LTC4097
12
4097f
Programming Charge Termination
The charge cycle terminates when the charge current
falls below the programmed termination threshold during
constant-voltage mode. This threshold is set by connect-
ing an external resistor, R
ITERM
, from the ITERM pin to
ground.
The charge termination current threshold (I
TERMINATE
) is
set by the following equation:
R
V
I
I
V
R
ITERM
TERMINATE
TERMINATE
ITERM
==
100 100
,
The termination condition is detected by using an internal
fi ltered comparator to monitor the ITERM pin. When the
ITERM pin voltage drops below 100mV* for longer than
t
TERMINATE
(typically 3ms), the charge cycle terminates,
charge current latches off and the LTC4097 enters standby
mode. When charging, transient loads on the BAT pin can
cause the ITERM pin to fall below 100mV for short periods
of time before the DC charge current has dropped below
the programmed termination current. The 3ms fi lter time
(t
TERMINATE
) 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 the programmed termination threshold, the
LTC4097 terminates the charge cycle and ceases to provide
any current out of the BAT pin. In this state, any load on
the BAT pin must be supplied by the battery.
Low-Battery Charge Conditioning (Trickle Charge)
This feature ensures that deeply discharged batteries are
gradually charged before applying full charge current. If
the BAT pin voltage is below 2.9V, the LTC4097 supplies
1/10th of the full charge current to the battery until the
BAT pin rises above 2.9V. For example, if the charger is
OPERATION
*Any external sources that hold the ITERM pin above 100mV will prevent the LTC4097 from
terminating a charge cycle.
programmed to charge at 800mA from the wall adapter
input and 500mA from the USB input, the charge current
during trickle charge mode would be 80mA and 50mA,
respectively.
Automatic Recharge
In standby mode, the charger sits idle and monitors the
battery voltage using a comparator with a 1.6ms fi lter time
(t
RECHRG
). A charge cycle automatically restarts when the
battery voltage falls below 4.1V (which corresponds to
approximately 80%-90% battery capacity). This ensures
that the battery is kept at, or near, a fully charged condi-
tion and eliminates the need for periodic charge cycle
initiations. If the battery is removed from the charger, a
sawtooth waveform appears at the battery output. This
is caused by the repeated cycling between termination
and recharge events. This cycling results in pulsing at the
⎯
C
⎯
H
⎯
R
⎯
G output; an LED connected to this pin will exhibit
a blinking pattern, indicating to the user that a battery is
not present. The frequency of the sawtooth is dependent
on the amount of output capacitance.
Status Indicators
The charge status output (
⎯
C
⎯
H
⎯
R
⎯
G) has two states: pull-down
and high impedance. The pull-down state indicates that
the LTC4097 is in a charge cycle. Once the charge cycle
has terminated or the LTC4097 is disabled, the pin state
becomes high impedance. The pull-down state is capable
of sinking up to 10mA.
The power present output (VNTC) has two states: DCIN/
USBIN voltages and high impedance. The high impedance
state indicates that suffi cient voltage is not present at either
DCIN or USBIN, therefore no charging will occur. The VNTC
output is capable of sourcing up to 120mA steady state
and includes short circuit protection.
