LTC4056-4.2_15 Datasheet, PDF (14/16 Pages) Linear Technology – Linear Li-Ion Charger with Termination in ThinSOT

LTC4056-4.2
TYPICAL APPLICATIO S
It is important to keep in mind that the LTC4056 can only
control charge current. If the system load is less than
500mA, then the LTC4056 can simply reduce the charge
current by an amount equal to the system load current and
the USB specification can be met. For instance, if the sys-
tem load is 150mA, then the charge current will be reduced
from 490mA to 340mA and the total USB input current will
remain at 490mA, thereby meeting the specification.
However, if the system load is increased beyond 500mA
the LTC4056 will reduce the charge current to zero, and all
of the system load will be provided by the USB input. This
scenario will violate the USB power specification. In order
to avoid this situation, it is important to ensure that the
system load never exceeds 500mA.
The LTC4412 provides automatic switchover of the sys-
tem load between a battery and the USB input supply. This
feature reduces the current drain on the battery to just a
few microamps when a USB input is present. Figure 5
shows a dual FET solution to minimize voltage drop
between the USB input voltage and the system load and
Figure 6 uses a Schottky diode to simplify the design.
Please refer to the LTC4412 data sheet for more informa-
tion on the operation of the Ideal Diode Controller.
In both designs all USB input current passes through the
sense resistor of the LTC4056 to ensure that the maximum
current drawn from the USB input supply is limited to less
than 500mA (assuming the system load is less than
500mA).
In Figure 6, P-channel MOSFET, M3, provides drive for the
green LED that illuminates when the USB input supply is
present. In both designs, the CHRG pin of the LTC4056
drives the red LED to indicate charging. Keep in mind that
the Li-Ion battery will charge at a reduced rate if a signifi-
cant system load is present. This is due to the fact that the
490mA charge current is split between the battery and the
system load.
Optional N-channel MOSFET, M1, can be used to shut
down the LTC4056 thereby reducing its input supply
current to about 40µA. This will automatically turn off the
red LED. However, since voltage will still be present on the
USB input (and therefore the I_SENSE pin), Figure 6 will
continue to draw power through the green LED. The green
14
LED should not be used without additional control logic if
a low current standby mode is required.
NiCd or NiMH Charging
The application circuit in Figure 7 shows how to use an
LTC4056 to charge Nickel chemistry batteries with user
termination. NiCd or NiMH batteries require constant
current charging regardless of the battery voltage. To
disable the voltage mode of the LTC4056 it is necessary to
connect the BAT pin to a voltage between the trickle charge
threshold and the final float voltage.
Assuming a reasonably well controlled input voltage, this
can be accomplished with a simple resistor divider con-
nected between the input supply and the BAT pin. In
Figure␣ 7, resistors R3 and R4 keep the BAT pin voltage
between the required voltage levels provided the input
voltage is between 4.5V and 6.1V (encompassing nearly
the entire specified operating input supply range of 4.5V to
6.5V). The LTC4056 has an internal impedance of approxi-
mately 2MΩ to GND on the BAT pin, so it is important to
keep the impedance of the resistor divider considerably
below that value. Furthermore, a 0.1µF bypass capacitor
may be required between the BAT pin and GND. If the input
voltage rises above 6.1V then it is possible that the battery
charge current will decrease due to the voltage mode
amplifier of the LTC4056.
The TIMER/SHDN manual shutdown threshold of the
LTC4056 is typically 0.82V, allowing the I/O port of a
microcontroller to drive this pin with standard logic levels
to manually control termination. Holding the TIMER/SHDN
pin high simultaneously enables the charger and disables
the internal timer function. A programmed constant cur-
rent will be provided to the battery until the I/O port pulls
the TIMER/SHDN pin to GND.
V_IN
4.5V TO 6.1V
R1
RED
750Ω
I/O
µCONTROL ^{TERM CHRG}
R4
68.1k
R3
31.6k ₈
LTC4056
1
CHRG
V_CC
7
2
TIMER/SHDN I_SENSE
6
3
BAT
DRIVE
5
PROG
R2
4
GND
1.3k
Q1
ZXT1M322
C3
NiCd
1µF
4056-4.2 F07
Figure 7. Nickel Chemistry Battery Charging
405642f