Datasheet

LTC3553
17
3553fb
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 bat ter y 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 cur-
rent if the die temperature rises to approximately 110°C.
Thermal regulation protects the LTC3553 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 or external
components. The benefi t of the LTC3553 thermal regula-
tion loop is that charge current can be set according to the
d e s i r e d c h a r g e r a t e r a t h e r t h a n w o r s t - c a s e c o n d i t i o n s w i t h
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 LTC3553 reduces the charge current due
to excess load on the V
OUT
pin. This prevents false end
of charge indications due to insuffi cient 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’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 1F from BAT to
GND. Furthermore, a 100F 1210 ceramic capacitor in
series with a 0.3 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 22F may
be used in parallel with a battery, but larger ceramics
should be decoupled with 0.2 to 1 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
≤
1
2π • 100kHz • C
PROG
OPERATION