LTC4054L-4.2
8
4054l42fa
The LTC4054L is a single cell lithium-ion battery charger
using a constant-current/constant-voltage algorithm. Its
ability to control charge currents as low as 10mA make it
well-suited for charging low capacity lithium-ion coin cell
batteries. The LTC4054L includes an internal P-channel
power MOSFET and thermal regulation circuitry. No block-
ing diode or external sense resistor is required; thus, the
basic charger circuit requires only three external compo-
nents. Furthermore, the LTC4054L is capable of operating
from a USB power source.
Normal Charge Cycle
The charge cycle begins when the voltage at the V
CC
pin
rises above the UVLO level and a 1% program resistor is
connected from the PROG pin to ground. If the BAT pin is
less than 2.9V, the charger enters trickle charge mode. In
this mode, the LTC4054L supplies approximately 1/10 the
programmed charge current in order to bring the battery
voltage up to a safe level for full current charging.
When the BAT pin voltage rises above 2.9V, the charger
enters constant-current mode, where the programmed
charge current is supplied to the battery. When the BAT
pin approaches the final float voltage (4.2V), the LTC4054L
enters constant-voltage mode and the charge current
begins to decrease. When the charge current drops to
1/10 of the programmed value, the charge cycle ends.
Programming Charge Current
The charge current is programmed using a single resistor
from the PROG pin to ground. The battery charge current
is 150 times the current out of the PROG pin. The program
resistor and the charge current are calculated using the
following equations:
R
PROG
=
I
CHG
, I
CHG
=
R
PROG
operaTion
The charge current out of the BAT pin can be determined
at any time by monitoring the PROG pin voltage using
the following equation:
I
BAT
=
PROG
R
PROG
• 150
Charge Termination
The charge cycle is terminated when the charge current
falls to 1/10th the programmed value after the final float
voltage is reached. This condition is detected by using an
internal, filtered comparator to monitor the PROG pin. When
the PROG pin voltage falls below 100mV
1
for longer than
t
TERM
(typically 1ms), charging is terminated. The charge
current is latched off and the LTC4054L enters standby
mode, where the input supply current drops to 200µA.
(Note: C/10 termination is disabled in trickle charging and
thermal limiting modes.)
While charging, transient loads on the BAT pin can cause
the PROG pin to fall below 100mV for short periods of
time before the DC charge current has dropped to 1/10th
the programmed value. The 1ms filter time (t
TERM
) on the
termination comparator ensures that transient loads of this
nature do not result in premature charge cycle termination.
Once the average charge current drops below 1/10th the
programmed value for longer than t
TERM
, the LTC4054L
terminates the charge cycle and ceases to provide any
current through the BAT pin. In this state, all loads on the
BAT pin must be supplied by the battery.
The LTC4054L constantly monitors the BAT pin voltage
in standby mode. If this voltage drops below the 4.05V
recharge threshold (V
RECHRG
), another charge cycle be-
gins and current is once again supplied to the battery. To
manually restart a charge cycle when in standby mode,
the input voltage must be removed and reapplied, or
the charger must be shut down and restarted using the
PROG pin. Figure 1 shows the state diagram of a typical
charge cycle.
1
Any external sources that hold the PROG pin above 100mV will prevent the LTC4054L from
terminating a charge cycle.
