Datasheet

LTC3677-3
23
36773f
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
V
R
BAT
PROG
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 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 LTC3677-3 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 LTC3677-3 or external components. The
benefit of the LTC3677-3 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 en-
ters 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 LTC3677-3 is in input
current limit. This prevents false end-of-charge indica-
tions 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 LTC3677-3’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.
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
kHz C
PROG
PROG
≤
1
2 100π • •
NTC Thermistor and Battery Voltage Reduction
The battery temperature is measured by placing a nega-
tive temperature coefficient (NTC) thermistor close to
the battery pack. To use this feature connect the NTC
thermistor, R
NTC
, between the NTC pin and ground and a
bias resistor, R
NOM
, from NTCBIAS to NTC. R
NOM
should
be a 1% resistor with a value equal to the value of the
chosen NTC thermistor at 25°C (R25). The LTC3677-3 will