LTC4069-4.4 Linear Technology, LTC4069-4.4 Datasheet - Page 9

• Manufacturer Part Number: LTC4069-4.4
• Description: Standalone 750mA Li-Ion Battery Charger in 2 x 2 DFN
• Manufacturer: Linear Technology

OPERATION
battery voltage does not exceed the recharge threshold
voltage when the timer ends, the timer resets and a 2.25
hour recharge cycle begins. 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 discharged battery less than 2.9V), simply remove
the input voltage and reapply it or momentarily fl oat the
PROG pin and reconnect it.
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 Battery Temperature Monitoring)
and high impedance. The pull-down state indicates that the
LTC4069-4.4 is in a charge cycle. A high impedance state
indicates that the charge current has dropped below 10%
of the full-scale current, the timer has ended the charge
cycle, or the LTC4069-4.4 is disabled. Figure 2 shows the
CHRG status under various conditions.
Charge Current Soft-Start and Soft-Stop
The LTC4069-4.4 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
170μs. Likewise, internal circuitry slowly ramps the charge
current 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 LTC4069-4.4 uses a unique architecture to charge
a battery in a constant-current, constant-voltage and
constant-temperature fashion. Figure 1 shows a Simplifi ed
Block Diagram of the LTC4069-4.4. Three of the amplifi er
feedback loops shown control the constant-current (CA),
constant-voltage (VA), and constant-temperature (TA)
modes. A fourth amplifi er 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.
Amplifi ers 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 amplifi er 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 4.4V.
The PROG pin voltage gives an indication of the charge
current during constant-voltage mode as discussed in
“Programming Charge Current”.
Transconductance amplifi er, TA, limits the die temperature
to approximately 115°C when in constant-temperature
mode. Diode D3 ensures that TA does not affect the charge
current when the die temperature is below approximately
115°C. 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 1000V/R
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 LTC4069-4.4 either returns to
constant-current mode or enters constant-voltage mode
PROG
. If the power dissipation of the LTC4069-4.4
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