Datasheet

8
LTC4068-4.2/LTC4068X-4.2
406842fa
OPERATIO
U
threshold (V
RECHRG
), another charge cycle begins and
charge current is once again supplied to the battery. To
manually restart a charge cycle when in standby mode, the
input voltage must be removed and reapplied or the charger
must be shut down and restarted using the EN pin. Figure 1
shows the state diagram of a typical charge cycle.
Thermal Limiting
An internal thermal feedback loop reduces the programmed
charge current if the die temperature attempts to rise above
a preset value of approximately 120°C. This feature protects
the LTC4068 from excessive temperature and allows the
user to push the limits of the power handling capability of
a given circuit board without risk of damaging the LTC4068.
The charge current can be set according to typical (not worst
case) ambient temperature with the assurance that the
charger will automatically reduce the current in worst-case
conditions. DFN power considerations are discussed fur-
ther in the Applications Information section.
Undervoltage Lockout (UVLO)
An internal undervoltage lockout circuit monitors the input
voltage and keeps the charger in shutdown mode until V
CC
rises above the undervoltage lockout threshold. The UVLO
circuit has a built-in hysteresis of 200mV. Furthermore, to
protect against reverse current in the power MOSFET, the
UVLO circuit keeps the charger in shutdown mode if V
CC
falls to within 30mV of the BAT voltage. If the UVLO com-
parator is tripped, the charger will not come out of shut-
down mode until V
CC
rises 100mV above the BAT voltage.
Manual Shutdown
At any point in the charge cycle, the LTC4068 can be put
into shutdown mode by driving the EN pin high. This
reduces the battery drain current to less than 2µA and the
supply current to less than 50µA. When in shutdown
mode, the CHRG pin is in the high impedance state. A new
charge cycle can be initiated by driving the EN pin low. An
internal resistor pull-down on this pin forces the LTC4068
to be enabled if the pin is allowed to float.
Automatic Recharge
Once the charge cycle is terminated, the LTC4068 continu-
ously monitors the voltage on the BAT pin using a com-
parator with a 2ms filter time (t
RECHARGE
). A charge cycle
restarts when the battery voltage falls below 4.10V (which
corresponds to approximately 80% to 90% battery capac-
ity). This ensures that the battery is kept at, or near, a fully
Figure 1. State Diagram of a Typical Charge Cycle
TRICKLE CHARGE
MODE
1/10TH FULL CURRENT
BAT > 2.9V
BAT < 2.9V
BAT > 2.9V
LTC4068
ONLY
CHRG: STRONG
PULL-DOWN
CHARGE MODE
FULL CURRENT
CHRG: STRONG
PULL-DOWN
SHUTDOWN MODE
CHRG: Hi-Z
EN DRIVEN LOW
OR
UVLO CONDITION
STOPS
EN DRIVEN HIGH
OR
UVLO CONDITION
I
CC
DROPS TO <25µA
POWER ON
ITERM < 100mV
STANDBY MODE
NO CHARGE CURRENT
CHRG: Hi-Z
2.9V < BAT < 4.1V
406842 F01
Charge Status Indicator (CHRG)
The charge status output has two states: pull-down and
high impedance. The pull-down state indicates that the
LTC4068 is in a charge cycle. Once the charge cycle has
terminated or the LTC4068 is disabled, the pin state
becomes high impedance.
Power Supply Status Indicator (ACPR)
The power supply status output has two states: pull-down
and high impedance. The pull-down state indicates that
V
CC
is above the UVLO threshold (3.8V) and is also 100mV
above the battery voltage. If these conditions are not met,
the ACPR pin is high impedance indicating that the LTC4068
is unable to charge the battery.