Datasheet

LTC3677-3
22
36773f
operaTion
V
BUS
Undervoltage Lockout (UVLO) and Undervoltage
Current Limit (UVCL)
An internal undervoltage lockout circuit monitors V
BUS
and keeps the input current limit circuitry off until V
BUS
rises above the rising UVLO threshold (3.8V) and at least
50mV above V
OUT
. Hysteresis on the UVLO turns off the
input current limit if V
BUS
drops below 3.7V or 50mV below
V
OUT
. When this happens, system power at V
OUT
will be
drawn from the battery via the ideal diode. To minimize the
possibility of oscillation in and out of UVLO when using
resistive input supplies, the input current limit is reduced
as V
BUS
falls below 4.45V (typ).
Battery Charger
The LTC3677-3 includes a constant-current/constant-
voltage battery charger with automatic recharge, automatic
termination by safety timer, low voltage trickle charging,
bad cell detection and thermistor sensor input for out of
temperature charge pausing. When a battery charge cycle
begins, the battery charger first determines if the battery
is deeply discharged. If the battery voltage is below V
TRKL
,
typically 2.85V, an automatic trickle charge feature sets the
battery charge current to 10% of the programmed value. If
the low voltage persists for more than 1/2 hour, the battery
charger automatically terminates. Once the battery voltage
is above 2.85V, the battery charger begins charging in full
power constant-current mode. The current delivered to
the battery will try to reach 1000V/R
PROG
. Depending on
available input power and external load conditions, the
battery charger may or may not be able to charge at the
full programmed rate. The external load will always be
prioritized over the battery charge current. The USB cur-
rent limit programming will always be observed and only
additional current will be available to charge the battery.
When system loads are light, battery charge current will
be maximized.
Charge Termination
The battery charger has a built-in safety timer. When
the battery voltage approaches the float voltage, the
charge current begins to decrease as the LTC3677-3
enters constant-voltage mode. Once the battery charger
detects that it has entered constant-voltage mode, the
four hour safety timer is started. After the safety timer
expires, charging of the battery will terminate and no
more current will be delivered.
Automatic Recharge
After the battery charger terminates, it will remain off
drawing only microamperes of current from the battery.
If the portable product remains in this state long enough,
the battery will eventually self discharge. To ensure that
the battery is always topped off, a charge cycle will au-
tomatically begin when the battery voltage falls below
V
RECHRG
(typically 4.1V for LTC3677-3). In the event
that the safety timer is running when the battery voltage
falls below V
RECHRG
, the timer will reset back to zero. To
prevent brief excursions below V
RECHRG
from resetting
the safety timer, the battery voltage must be below V
RECHRG
for more than 1.3ms. The charge cycle and safety timer
will also restart if the V
BUS
UVLO cycles low and then high
(e.g., V
BUS
, is removed and then replaced).
Charge Current
The charge current is programmed using a single resistor
from PROG to ground. 1/1000th of the battery charge cur-
rent is delivered to PROG which will attempt to servo to
1.000V. Thus, the battery charge current will try to reach
1000 times the current in the PROG pin. The program
resistor and the charge current are calculated using the
following equations:
R
V
I
I
V
R
PROG
CHG
CHG
PROG
= =
1000 1000
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