ltc4069edc-trpbf Linear Technology Corporation, ltc4069edc-trpbf Datasheet - Page 9

• Manufacturer Part Number: ltc4069edc-trpbf
• Description: Standalone 750ma Li-ion Battery Charger In 2 X 2 Dfn With Ntc Thermistor Input
• Manufacturer: Linear Technology Corporation

OPERATIO
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 dis-
charged battery), simply remove the input voltage and
reapply it or momentarily float 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 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 LTC4069 is disabled. Figure
2 shows the CHRG status under various conditions.
Charge Current Soft-Start and Soft-Stop
The LTC4069 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 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.
U
Constant-Current/Constant-Voltage/
Constant-Temperature
The LTC4069 uses a unique architecture to charge a
battery in a constant-current, constant-voltage and con-
stant-temperature fashion. Figure 1 shows a Simplified
Block Diagram of the LTC4069. 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”.
Transconductance amplifier, TA, limits the die tempera-
ture to approximately 115°C when in constant-tempera-
ture mode. Diode D3 ensures that TA does not affect the
charge current when the die temperature is below approxi-
mately 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
LTC4069 results in the junction temperature approaching
115°C, the amplifier (TA) will begin decreasing the charge
current to limit the die temperature to approximately
115°C. As the battery voltage rises, the LTC4069 either
returns to constant-current mode or enters constant-
voltage mode straight from constant-temperature mode.
PROG
. If the power dissipation of the
LTC4069
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