Datasheet

8
LTC4050
4050f
APPLICATIONS INFORMATION
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Figure 1. Microprocessor Interface
Charger Conditions
The charger is off when any of the following conditions exist:
the voltage at the V
CC
pin is below 4V, the voltage at the V
CC
pin is greater than 4V but is less than 54mV above V
BAT
,
the PROG pin is floating, the timer has timed out or the
thermistor temperature is outside the acceptable range. In
this condition, the DRV pin is pulled to V
CC
and the internal
resistor divider is disconnected to reduce the current drain
on the battery.
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 of 4V. To
prevent oscillation around V
CC
= 4V, the UVLO circuit has
built-in hysteresis.
Trickle Charge and Defective Battery Detection
At the beginning of the charging sequence, if the battery
voltage is low (below 2.49V), the charger goes into trickle
mode. In this mode, the charge current is reduced to 10%
of the full-scale current. If the low cell voltage persists for
one quarter of the total charge time, the battery is consid-
ered defective, the charge cycle is terminated and the
CHRG pin output becomes high impedance.
Shutdown
The LTC4050 can be forced into shutdown by floating the
PROG pin and allowing the internal 2.3µA current source
to pull the pin above the 3.6V shutdown threshold voltage.
In shutdown, the DRV pin is pulled up to V
CC
, turning off
the external P-channel MOSFET and resetting the internal
timer.
Programming Charge Current
The formula for the battery charge current (see Block
Diagram) is:
I
BAT
= (I
PROG
)(800Ω/R
SENSE
)
= (2.47V/R
PROG
)(800Ω/R
SENSE
) or
R
PROG
= (2.47V/I
BAT
)(800Ω/R
SENSE
)
where R
PROG
is the total resistance from the PROG pin to
ground.
For example, if 0.5A charge current is needed, select a
value for R
SENSE
that will drop 100mV at the maximum
charge current. R
SENSE
= 0.1V/0.5A = 0.2Ω, then calculate:
R
PROG
= (2.47V/500mA)(800Ω/0.2Ω) = 19.76k
For best stability over temperature and time, 1% resistors
are recommended. The closest 1% resistor value is 19.6k.
Programming the Timer
The programmable timer terminates the charge cycle.
Typically when charging at a 1C rate, a discharged Li-Ion
battery will become fully charged in 3 hours. For lower
charge current rates, extend the time accordingly. The
length of the timer is programmed by an external capaci-
tor at the TIMER pin. The total charge time is:
Time (Hours) = (3 Hours) • (C
TIMER
/0.1µF) or
C
TIMER
= 0.1µF • Time (Hours)/3 Hours
The timer starts when an input voltage greater than 4V is
applied and the program resistor is connected to ground.
After a time-out occurs, the CHRG output will go high
impedance to indicate that charging has stopped. To dis-
able the timer function, short the TIMER pin to GND.
2k
400k
CHRG
4050 F01
V
CC
LTC4050
V
+
OUT
µPROCESSOR
IN
3
8
V
DD