Rainbow Electronics MAX8903A User Manual
Page 19
MAX8903A
2A 1-Cell Li+ DC-DC Charger for USB*
and Adapter Power
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19
state when the thermistor resistance falls below 3.97k
Ω
(too hot) or rises above 28.7k
Ω (too cold). This corre-
sponds to a 0°C to +50°C range when using a 10k
Ω
NTC thermistor with a beta of 3500. The general relation
of thermistor resistance to temperature is defined by
the following equation:
where:
R
T
= The resistance in
Ω of the thermistor at tempera-
ture T in Celsius
R
25
= The resistance in
Ω of the thermistor at +25°C
ß = The material constant of the thermistor, which typi-
cally ranges from 3000K to 5000K
T = The temperature of the thermistor in °C
Table 4 shows the MAX8903A THM temperature limits
for different thermistor material constants.
Some designs might prefer other thermistor tempera-
ture limits. Threshold adjustment can be accommodat-
ed by changing R
TB
, connecting a resistor in series
and/or in parallel with the thermistor, or using a thermis-
tor with different ß. For example, a +45°C hot threshold
and 0°C cold threshold can be realized by using a ther-
mistor with a ß of 4250 and connecting 120k
Ω in paral-
lel. Since the thermistor resistance near 0°C is much
higher than it is near +50°C, a large parallel resistance
lowers the cold threshold, while only slightly lowering
the hot threshold. Conversely, a small series resistance
R
R
e
T
T
=
×
+
−
⎛
⎝⎜
⎞
⎠⎟
⎧
⎨
⎩
⎫
⎬
⎭
25
1
273
1
298
β
THM
GND
THM
OUT OF
RANGE
DISABLE
CHARGER
VL
CEN
VL
0.74 VL
0.28 VL
0.03 VL
COLD
HOT
RTB
THERMISTOR
CIRCUITRY
ENABLE THM
R
T
RTS
RTP
ALTERNATE
THERMISTOR
CONNECTION
R
T
ALL COMPARATORS
60mV HYSTERESIS
MAX8903A
Figure 7. Thermistor Monitor Circuitry
Thermistor ß (K)
3000
3250
3500
3750
4250
R
TB
(k
Ω) (Figure 7)
10
10
10
10
10
Resistance at +25°C
(k
Ω)
10
10
10
10
10
Resistance at +50°C
(k
Ω)
4.59
4.30
4.03
3.78
3316
Resistance at 0°C (k
Ω) 25.14 27.15 29.32 31.66 36.91
Nominal Hot Trip
Temperature (°C)
55
53
50
49
46
Nominal Cold Trip
Temperature (°C)
-3
-1
0
2
4.5
Table 3. Fault Temperatures for Different
Thermistors