LTC3577-3 Datasheet, PDF (25/52 Pages) Linear Technology – Highly Integrated Portable Product PMIC

LTC3577-3/LTC3577-4
OPERATION
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 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 thermistor 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 oscillation 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.
NTCBIAS
34
R_NOM
100k
NTC
35
NTC BLOCK
0.76 • NTCBIAS
–
+
R_NTC
100k
–
0.35 • NTCBIAS
+
LTC3577-3/
LTC3577-4
TOO_COLD
TOO_HOT
0.26 • NTCBIAS
+
–
BATTERY
OVERTEMP
357734 F03
Figure 3. Typical NTC Thermistor Circuit
357734fa
25