ADT7466 Просмотр технического описания (PDF) - ON Semiconductor

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ADT7466

dBCool® Remote Thermal Controller and Voltage Monitor

ON Semiconductor 

Thermistor Linearization

A linear transfer function can be obtained over a limited

temperature range by connecting the thermistor in series with

an optimum resistor. Placing a resistor in series with the

thermistor as shown in Figure 26 produces an S-shaped error

curve as shown in Figure 27. The overall error across the range

can be reduced by calculating the external resistor so that the

error is 0 at the ends of the range. REXT is calculated as follows:

R EXT

=

R MID

× (R MIN + R MAX ) − (2 × R MIN × R MAX )

(R MIN + RMAX − 2 × R MID )

where:

RMIN is the thermistor value at TMIN.

RMAX is the thermistor value at TMAX.

RMID is the thermistor value at TMIN +TMAX

2

Figure 27 shows the linearity error using a 100 kΩ thermistor

with a B value of 3500 and a 14400 Ω resistor. Using the

specified thermistor and resistor, the error over a temperature

range of 30°C to 100°C is less than ±2°C. Other thermistors can

be used, but the resistor value is different. A smaller error can be

achieved over a narrower temperature range; conversely, a

wider temperature range can be used, but the error is greater. In

both cases, the optimum resistor value is different.

2

1

0

–1

–2

30

40

50

60

70

80

90

100

TEMPERATURE (°C)

Figure 27. Linearity Error Using Specified Components

ADT7466

Thermistor Normalization

Even when the thermistor is linearized, it does not provide an

output to the ADC that gives a direct temperature reading in

degrees Celsius. The linearized data is proportional to the

voltage applied; however, normalization is needed to use the

value as a temperature reading.

To overcome this problem, when an analog input is configured

for use with a thermistor, the output of the ADC is scaled and

offset so that it produces the same output (for example, 1 LSB =

0.25°C) as from the thermal diode input, when REXT is chosen to

linearize the thermistor over 30°C to 100°C.

Normalization can be chosen for 10 kΩ thermistors by setting

Bit 0 of Configuration Register 2 (0x01) or for 100 kΩ

thermistors by clearing this bit (default setting).

READING TEMPERATURE FROM THE ADT7466

It is important to note that temperature can be read from the

ADT7466 as an 8-bit value (with 1°C resolution) or as a 10-bit

value (with 0.25°C resolution). If only 1°C resolution is

required, the temperature readings can be read at any time

and in no particular order.

If the 10-bit measurement is required, this involves a 2-register

read for each measurement. The extended resolution registers

(0x08 and 0x09) should be read first. This causes all temperature

reading registers to be frozen until all temperature reading

registers have been read. This prevents an MSB reading from

being updated while its 2 LSBs are being read and vice versa.

Measurement Sequence

The ADT7466 automatically measures each analog and

temperature channel in the following round-robin sequence:

1. AIN1/TH1

2. AIN2(TH2)

3. VCC

4. Remote Temperature 1 (D1)

5. Local Temperature

If AIN1 and AIN2 are configured for a second thermal diode,

this is measured instead of the AIN1 and AIN 2 measurements,

and the result stored in the AIN1 reading register (0x0A).

Rev. 3 | Page 21 of 48 | www.onsemi.com

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