LTC4076
10
4076fa
*Any external sources that hold the ITERM pin above 100mV will prevent the LTC4076 from
terminating a charge cycle.
The termination condition is detected by using an internal
filtered comparator to monitor the ITERM pin. When the
ITERM pin voltage drops below 100mV
*
for longer than
t
TERMINATE
(typically 1.5ms), the charge cycle terminates,
charge current latches off and the LTC4076 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 1.5ms filter 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
LTC4076 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 LTC4076 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
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 6ms filter 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 of approximately 100mV 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.
Manual Shutdown
The
E
N pin has a 2MΩ pulldown resistor to GND. A logic
low enables the charger and logic high disables it (the
pulldown defaults the charger to the charging state).
The DCIN input draws 20µA when the charger is in shut-
down. The USBIN input draws 18µA during shutdown if
no power is applied to DCIN, but draws only 10µA when
V
DCIN
> V
USBIN
.
Charge Current Soft-Start and Soft-Stop
The LTC4076 includes a soft-start circuit to minimize
the inrush current at the start of a charge cycle. When a
charge cycle is initiated, the charge current ramps from
zero to full-scale current over a period of 250µs. Likewise,
internal circuitry slowly ramps the charge current from
full-scale to zero in a period of approximately 30µs when
the charger shuts down or self terminates. This minimizes
the transient current load on the power supply during
start-up and shut-off.
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 LTC4076 is in a charge cycle. Once the charge cycle
has terminated or the LTC4076 is disabled, the pin state
becomes high impedance. The pull-down state is capable
of sinking up to 10mA.
