8
LTC4054-4.2/LTC4054X-4.2
405442xf
OPERATIO
U
The LTC4054 is a single cell lithium-ion battery charger
using a constant-current/constant-voltage algorithm. It
can deliver up to 800mA of charge current (using a good
thermal PCB layout) with a final float voltage accuracy of
±1%. The LTC4054 includes an internal P-channel power
MOSFET and thermal regulation circuitry. No blocking
diode or external current sense resistor is required; thus,
the basic charger circuit requires only two external com-
ponents. Furthermore, the LTC4054 is capable of operat-
ing from a USB power source.
Normal Charge Cycle
A charge cycle begins when the voltage at the V
CC
pin rises
above the UVLO threshold level and a 1% program resistor
is connected from the PROG pin to ground or when a
battery is connected to the charger output. If the BAT pin
is less than 2.9V, the charger enters trickle charge mode.
In this mode, the LTC4054 supplies approximately 1/10
the programmed charge current to bring the battery volt-
age up to a safe level for full current charging. (Note: The
LTC4054X does not include this trickle charge feature).
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 LTC4054
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 1000 times the current out of the PROG pin. The
program resistor and the charge current are calculated
using the following equations:
R
V
I
I
V
R
PROG
CHG
CHG
PROG
==
1000 1000
,
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
V
R
BAT
PROG
PROG
= •1000
Charge Termination
A 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 LTC4054 enters standby
mode, where the input supply current drops to 200µA.
(Note: C/10 termination is disabled in trickle charging and
thermal limiting modes).
When 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 termi-
nation. Once the
average
charge current drops below
1/10th the programmed value, the LTC4054 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 LTC4054 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.
Charge Status Indicator (CHRG)
The charge status output has three different states: strong
pull-down (~10mA), weak pull-down (~20µA) and high
impedance. The strong pull-down state indicates that the
LTC4054 is in a charge cycle. Once the charge cycle has
terminated, the pin state is determined by undervoltage
Note 1:
Any external sources that hold the PROG pin above 100mV will prevent the LTC4054
from terminating a charge cycle.
