LT3651-8.2/LT3651-8.4
17
36518284fa
For more information www.linear.com/LT3651-8.2
APPLICATIONS INFORMATION
Termination at the end of the timer cycle only occurs if the
charge cycle was successful. A successful charge cycle
occurs when the battery is charged to within 2.5% of the
full-charge float voltage. If a charge cycle is not success
-
ful at end-of-cycle, the timer cycle resets and charging
continues for another full-timer cycle.
When
V
BAT
drops below 97.5% of the full-charge float
voltage, whether by battery loading or replacement of the
battery, the charger automatically re-engages and starts
charging.
Precondition and Bad Battery Fault
A LT3651-8.2/LT3651-8.4 charger has a precondition
mode, in which charge current is limited to 15% of the
programmed I
MAX
, as set by R
SENSE
. The precondition
current corresponds to 14mV across R
SENSE
.
Precondition mode is engaged while the voltage on the
BAT pin is below the precondition threshold (V
BAT(PRE)
).
Once the BAT voltage rises above the precondition thresh-
old, normal
full-current charging can commence. The
LT3651-8.2/LT3651-8.4
incorporate 2.5% of threshold
for hysteresis to prevent mode glitching.
When the internal timer is used for termination, bad bat
-
tery detection
is engaged. This fault detection feature
is designed to identify failed cells. A bad battery fault is
triggered
when the voltage on BAT remains below the
precondition threshold for greater than one-eighth of a full
timer cycle (one-eighth end-of-cycle). A bad battery fault
is also triggered if a normally charging battery re-enters
precondition mode after one-eighth end-of-cycle.
When a bad battery fault is triggered, the charge cycle
is suspended, so the CHRG status pin becomes high
impedance. The FAULT pin is pulled low to signal a fault
detection. The RNG/SS pin is also pulled low during this
fault, to accommodate a graceful restart, in the event that
a soft-start function is incorporated (see the RNG/SS:
Soft-Start section).
Cycling the charger’s power or SHDN function initiates a
new charge cycle, but a LT3651-8.2/LT3651-8.4 charger
does not require a reset. Once a bad battery fault is de
-
tected, a
new timer charge cycle initiates when the B
AT pin
exceeds the precondition threshold voltage. During a bad
battery fault, 1mA is sourced from the charger. Removing
the failed battery allows the charger output voltage to rise
and initiate a charge cycle reset. In that way removing a
bad battery resets the LT3651
-8.2/LT3651-8.4. A
new
charge cycle is started by connecting another battery to
the charger output.
Battery Temperature Fault: NTC
The LT3651-8.2/LT3651-8.4 can accommodate battery tem
-
perature monitoring
by
using an NTC (negative temperature
coefficient) thermistor close to the battery pack. The temper-
ature monitoring
function
is enabled by connecting a 10kΩ,
B = 3380 NTC thermistor
from the NTC pin to ground. If
the NTC function is not desired, leave the pin unconnected.
The NTC pin sources 50µA and monitors the voltage
dropped across the 10kΩ thermistor. When the voltage on
this pin is above 1.36V (0°C) or below 0.29V (40°C), the
battery temperature is out of range, and the LT3651-8.2/
LT3651-8.4 trigger an NTC fault. The NTC fault condition
remains until the voltage on the NTC pin corresponds to
a temperature within the 0°C to 40°C range. Both hot and
cold thresholds incorporate hysteresis that corresponds
to 2.5°C.
During an NTC fault, charging is halted and both status
pins are pulled low. If timer termination is enabled, the
timer count is suspended and held until the fault condition
is relieved. The RNG/SS pin is also pulled low during this
fault, to accommodate a graceful restart in
the event that
a soft-start function is being incorporated (see the RNG/
SS: Soft-Start section).
