LTC4056-4.2_15 Datasheet, PDF (7/16 Pages) Linear Technology – Linear Li-Ion Charger with Termination in ThinSOT

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OPERATIO
The LTC4056 is a linear battery charger controller with a
programmable charge termination timer. Operation can
be understood by referring to the Block Diagram. A charge
cycle begins when V_CC rises above the UVLO (undervoltage
lockout) threshold V_UVLOI (nominally 4.4V), an external
current programming resistor is connected between the
PROG pin and ground and the TIMER/SHDN pin is allowed
to rise above the shutdown threshold V_MSDT (nominally
0.82V).
If the battery voltage is below V_TRIKL (2.8V) at the begin-
ning of the charge cycle, the charger goes into trickle
charge mode to bring the cell voltage up to a safe level for
charging at full current. In this mode, an internal current
source provides approximately 2% of the programmed
charge current to the BAT pin. The charger goes into the
full charge constant current mode once the voltage on the
BAT pin rises above V_TRIKL + ∆V_TRIKL (2.9V).
During full current charging, the collector of the external
PNP provides the charge current. The PNP emitter current
flows through the I_SENSE pin and through the internal
110mΩ current sense resistor. This current is close in
magnitude, but slightly more than the collector current
since it includes base current. Amplifier A1 forces 1V on
the PROG pin. Therefore, a current equal to 1V/R_PROG will
flow through the internal 100Ω resistor. Amplifier CA will
force the same voltage that appears across the 100Ω
resistor to appear across the internal 110mΩ resistor.
This amplifier ensures that the current flowing out of the
I_SENSE pin is equal to 915 times the current flowing out of
the PROG pin. Therefore, neglecting base current, the
charge current will be 915V/R_PROG. This region of opera-
tion is referred to as constant current mode.
LTC4056-4.2
As the battery accepts charge, its voltage rises. When it
reaches the preset float voltage of 4.2V, a precisely divided
down version of this voltage (1.2V) is compared to the
1.2V internal reference voltage by amplifier VA. If the
battery voltage attempts to exceed 4.2V (1.2V at the input
of amplifier VA), the amplifier will divert current away from
the output driver thus limiting charge current to maintain
4.2V on the battery. This is the constant voltage mode.
An external capacitor on the TIMER/SHDN pin and the
resistance between the PROG pin and ground set the total
charge time. When this time elapses, the charge cycle
terminates and the CHRG pin transitions from a strong
pull-down to a weak 12µA pull-down. To restart the charge
cycle, simply remove the input voltage and reapply it or
momentarily force the TIMER/SHDN pin to ground. The
charge cycle will also restart if the BAT pin voltage falls
below the recharge threshold (V_RECHRG is nominally 4.05V).
When V_CC is applied, pulling the TIMER/SHDN pin to
ground will manually shut down the charger and reset the
timer. When this pin is released an internal 7µA current
source pulls the TIMER/SHDN pin above the 0.82V shut-
down threshold to resume charging.
Fault conditions such as overheating of the die or exces-
sive DRIVE pin or PROG pin current are monitored and
limited.
When input power is removed or manual shutdown is
entered, the charger will drain only tiny leakage currents
(<1µA) from the battery, thus maximizing battery standby
time. With V_CC removed the external PNP base is con-
nected to the battery by the charger. In manual shutdown
the base is connected to V_CC by the charger.
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