LTC3577-4_15 Datasheet, PDF (23/52 Pages) Linear Technology – Highly Integrated Portable Product PMIC

LTC3577-3/LTC3577-4
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 LTC3577-3/LTC3577-4 include 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 current limit programming will always be observed
and only additional current will be available to charge the
battery. When system loads are light, battery charge cur-
rent 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 LTC3577-3/LTC3577-4
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 automati-
cally begin when the battery voltage falls below V_RECHRG
(typically 4.1V for LTC3577-3 and 4V for LTC3577-4). 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 re-
setting 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_PROG
=
1000V
I_CHG
,
I_CHG
=
1000V
R_PROG
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