA when
enabled.
BAT (Pin 3): Battery Voltage Monitor Pin. This pin moni
-
tors battery voltage. A Kelvin connection is made to the
batter
y from this pin and a decoupling capacitor (C
BAT
)
is placed from this pin to ground.
The charge function operates to achieve the final float
voltage at this pin. The auto-restart feature initiates a new
charging cycle when the voltage at the BAT pin falls 2.5%
below this float voltage.
Once the charge cycle is termi-
nated,
the input bias current of the B
AT pin is reduced to
<0.1µA to minimize battery discharge while the charger
remains connected.
SENSE (Pin 4): Charge Current Sense Pin. The charge
current is monitored with a sense resistor (R
SENSE
) con-
nected between
this pin and the BAT pin. The inductor
current flows through R
SENSE
to the battery. The voltage
across this resistor sets the average charge current. The
maximum average charge current (I
MAX
) corresponds to
95mV across the sense resistor.
BOOST (Pin 5): Bootstrapped Supply Rail for Switch
Drive. This pin facilitates saturation of the high side switch
transistor. Connect a 1µF or greater capacitor from the
BOOST pin to the SW pin. The operating range of this pin
is 0V to 8.5V, referenced to the SW pin when the switch is
high. The voltage on the decoupling capacitor is refreshed
through a rectifying diode, with the anode connected to
either the battery output voltage or an external source,
and the cathode connected to the BOOST pin.
GND (Pins 6, 23, 31, 37): Ground. These pins are the
ground pins for the part. Pins 31 and 37 must be connected
together. Pins 6 and 23 are connected via the leadframe
to
the
exposed backside Pin 37. Solder the exposed backside
to the PCB for good thermal and electrical connection.
SW (Pins 7, 11-18, 22, 38): Switch Output Pin. These
pins are the output of the charger switches. An inductor is
connected between these pins and the SENSE pin. When
the switcher is active, the inductor is charged by the high
side switch from V
IN
and discharged by the bottom side
switch to GND. Solder the exposed backside, Pin 38, to
the PCB for good thermal connection.
NC (Pins 8-10,19-21): No Connect. These pins can be left
floating (not connected).
TIMER (Pin 24): End-Of-Cycle Timer Programming Pin.
A capacitor on this pin to ground determines the full
charge end-of-cycle time. Full charge end-of-cycle time is
programmed with this capacitor. A 3 hour charge cycle is
obtained with a 0.68µF capacitor. This timer also controls
the bad battery fault that is generated if the battery does not
reach the precondition threshold voltage within one-eighth
of a full cycle (22.5 minutes for a 3 hour charge cycle).
The timer based termination is disabled by connecting the
TIMER pin to ground. With the timer function disabled,
charging terminates when the charge current drops below a
C/10
rate, or approximately 10% of maximum charge rate.
F
AULT (Pin 25): Open-Collector Fault Status Output. This
pin indicates charge cycle fault conditions during a battery
charging cycle. Typically a resistor pull-up is used on this
pin. This status pin can be pulled up to voltages as high
as V
IN
when disabled, and can sink currents up to 10mA
when enabled. A temperature fault causes this pin to be
pulled low. If the internal timer is used for termination,
a bad battery fault also causes this pin to be pulled low.
If no fault conditions exist, the FAULT pin remains high
impedance.
CHRG (Pin 26): Open-Collector Charger Status Output.
This pin indicates the battery charging status. Typically
a resistor pull-up is used on this pin. This status pin can
be pulled up to voltages as high as V
IN
when disabled,
