11
LTC4065/LTC4065A
4065fb
Once the charge cycle terminates, the LTC4065 continu-
ously monitors the BAT pin voltage using a comparator
with a 2ms filter time. When the average battery voltage
falls below 4.1V (which corresponds to 80% to 90%
battery capacity), a new charge cycle is initiated and a 2.25
hour timer begins. This ensures that the battery is kept at,
or near, a fully charged condition and eliminates the need
for periodic charge cycle initiations. The CHRG output
assumes a strong pull-down state during recharge cycles
until C/10 is reached when it transitions to a high
impendance state.
Trickle Charge and Defective Battery Detection
At the beginning of a charge cycle, if the battery voltage is
low (below 2.9V), the charger goes into trickle charge,
reducing the charge current to 10% of the full-scale
current. If the low-battery voltage persists for one quarter
of the total time (1.125 hour), the battery is assumed to be
defective, the charge cycle is terminated and the CHRG 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 defective battery is replaced with a dis-
charged battery), simply remove the input voltage and
reapply it, temporarily pull the EN pin above the shutdown
threshold (LTC4065), or momentarily float the PROG pin
and reconnect it (LTC4065A).
CHRG Status Output Pin
The charge status indicator pin has three states: pull-
down, pulse at 2Hz (see Trickle Charge and Defective
Battery Detection) and high impedance. The pull-down
state indicates that the LTC4065 is in a charge cycle. A high
impedance state indicates that the charge current has
dropped below 10% of the full-scale current or the LTC4065
is disabled. Figure 2 shows the CHRG status under various
conditions.
Power Supply Status Indicator
(ACPR, LTC4065A Only)
The power supply status output has two states: pull-down
and high impedance. The pull-down state indicates that
V
CC
is above the undervoltage lockout threshold (see
Undervoltage Lockout). When this condition is not met,
the ACPR pin is high impedance indicating that the
LTC4065A is unable to charge the battery.
Charge Current Soft-Start and Soft-Stop
The LTC4065 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 current ramps from zero to the
full-scale current over a period of approximately 180µs.
Likewise, internal circuitry slowly ramps the charge cur-
rent from full-scale to zero when the charger is shut off or
self terminates. This has the effect of minimizing the
transient current load on the power supply during start-up
and charge termination.
Constant-Current/Constant-Voltage/
Constant-Temperature
The LTC4065/LTC4065A use a unique architecture to
charge a battery in a constant-current, constant-voltage
and constant-temperature fashion. Figures 1a and 1b
show simplified block diagrams of the LTC4065 and
LTC4065A, respectively. Three of the amplifier feedback
loops shown control the constant-current, CA, constant-
voltage, VA, and constant-temperature, TA modes. A
fourth amplifier feedback loop, MA, is used to increase the
output impedance of the current source pair; M1 and M2
(note that M1 is the internal P-channel power MOSFET). It
ensures that the drain current of M1 is exactly 1000 times
greater than the drain current of M2.
Amplifiers CA and VA are used in separate feedback loops
to force the charger into constant-current or constant-
voltage mode, respectively. Diodes D1 and D2 provide
priority to either the constant-current or constant-voltage
loop; whichever is trying to reduce the charge current the
most. The output of the other amplifier saturates low
which effectively removes its loop from the system. When
in constant-current mode, CA servos the voltage at the
PROG pin to be precisely 1V. VA servos its inverting input
to an internal reference voltage when in constant-voltage
mode and the internal resistor divider, made up of R1 and
R2, ensures that the battery voltage is maintained at 4.2V.
The PROG pin voltage gives an indication of the charge
current during constant-voltage mode as discussed in
“Programming Charge Current”.
OPERATIO
U
