LTC3577_15 Datasheet, PDF (26/54 Pages) Linear Technology – Highly Integrated 6-Channel Portable PMIC

LTC3577/LTC3577-1
OPERATION
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 4.0V for LTC3577-1 and 4.1V for LTC3577). 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
In either the constant-current or constant-voltage charging
modes, the PROG pin voltage will be proportional to the
actual charge current delivered to the battery. Therefore,
the actual charge current can be determined at any time
by monitoring the PROG pin voltage and using the fol-
lowing equation:
I_BAT
=
V_PROG
R_PROG
•
1000
In many cases, the actual battery charge current, I_BAT, will
be lower than I_CHG due to limited input current available and
prioritization with the system load drawn from V_OUT.
26
Thermal Regulation
To prevent thermal damage to the IC or surrounding
components, an internal thermal feedback loop will
automatically decrease the programmed charge current
if the die temperature rises to approximately 110°C.
Thermal regulation protects the LTC3577 from excessive
temperature due to high power operation or high ambient
thermal conditions and allows the user to push the limits
of the power handling capability with a given circuit board
design without risk of damaging the LTC3577 or external
components. The benefit of the LTC3577 thermal regula-
tion loop is that charge current can be set according to
actual conditions rather than worst-case conditions with
the assurance that the battery charger will automatically
reduce the current in worst-case conditions.
Charge Status Indication
The CHRG pin indicates the status of the battery charger. An
open-drain output, the CHRG pin can drive an indicator LED
through a current limiting resistor for human interfacing or
simply a pull-up resistor for microprocessor interfacing.
When charging begins, CHRG is pulled low and remains
low for the duration of a normal charge cycle. When charg-
ing is complete, i.e., the charger enters constant voltage
mode and the charge current has dropped to one-tenth
of the programmed value, the CHRG pin is released (high
impedance). The CHRG pin does not respond to the C/10
threshold if the LTC3577 is in input current limit. This
prevents false end-of-charge indications due to insufficient
power available to the battery charger. Even though charg-
ing is stopped during an NTC fault, the CHRG pin will stay
low indicating that charging is not complete.
Battery Charger Stability Considerations
The LTC3577’s battery charger contains both a constant-
voltage and a constant-current control loop. The constant-
voltage loop is stable without any compensation when a
battery is connected with low impedance leads. Excessive
lead length, however, may add enough series inductance
to require a bypass capacitor of at least 1μF from BAT to
GND. Furthermore, a 4.7μF capacitor in series with a 0.2Ω
to 1Ω resistor from BAT to GND is required to keep ripple
voltage low when the battery is disconnected.
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