Datasheet

LTC3677-3
24
36773f
operaTion
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 LTC3677-3 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.
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 LTC3677-3 provides 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 thermistor (R25)
the upper and lower temperatures are pre-programmed
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
Figure 3. Typical NTC Thermistor Circuit
–
+
–
+
R
NOM
100k
R
NTC
100k
NTC
NTCBIAS
34
0.26 • NTCBIAS
BATTERY
OVERTEMP
36773 F03
NTC BLOCK
LTC3677-3
TOO_COLD
TOO_HOT
0.76 • NTCBIAS
0.35 • NTCBIAS
–
+
35