LTC4057-4.2
6
4057f
The LTC4057 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 PC board layout) with a final float voltage accuracy
of ±1%. The LTC4057 includes an internal P-channel
power MOSFET and thermal regulation circuitry. No block-
ing diode or external current sense resistor is required and
the LTC4057 is capable of operating from a USB power
source.
Normal Charge
Charging begins when SHDN is high, the voltage at the V
CC
pin rises above the UVLO threshold level and a program
resistor is connected from the PROG pin to ground. If the
BAT pin voltage is below 2.9V, the charger enters trickle-
charge mode. In this mode, the LTC4057 supplies ap-
proximately 1/10 the programmed charge current 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 LTC4057
enters constant-voltage mode, and the charge current
begins to decrease.
Programming Charge Current
The charge current is programmed using a single resistor
from the PROG pin to ground. The 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 and using
the following equation:
I
V
R
BAT
PROG
PROG
= •1000
Thermal Limiting
An internal thermal feedback loop reduces the programmed
charge current if the die temperature attempts to rise
above a preset value of approximately 120°C. This feature
protects the LTC4057 from excessive temperature and
allows the user to push the limits of the power handling
capability of a given circuit board without risk of damaging
the LTC4057. The charge current can be set according to
typical (not worst-case) ambient temperature with the
assurance that the charger will automatically reduce the
current in worst-case conditions. ThinSOT power consid-
erations are discussed further in the Applications Informa-
tion section.
Undervoltage Lockout (UVLO)
An internal undervoltage lockout circuit monitors the input
voltage and keeps the charger in shutdown mode until V
CC
rises above the undervoltage lockout threshold. The UVLO
circuit has a built-in hysteresis of 200mV. Furthermore, to
protect against reverse current in the power MOSFET, the
UVLO circuit keeps the charger in shutdown mode if V
CC
falls to within 30mV of the battery voltage. If the UVLO
comparator is tripped, the charger will not come out of
shutdown mode until V
CC
rises 100mV above the battery
voltage.
Shutdown Mode
The LTC4057 can also be put into shutdown mode at any
time by applying logic “low” to the SHDN pin (V
SHDN
<
0.4V). This reduces the battery drain current to less than
2µA and the input supply current to less than 50µA.
Charging will resume when applying a logic “high” to the
SHDN pin (V
SHDN
> 1V).
OPERATIO
U
