Datasheet

LTC4081
12
4081f
If the die temperature starts to creep up above 115°C
due to internal power dissipation, the transconductance
amplifi er, TA, limits the die temperature to approximately
115°C by reducing the charge current. Diode D3 ensures
that TA does not affect the charge current when the die
temperature is below 115°C. In thermal regulation, the
PROG pin voltage continues to give an indication of the
charge current.
In typical operation, the charge cycle begins in constant-
current mode with the current delivered to the battery equal
to 400V/R
PROG
. If the power dissipation of the LTC4081
results in the junction temperature approaching 115°C, the
amplifi er (TA) will begin decreasing the charge current to
limit the die temperature to approximately 115°C. As the
battery voltage rises, the LTC4081 either returns to full
constant-current mode or enters constant-voltage mode
straight from constant-temperature mode.
Battery Charger Undervoltage Lockout (UVLO)
An internal undervoltage lockout circuit monitors the V
CC
input voltage and keeps the battery charger off
until V
CC
rises above 3.6V and approximately 82mV above the BAT
pin voltage. The 3.6V UVLO circuit has a built-in hysteresis
of approximately 0.6V, and the 82mV automatic shutdown
threshold has a built-in hysteresis of approximately 50mV.
During undervoltage lockout conditions, maximum battery
drain current is 5
μ A and maximum supply current is 10μA.
Undervoltage Charge Current Limiting (UVCL)
The battery charger in the LTC4081 includes undervoltage
charge current limiting that prevents full charge current
until the input supply voltage reaches approximately 300mV
above the battery voltage (ΔV
UVCL1
). This feature is particu-
larly useful if the LTC4081 is powered from a supply with
long leads (or any relatively high output impedance). See
Applications Information section for further details.
Trickle Charge and Defective Battery Detection
At the beginning of a charge cycle, if the battery volt-
age is below 2.9V, the battery charger goes into trickle
charge mode, reducing the charge current to 10% of the
programmed current. If the low battery voltage persists
for one quarter of the total time (1.125 hr), the battery is
assumed to be defective, the charge cycle terminates and
the
⎯
C
⎯
H
⎯
R
⎯
G pin output pulses at a frequency of 2Hz with
a 75% duty cycle. If, for any reason, the battery voltage
rises above 2.9V, the charge cycle will be restarted. To
restart the charge cycle (i.e., when the dead battery is
replaced with a discharged battery less than 2.9V), the
charger must be reset by removing the input voltage and
reapplying it or temporarily pulling the
⎯
E
⎯
N
⎯
_
⎯
C
⎯
H
⎯
R
⎯
G pin above
the shutdown threshold.
Battery Charger Shutdown Mode
The LTC4081’s battery charger can be disabled by pulling
the
⎯
E
⎯
N
⎯
_
⎯
C
⎯
H
⎯
R
⎯
G pin above the shutdown threshold (V
IH
).
In shutdown mode, the battery drain current is reduced
to about 2μA and the V
CC
supply current to about 5μA
provided the regulator is off. When the input voltage is
not present, the battery charger is in shutdown and the
battery drain current is less than 5μA.
⎯
C
⎯
H
⎯
R
⎯
G Status Output Pin
The charge status indicator pin has three states: pulldown,
pulsing at 2Hz (see Trickle Charge and Defective Battery
Detection and Battery Temperature Monitoring) and high
impedance. The pulldown state indicates that the battery
charger is in a charge cycle. A high impedance state indi-
cates that the charge current has dropped below 10% of
the full-scale current or the battery charger is disabled.
When the timer runs out (4.5 hrs), the
⎯
C
⎯
H
⎯
R
⎯
G pin is also
forced to the high impedance state. If the battery charger
is not in constant-voltage mode when the charge current
is forced to drop below 10% of the full-scale current by
UVCL,
⎯
C
⎯
H
⎯
R
⎯
G will stay in the strong pulldown state.
Charge Current Soft-Start
The LTC4081’s battery charger includes a soft-start circuit
to minimize the inrush current at the start of a charge
cycle. When a charge cycle is initiated, the charge cur-
rent ramps from zero to full-scale current over a period
of approximately 180μs. This has the effect of minimizing
the transient current load on the power supply during
start-up.
OPERATIO
U