LTC4077
10
4077fa
*Any external sources that hold the pin above 100mV will prevent the LTC4077 from terminating
a charge cycle.
Programming Charge Termination
The charge cycle terminates when the voltage at the IUSB,
IUSBL and IDC pins falls below 100mV. For wall adapter
or high power USB use the charge termination is set to
1/10th of the programmed charge current. For low power
USB use (HPWR = 0V) the charge termination is set to
1/2 of the programmed charge current.
The termination condition is detected by using an internal
filtered comparator to monitor the IDC, IUSB and IUSBL
pin voltages. When one of these voltages drops below
100mV
*
for longer than t
TERMINATE
(typically 1.5ms), the
charge cycle terminates, charge current latches off and
the LTC4077 enters standby mode.
When charging, transient loads on the BAT pin can cause
the active charge current program pin to fall below 100mV
for short periods of time before the steady-state charge
current has dropped below the programmed termination
current. The 1.5ms filter time (t
TERMINATE
) on the termina-
tion comparator ensures that transient loads of this nature
do not result in premature charge cycle termination. Once
the average charge current drops below the termination
threshold, the LTC4077 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 reapplying full charge cur-
rent. If the BAT pin voltage is below 2.9V, the LTC4077
supplies 1/10th of the full charge current to the battery,
when HPWR = high or 1/2 of the full charge current
(programmed by R
IUSBL
) to the battery when HPWR =
low, 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 when
HPWR is high or 50mA if HPWR is low, the charge current
during trickle charge mode would be 80mA, 50mA and
25mA 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 LTC4077 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.
