Rainbow-electronics MAX8903A Manual do Utilizador descarregar pdf (Página 19)

MAX8903A

2A 1-Cell Li+ DC-DC Charger for USB*

and Adapter Power

______________________________________________________________________________________ 19

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:

where:

= The resistance in Ω of the thermistor at tempera-

ture T in Celsius

= 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

, 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 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

RR e

=×

−

⎛

⎝

⎜

⎞

⎠

⎟

⎧

⎨

⎩

⎫

⎬

⎭

273

298

THM

GND

THM

OUT OF

RANGE

DISABLE

CHARGER

CEN

0.74 VL

0.28 VL

0.03 VL

COLD

HOT

RTB

THERMISTOR

CIRCUITRY

ENABLE THM

RTS

RTP

ALTERNATE

THERMISTOR

CONNECTION

ALL COMPARATORS

60mV HYSTERESIS

MAX8903A

Figure 7. Thermistor Monitor Circuitry

Thermistor ß (K) 3000 3250 3500 3750 4250

(kΩ) (Figure 7) 10 10 10 10 10

Resistance at +25°C

(kΩ)

10 10 10 10 10

Resistance at +50°C

(kΩ)

4.59 4.30 4.03 3.78 3316

Resistance at 0°C (kΩ) 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

1 2 ... 14 15 16 17 18 19 20

MAX8903A 1

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