LTC4089-1
16
40891fa
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 negative
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% resistor with
a value equal to the value of the chosen NTC thermistor at
25°C (this value is 10k for a Vishay NTHS0603N02N1002J
thermistor). The LTC4089-1 goes into hold mode when
the resistance (R
HOT
) of the NTC thermistor drops to 0.48
times the value of R
NOM
, or approximately 4.8k, which
should be at 45°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 LTC4089-1
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 2°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 LTC4089-1 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
4089-1 F04
LTC4089-1
TOO_COLD
TOO_HOT
0.738 •
VNTC
0.326 • VNTC
–
+
5
OPERATION
