LTC4059_15 Datasheet, PDF (8/12 Pages) Linear Technology – 900mA Linear Li-Ion Battery Chargers with Thermal Regulation in 2 2 DFN

LTC4059/LTC4059A
APPLICATIO S I FOR ATIO
Shutdown Mode
Charging can be terminated by pulling the EN pin above the
shutdown threshold (approximately 0.92V). In shutdown
mode, the battery drain current is reduced to less than 1µA
and the supply current to 10µA.
USB and Wall Adapter Power
Although the LTC4059/LTC4059A allow charging from a
USB port, a wall adapter can also be used to charge Li-Ion
batteries. Figure 3 shows an example of how to combine
wall adapter and USB power inputs. A P-channel MOSFET,
MP1, is used to prevent back conducting into the USB port
when a wall adapter is present and Schottky diode, D1, is
used to prevent USB power loss through the 1k pull-down
resistor.
Typically a wall adapter can supply significantly more
current than the 500mA limited USB port. Therefore, an
N-channel MOSFET, MN1, and an extra program resistor
are used to increase the charge current to 850mA when the
wall adapter is present.
5V WALL
ADAPTER
850mA I_CHG
USB
POWER
500mA I_CHG
MP1
I_CHG
3
BAT
D1
LTC4059
4
V_CC
5⁺
PROG
SYSTEM
LOAD
Li-Ion
BATTERY
1k
MN1 ^3.4k
2.43k
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Figure 3. Combining Wall Adapter and USB Power
Constant Current/Constant Voltage/
Constant Temperature
The LTC4059/LTC4059A use a unique architecture to
charge a battery in a constant-current, constant-voltage
and constant-temperature fashion. Figures 1 and 2 show
simplified block diagrams of the LTC4059 and LTC4059A
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 imped-
ance 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 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 1.21V. VA servos its inverting input to
precisely 1.21V 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 the Pro-
gramming Charge Current section.
Transconductance amplifier, TA, limits the die tempera-
ture to approximately 115°C when in constant-tempera-
ture mode. TA acts in conjunction with the constant-current
loop. When the die temperature exceeds approximately
115°C, TA sources current through R3. This causes CA to
reduce the charge current until the PROG pin voltage plus
the voltage across R3 equals 1.21V. Diode D3 ensures that
TA does not affect the charge current when the die tem-
perature 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 1210V/R_PROG. If the power dissipation of the
LTC4059/LTC4059A results in the junction temperature
approaching 115°C, the amplifier (TA) will begin decreas-
ing the charge current to limit the die temperature to
approximately 115°C. As the battery voltage rises, the
LTC4059/LTC4059A either return to constant-current mode
or enter constant-voltage mode straight from constant-
temperature mode. Regardless of mode, the voltage at the
PROG pin is proportional to the current delivered to the
battery.
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