LTC4088 LINER [Linear Technology], LTC4088 Datasheet - Page 15

• Manufacturer Part Number: LTC4088
• Description: High Effi ciency Battery Charger/USB Power Manager
• Manufacturer: LINER [Linear Technology]

OPERATIO
NTC Thermistor
The battery temperature is measured by placing a nega-
tive temperature coeffi cient (NTC) thermistor close to
the battery pack. The NTC circuitry is shown in the Block
Diagram.
To use this feature, connect the NTC thermistor, R
between the NTC pin and ground and a bias resistor, R
from V
a value equal to the value of the chosen NTC thermistor
at 25°C (R25). A 100k thermistor is recommended since
thermistor current is not measured by the LTC4088 and
will have to be considered for USB compliance.
The LTC4088 will pause charging when the resistance of
the NTC thermistor drops to 0.54 times the value of R25 or
approximately 54k (for a Vishay “Curve 1” thermistor, this
corresponds to approximately 40°C). If the battery charger
is in constant voltage (fl oat) mode, the safety timer also
pauses until the thermistor indicates a return to a valid
temperature. As the temperature drops, the resistance of
the NTC thermistor rises. The LTC4088 is also designed
to pause charging when the value of the NTC thermistor
increases to 3.25 times the value of R25. For a Vishay
“Curve 1” thermistor, this resistance, 325k, corresponds
to approximately 0°C. The hot and cold comparators each
have approximately 3°C of hysteresis to prevent oscillation
about the trip point. Grounding the NTC pin disables all
NTC functionality.
BUS
to NTC. R
U
NOM
should be a 1% resistor with
NOM
NTC
,
,
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 LTC4088 from excessive temperature due to
high power operation or high ambient thermal conditions,
and allows the user to push the limits of the power han-
dling capability with a given circuit board design without
risk of damaging the LTC4088 or external components.
The benefi t of the LTC4088 thermal regulation loop is that
charge current can be set according to actual conditions
rather than worst-case conditions for a given application
with the assurance that the charger will automatically
reduce the current in worst-case conditions.
Shutdown Mode
For autonomous battery charger operation, D2 should
be permanently grounded. However, for more control
via software the LTC4088’s battery charger can be inde-
pendently disabled by bringing the D2 pin above V
must also be brought high to enable high power (2.5mA)
suspend mode.
The input switching regulator is enabled whenever V
is above the UVLO voltage and the LTC4088 is not in one
of the two USB suspend modes (500µA or 2.5mA).
The ideal diode is enabled at all times and cannot be
disabled.
LTC4088
15
IH
. D2
BUS
4088f