LTC4007_1 Datasheet, PDF (9/24 Pages) Linear Technology – 4A, High Efficiency, Standalone Li-Ion Battery Charger

TEST CIRCUIT
CHEM
16
3C4C
7
V_REF
DIVIDER/
MUX
CSP
BAT
14
15
+
EA
–
LTC4007
ITH
10
LT1055
0.6V
4007 TC
LTC4007
U
OPERATIO
Overview
The LTC4007 is a synchronous current mode PWM step-
down (buck) switcher battery charger controller. The
charge current is programmed by the combination of a
program resistor (R_PROG) from the PROG pin to ground
and a sense resistor (R_SENSE) between the CSP and BAT
pins. The final float voltage is programmed to one of four
values (12.3V, 12.6V, 16.4V, 16.8V) with ±1% maximum
accuracy using pins 3C4C and CHEM. Charging begins
when the potential at the DCIN pin rises above the voltage
at BAT (and the UVLO voltage) and the SHDN pin is low; the
CHG pin is set low. At the beginning of the charge cycle, if
the cell voltage is below 2.5V (2.44V if CHEM is low), the
LOBAT pin will be low. The LOBAT indicator can be used
to reduce the charging current to a low value, typically
10% of full scale. If the cell voltage stays below 2.5V for
25% of the total charge time, the charge sequence will be
terminated immediately and the FAULT pin will be set low.
An external thermistor network is sampled at regular
intervals. If the thermistor value exceeds design limits,
charging is suspended and the FAULT pin is set low. If the
thermistor value returns to an acceptable value, charging
resumes and the FAULT pin is set high. An external resistor
on the R_T pin sets the total charge time. The timer can be
defeated by forcing the CHG pin to a low voltage.
As the battery approaches the final float voltage, the
charge current will begin to decrease. When the current
drops to 10% of the full-scale charge current, an internal
C/10 comparator will indicate this condition by latching
the FLAG pin low. The charge timer is also reset to 1/4 of
the total charge time when FLAG goes low. If this condition
is caused by an input current limit condition, described
below, then the FLAG indicator will be inhibited. When a
time-out occurs, charging is terminated immediately and
the CHG pin is forced to a high impedance state. The
charger will automatically restart if the cell voltage is
below 3.9V (or 3.81V if CHEM is low). To restart the charge
cycle manually, simply remove the input voltage and
reapply it, or set the SHDN pin high momentarily. When
the input voltage is not present, the charger goes into a
sleep mode, dropping battery current drain to 15µA. This
greatly reduces the current drain on the battery and
increases the standby time. The charger is inhibited any
time the SHDN pin is high.
Input FET
The input FET circuit performs two functions. It enables
the charger if the input voltage is higher than the CLN pin
and provides the logic indicator of AC present on the ACP
pin. It controls the gate of the input FET to keep a low
forward voltage drop when charging and also prevents
reverse current flow through the input FET.
If the input voltage is less than V_CLN, it must go at least
170mV higher than V_CLN to activate the charger. When this
occurs the ACP pin is released and pulled up with an
external load to indicate that the adapter is present. The
gate of the input FET is driven to a voltage sufficient to keep
4007fa
9