Data Sheet

LT3652
20
3652fe
For more information www.linear.com/LT3652
APPLICATIONS INFORMATION
full-charge float voltage. If a charge cycle is not successful
at EOC, 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 reengages and starts
charging.
Preconditioning and Bad Battery Fault
A LT3652 has a precondition mode, where charge current
is limited to 15% of the programmed I
CHG(MAX)
, as set by
R
SENSE
. The precondition current corresponds to 15mV
across R
SENSE
.
Precondition mode is engaged while the voltage on the
V
FB
pin is below the precondition threshold (2.3V, or
0.7 • V
BAT(FLT)
). Once the V
FB
voltage rises above the
precondition threshold, normal full-current charging can
commence. The LT3652 incorporates 70mV of threshold
hysteresis to prevent mode glitching.
When the internal timer is used for termination, bad battery
detection is engaged. There is no provision for bad battery
detection if C/10 termination is used. A bad battery fault
is triggered when the voltage on V
FB
remains below the
precondition threshold for greater than 1/8 of a full timer
cycle (1/8 EOC). A bad battery fault is also triggered if a
normally charging battery re-enters precondition mode
after 1/8 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.
Cycling the charger’s power or SHDN function initiates a
new charging cycle, but a LT3652 charger does not re
-
quire a reset. Once a bad battery fault is detected, a new
timer charging cycle initiates when the V
FB
pin exceeds
the precondition threshold voltage. During a bad battery
fault, 0.5mA is sourced from the charger, so removing
the failed battery allows the charger output voltage to rise
and initiate a charge cycle reset. As such, removing a bad
battery resets the LT3652, so a new charge cycle is started
by connecting another battery to the charger output.
Battery Temperature Monitor and Fault
The LT3652 can accommodate battery temperature moni
-
toring by using an NTC (negative temperature co-efficient)
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 LT3652
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 correspond to 5°C.
If higher operational charging temperatures are desired,
the temperature range can be expanded by adding series
resistance to the 10k NTC resistor. Adding a 0.91k resistor
will increase the effective hot temperature 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 condi
-
tion is relieved.
Thermal Foldback
The LT3652 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 excessive tem
-
perature conditions are relieved with only slight reductions
in maximum charger current.