LTC3553-2_15 Datasheet, PDF (17/36 Pages) Linear Technology – Micropower USB Power Manager with Li-Ion Charger, Always-On LDO and Buck Regulator

LTC3553-2
OPERATION
In either the constant-current or constant-voltage charg-
ing modes, the PROG pin voltage will be proportional to
the actual charge current delivered to the battery. There-
fore, the actual charge current can be determined at any
time by monitoring the PROG pin voltage and using the
following equation:
I _BAT
=
V _PROG
R _PROG
•
750
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 compo-
nents, an internal thermal feedback loop will automatically
decrease the programmed charge current if the die tem-
perature rises to approximately 110°C. Thermal regulation
protects the LTC3553-2 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 LTC3553-2 or external components. The
benefit of the LTC3553-2 thermal regulation loop is that
charge current can be set according to the desired charge
rate rather than worst-case conditions with the assurance
that the battery charger will automatically reduce the cur-
rent 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 LTC3553-2 reduces the charge current due
to excess load on the V_OUT pin. 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 LTC3553-2’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 100μF 1210 ceramic capacitor in
series with a 0.3Ω resistor from BAT to GND is required
to keep ripple voltage low if operation with the battery
disconnected is allowed.
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
35532f
17