MAX8903A Maxim Integrated Products, MAX8903A Datasheet - Page 19

• Manufacturer Part Number: MAX8903A
• Description: 2A 1-Cell Li+ DC-DC Charger for USB* and Adapter Power
• Manufacturer: Maxim Integrated Products

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state when the thermistor resistance falls below 3.97kΩ
(too hot) or rises above 28.7kΩ (too cold). This corre-
sponds to a 0°C to +50°C range when using a 10kΩ
NTC thermistor with a beta of 3500. The general relation
of thermistor resistance to temperature is defined by
the following equation:
Figure 7. Thermistor Monitor Circuitry
Table 3. Fault Temperatures for Different
Thermistors
Thermistor ß (K)
R
Resistance at +25°C
(kΩ)
Resistance at +50°C
(kΩ)
Resistance at 0°C (kΩ)
Nominal Hot Trip
Temperature (°C)
Nominal Cold Trip
Temperature (°C)
TB
(kΩ) (Figure 7)
RTS
______________________________________________________________________________________
R
CONNECTION
THERMISTOR
T
ALTERNATE
THERMISTOR
25.14
CIRCUITRY
RTP
3000
4.59
10
10
55
-3
2A 1-Cell Li+ DC-DC Charger for USB*
27.15
3250
4.30
10
10
53
RTB
-1
R
T
CEN
29.32 31.66 36.91
3500
4.03
10
10
50
VL
0
GND
THM
3750
3.78
VL
10
10
49
2
0.74 VL
0.03 VL
0.28 VL
4250
3316
4.5
10
10
46
ALL COMPARATORS
60mV HYSTERESIS
where:
R
ture T in Celsius
R
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
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 120kΩ 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
T
25
= The resistance in Ω of the thermistor at tempera-
= The resistance in Ω of the thermistor at +25°C
COLD
HOT
ENABLE THM
MAX8903A
and Adapter Power
R
T
THM
OUT OF
RANGE
=
TB
R
, connecting a resistor in series
25
×
e
⎧
⎨
⎩
DISABLE
CHARGER
β
⎛
⎜
⎝
T
+
1
273
−
298
1
⎞
⎟
⎠
⎫
⎬
⎭
19