Rainbow-electronics MAX6691 Manual do Utilizador descarregar pdf (Página 6)

MAX6691

bypass V

to GND by placing a 0.1µF to 1.0µF

ceramic bypass capacitor close to the supply pin of the

devices.

Thermal Considerations

Self-heating degrades the temperature measurement

accuracy of thermistors. The amount of self-heating

depends on the power dissipated and the dissipation

constant of the thermistor. Dissipation constants

depend on the thermistor’s package and can vary con-

siderably.

A typical thermistor might have a dissipation constant

equal to 1mW/°C. For every milliwatt the thermistor dis-

sipates, its temperature rises by 1°C. For example, con-

sider a 10kΩ (at +25°C) NTC thermistor in series with a

5110Ω resistor operating +40°C with a constant 5V

bias. If it is one of the standard thermistors previously

mentioned, its resistance is 5325Ω at this temperature.

The power dissipated in the thermistor is:

(5V)

(5325Ω) / (5325Ω + 5110Ω)

= 1.22mW

This thermistor therefore has a self-heating error at

+40°C of 1.22°C. Because the MAX6691 uses a small

reference voltage and energizes each thermistor for

only about 25ms per conversion cycle, the self-heating

of the thermistor under the same conditions when used

with the MAX6691 is far less. Assuming one conversion

cycle every 5s, each thermistor is energized only 0.5%

of the time:

(1.22)

(5325)(0.005) / (5325 + 5110)

= 0.364µW, or

only about 0.00036°C self-heating error.

Chip Information

TRANSISTOR COUNT: 7621

PROCESS: BiCMOS

Four-Channel Thermistor Temperature-to-Pulse-

Width Converter

6 _______________________________________________________________________________________

Table 1. Standard Thermistors

MANUFACTURER PART WEBSITE

Betatherm 10K3A1 www.betatherm.com/indexna.htm

Dale 1M1002 www.vishay.com/brands/dale/main.html

Thermometrics C100Y103J www.thermometrics.com

REFERENCE

VOLTAGE-TO-PWM

CONVERTER

R-

R+

I/O

MAX6691

Functional Diagram

1 2 3 4 5 6 7

Width Converter 1

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