LT3650-4.1/LT3650-4.2
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Preconditioning and Bad-Battery Fault
An LT3650 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 15mV 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 threshold,
normal full-current charging can commence. The LT3650
incorporates 3% of threshold hysteresis to prevent mode
glitching.
When the internal timer is used for termination, bad-battery
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 EOC). A bad-battery fault is also triggered if
a normally charging battery re-enters precondition mode
after one-eighth EOC.
When a bad-battery fault is triggered, the charging 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 charging cycle, but an LT3650 charger does not
require a reset. Once a bad-battery fault is detected, a new
timer charging cycle initiates when the BAT pin exceeds
the precondition threshold voltage. During a bad-battery
fault, 0.5mA is sourced from the charger; removing the
failed battery allows the charger output voltage to rise and
initiate a charge cycle reset. As such, removing a bad bat-
tery resets the LT3650, so a new charging cycle is started
by connecting another battery to the charger output.
Battery Temperature Fault: NTC
The LT3650 can accommodate battery temperature moni-
toring by using an NTC (negative temperature coefficient)
thermistor close to the battery pack. The temperature
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 LT3650
triggers
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 5°C.
If higher operational charging temperatures are desired,
the temperature range can be expanded by adding se-
ries resistance to the 10k NTC resistor. Adding a 0.91k
resistor will increase the effective temperature threshold
to 45°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).
Thermal Foldback
The LT3650 contains a thermal foldback protection feature
that reduces maximum charger output current if the IC
junction temperature approaches 125°C. In most cases,
on-chip temperatures servo such that any overtempera-
ture conditions are relieved with only slight reductions in
maximum charger current.
In some cases, the thermal foldback
protection feature
can reduce charger currents below the C/10 threshold. In
applications that use C/10 termination (TIMER = 0V), the
LT3650 will suspend charging and enter standby mode
until the overtemperature condition is relieved.
applicaTions inForMaTion
