LTC3577-4_15 Datasheet, PDF (25/52 Pages) Linear Technology – Highly Integrated Portable Product PMIC

LTC3577-3/LTC3577-4
OPERATION
NTC Thermistor and Battery Voltage Reduction
The battery temperature is measured by placing a negative
temperature coefficient (NTC) thermistor close to the bat-
tery pack. To use this feature connect the NTC thermistor,
R_NTC, between the NTC pin and ground and a bias resis-
tor, 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 LTC3577-3/LTC3577-4 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 (float) 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 therm-
istor rises. The LTC3577-3/LTC3577-4 are 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 oscilla-
tion about the trip point. The typical NTC circuit is shown
in Figure 3.
To improve safety and reliability the battery voltage is re-
duced when the battery temperature becomes excessively
high. When the resistance of the NTC thermistor drops
to about 0.35 times the value of R25 or approximately
35k (for a Vishay Curve 1 thermistor, this corresponds to
approximately 50°C) the NTC enables circuitry to moni-
tor the battery voltage. If the battery voltage is above the
battery discharge threshold (about 3.9V) then the battery
discharge circuitry is enabled and draws about 140mA from
the battery when V_BUS = 0V and about 180mA when V_BUS
= 5V. The battery discharge current is disabled below the
battery discharge threshold.
When the charger is disabled an internal watchdog timer
samples the NTC thermistor for about 150μs every 150ms
and will enable the battery monitoring circuitry if the bat-
tery temperature exceeds the NTC TOO_HOT threshold.
If adding a capacitor to the NTC pin for filtering the time
constant must be much less than 150μs so that the NTC
pin can settle to its final value during the sampling period.
A time constant less than 10μs is recommended. Once
the battery monitoring circuitry is enabled it will remain
enabled and monitoring the battery voltage until the battery
temperature falls back below the discharge temperature
threshold. The battery discharge circuitry is only enabled
if the battery voltage is greater than the battery discharge
threshold.
Alternate NTC Thermistors and Biasing
The LTC3577-3/LTC3577-4 provide temperature qualified
charging if a grounded thermistor and a bias resistor are
connected to NTC. By using a bias resistor whose value is
equal to the room temperature resistance of the thermis-
tor (R25) the upper and lower temperatures are pre-pro-
grammed to approximately 40°C and 0°C, respectively
(assuming a Vishay Curve 1 thermistor).
The upper and lower temperature thresholds can be ad-
justed by either a modification of the bias resistor value
or by adding a second adjustment resistor to the circuit.
If only the bias resistor is adjusted, then either the upper
or the lower threshold can be modified but not both. The
other trip point will be determined by the characteristics
of the thermistor. Using the bias resistor in addition to an
adjustment resistor, both the upper and the lower tempera-
ture trip points can be independently programmed with
the constraint that the difference between the upper and
lower temperature thresholds cannot decrease. Examples
of each technique are given below.
NTC thermistors have temperature characteristics which
are indicated on resistance-temperature conversion tables.
The Vishay-Dale thermistor NTHS0603N011-N1003F, used
in the following examples, has a nominal value of 100k
and follows the Vishay Curve 1 resistance-temperature
characteristic.
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