NCT75 datasheet(5/16 Pages) ONSEMI

Part #	NCT75

Description	Industry Standard Digital Temperature Sensor with 2?릛ire Interface

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NCT75

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APPLICATION INFORMATION

Functional Description

The NCT75 temperature sensor converts an analog

temperature measurement to a digital representation by

using an on-chip measurement transistor and a 12 bit

Delta-Sigma ADC.

The device includes an open drain ALERT output which

can be used to signal that the programmed temperature limit

has been exceeded.

The two main modes of operation are normal and

shutdown mode. In normal mode the NCT75 performs

a new temperature conversion every 80 ms. This new value

is then updated to the temperature value register

(address 0x00) and also compared to the TOS register limit

(default = 80

°C). If the temperature value register is read

during the conversion sequence the value returned is the

previously stored value. A bus read does not affect the

conversion that is in progress.

In shutdown mode temperature conversion is disabled and

the temperature value register holds the last valid

temperature reading. The NCT75 can still be communicated

with in this mode as the interface is still active. The device

mode is controlled via bit 0 of the configuration register.

While in shutdown mode a conversion can be initiated by

writing an arbitrary value to the one-shot register (0x04).

This has the effect of powering up the NCT75, performing

a conversion, comparing the new temperature with the

programmed limit and then going back into shutdown mode.

The OS/ALERT pin can be configured in many ways to

allow it to be used in many different system configurations.

The overtemperature output can be configured to operate

as a comparator type output (which is self clearing once the

temperature has returned below the hysteresis value) or an

interrupt type output (which requires the master to read an

internal register AND the temperature to return below the

hysteresis value before going into an inactive state). The

ALERT pin can also be configured as an active high or active

low output.

Temperature Measurement Results

The results of the on chip temperature measurements are

stored in the temperature value register and compared with

the TOS and THYST limit register.

The temperature value, TOS and THYST registers are

16 bits wide and have a resolution of 0.0625

°C. The data is

stored as a 12 bit 2s complement word. The data is left

justified, D15 is the MSB and is the sign bit. The four LSBs

(D3 to D0) are always 0 as they are not part of the result.

While the ADC of the NCT75 can theoretically measure

temperatures in the range of −128

°C to 127°C, the NCT75

is guaranteed to measure from −55

°C to +125°C.

Table 6 shows the relevant temperature bits for a 12 bit

temperature reading. A 2-byte read is required to obtain the

full 12 bit temperature reading. If an 8 bit (1

°C resolution)

reading is required then a single byte read is sufficient.

Table 6. 12-BIT TEMPERATURE DATA FORMAT

Temperature

Binary Value

D15 to D4

Hex Value

−55

°C

1100 1001 0000

0xC90

−25

°C

1110 0111 0000

0xE70

−0.0625

°C

1111 1111 1111

0xFFF

0

°C

0000 0000 0000

0x000

+0.0625

°C

0000 0000 0001

0x001

+25

°C

0001 1001 0000

0x190

+75.25

°C

0100 1011 0100

0x4B4

+100

°C

0110 0100 0000

0x640

+125

°C

0111 1101 0000

0x7D0

Temperature Data Conversion

12-bit Temperature Data Format

Positive Temperature = ADC Code (decimal)/16

Example 190h = 400d/16 = +25

°C

Negative Temperature = (ADC Code(decimal) − 4096)/16

Example E70h = (3696d – 4096)/16 = −25

°C

One-shot Mode

One of the features of the NCT75 is a One-shot

Temperature Measurement Mode. This mode is useful if

reduced power consumption is a design requirement.

To enable one-shot mode bit 5 of the configuration

register needs to be set. Once, enabled, the NCT75 goes

immediately into shutdown mode. Here, the current

consumption is reduced to a typical value of 3

mA. Writing

address 0x04 to the address pointer register initiates a

one-shot temperature measurement. This powers up the

NCT75, carries out a temperature measurement, and then

powers down again. The data written to this register is

irrelevant and is not stored. It is the write operation that

causes the one-shot conversion.

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