Datasheet

LTC3577/LTC3577-1
28
3577fa
OPERATION
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 provides temperature qualifi ed 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 modifi cation 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 modifi ed 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 follows.
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.
In the following explanation, this notation is used.
R25 = Value of the thermistor at 25°C
R
NTC|COLD
= Value of thermistor at the cold trip point
R
NTC|HOT
= Value of the thermistor at the hot trip point
r
COLD
= Ratio of R
NTC|COLD
to R25
r
HOT
= Ratio of R
NTC|HOT
to R25
R
NOM
= Primary thermistor bias resistor (see Figure 9)
R1 = Optional temperature range adjustment resistor
(see Figure 9)
The trip points for the LTC3577’s temperature qualifi ca-
tion are internally programmed at 0.35 • V
NTC
for the hot
threshold and 0.76 • V
NTC
for the cold threshold.
Therefore, the hot trip point is set when:
R
NTC|HOT
R
NOM
+R
NTC|HOT
•NTCBIAS= 0.35 •NTCBIAS
and the cold trip point is set when:
R
NTC|COLD
R
NOM
+R
NTC|COLD
•NTCBIAS= 0.76 •NTCBIAS
Solving these equations for R
NTC|COLD
and R
NTC|HOT
results
in the following:
R
NTC|HOT
= 0.538 • R
NOM
and
R
NTC|COLD
= 3.17 • R
NOM
By setting R
NOM
equal to R25, the above equations result
in r
HOT
= 0.538 and r
COLD
= 3.17. Referencing these ratios
to the Vishay Resistance-Temperature Curve 1 chart gives
a hot trip point of about 40°C and a cold trip point of about
0°C. The difference between the hot and cold trip points
is approximately 40°C.
–
+
–
+
R
NOM
105k
R
NTC
100k
R1
12.7k
NTC
NTCBIAS
34
0.26 • NTCBIAS
BATTERY
OVERTEMP
3577 F09
NTC BLOCK
LTC3577
TOO_COLD
TOO_HOT
0.76 • NTCBIAS
0.35 • NTCBIAS
–
+
35
Figure 9. NTC Thermistor Circuit with Additional Bias Resistor