LTC3577-4_15 Datasheet, PDF (24/52 Pages) Linear Technology – Highly Integrated Portable Product PMIC

LTC3577-3/LTC3577-4
OPERATION
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.
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 LTC3577-3/LTC3577-4
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
LTC3577-3/LTC3577-4 or external components. The benefit
of the LTC3577-3/LTC3577-4 thermal regulation 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 indica-
tor LED through a current limiting resistor for human
interfacing or simply a pull-up resistor for microproces-
sor interfacing. When charging begins, CHRG is pulled
low and remains low for the duration of a normal charge
cycle. When charging 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 re-
spond to the C/10 threshold if the LTC3577-3/LTC3577-4
are in input current limit. This prevents false end-of-charge
indications due to insufficient power available to the battery
charger. Even though charging is stopped during an NTC
fault the CHRG pin will stay low indicating that charging
is not complete.
Battery Charger Stability Considerations
The LTC3577-3/LTC3577-4’s battery charger contains both
a constant-voltage and a constant-current control loop.
The constant-voltage loop is stable without any compen-
sation 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.
High value, low ESR multilayer ceramic chip capacitors
reduce the constant-voltage loop phase margin, possibly
resulting in instability. Ceramic capacitors up to 22μF may
be used in parallel with a battery, but larger ceramics should
be decoupled with 0.2Ω to 1Ω of series resistance.
In constant-current mode, the PROG pin is in the feed-
back loop rather than the battery voltage. Because of the
additional pole created by any PROG pin capacitance,
capacitance on this pin must be kept to a minimum. With
no additional capacitance on the PROG pin, the battery
charger is stable with program resistor values as high
as 25k. However, additional capacitance on this node
reduces the maximum allowed program resistor. The pole
frequency at the PROG pin should be kept above 100kHz.
Therefore, if the PROG pin has a parasitic capacitance,
C_PROG, the following equation should be used to calculate
the maximum resistance value for R_PROG:
R_PROG
≤
2π
•
1
100kHz
•
C_PROG
24
357734fb