LTC4089/LTC4089-5
16
40895fc
If, for some reason the charge current rises back above
the threshold, the CHRG pin will not resume the strong
pull-down state. The EOC latch can be reset by a recharge
cycle (i.e., V
BAT
drops below the recharge threshold) or
toggling the input power to the part.
NTC Thermistor—Battery Temperature Charge
Qualifi cation
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 Figure 4.
To use this feature, connect the NTC thermistor (R
NTC
)
between the NTC pin and ground and a resistor (R
NOM
)
from the NTC pin to VNTC. R
NOM
should be a 1% resis-
tor with a value equal to the value of the chosen NTC
thermistor at 25°C (this value is 10k for a Vishay NTH-
S0603N02N1002J thermistor). The LTC4089/LTC4089-5
goes into hold mode when the resistance (R
HOT
) of the
NTC thermistor drops to 0.41 times the value of R
NOM
,
or approximately 4.1k, which should be at 50°C. The hold
mode freezes the timer and stops the charge cycle until the
thermistor indicates a return to a valid temperature. As the
temperature drops, the resistance of the NTC thermistor
rises. The LTC4089/LTC4089-5 is designed to go into hold
mode when the value of the NTC thermistor increases to
2.82 times the value of R
NOM
. This resistance is R
COLD
.
For a Vishay NTHS0603N02N1002J thermistor, this value
is 28.2k which 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 will disable the NTC function.
Current Limit Undervoltage Lockout
An internal undervoltage lockout circuit monitors the
input voltage and disables the input current limit circuits
until V
IN
rises above the undervoltage lockout threshold.
The current limit UVLO circuit has a built-in hysteresis of
125mV. Furthermore, to protect against reverse current in
the power MOSFET, the current limit UVLO circuit disables
the current limit (i.e., forces the input power path to a high
impedance state) if V
OUT
exceeds V
IN
. If the current limit
UVLO comparator is tripped, the current limit circuits will
not come out of shutdown until V
OUT
falls 50mV below
the V
IN
voltage.
Charger Undervoltage Lockout
An internal undervoltage lockout circuit monitors the V
OUT
voltage and disables the battery charger circuits until
V
OUT
rises above the undervoltage lockout threshold. The
battery charger UVLO circuit has a built-in hysteresis of
125mV. Furthermore, to protect against reverse current
in the power MOSFET, the charger UVLO circuit keeps the
charger shut down if V
BAT
exceeds V
OUT
. If the charger
UVLO comparator is tripped, the charger circuits will
not come out of shutdown until V
OUT
exceeds V
BAT
by 50mV.
Suspend
The LTC4089/LTC4089-5 can be put in suspend mode by
forcing the SUSP pin greater than 2.3V. In suspend mode,
the ideal diode function from BAT to OUT is kept alive. If
power is applied to the HVIN pin, then charging will be
unaffected. Current drawn from the IN pin is reduced to
50μA. Suspend mode is intended to comply with the USB
power specifi cation mode of the same name.
Figure 4. NTC Circuit
–
+
–
+
R
NOM
10k
R
NTC
10k
NTC
VNTC
6
0.1V
NTC_ENABLE
4089 F04
LTC4089
TOO_COLD
TOO_HOT
0.74 •
VNTC
0.29 • VNTC
–
+
5
OPERATION
