ADT7481ARMZ-R7 Datasheet ADT7481ARMZ-1RL, ADT7481ARMZ-REEL, ADT7481ARMZ | P7-P9

ADT7481

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Theory of Operation

The ADT7481 is a local and dual remote temperature

sensor and over/under temperature alarm. When the

ADT7481 is operating normally, the on-board ADC

operates in a free-running mode. The analog input

multiplexer alternately selects either the on-chip

temperature sensor to measure its local temperature, or

either of the remote temperature sensors. The ADC digitizes

these signals and the results are stored in the local, Remote 1,

and Remote 2 temperature value registers.

The local and remote measurement results are compared

with the corresponding high, low, and THERM

temperature

limits, stored in on-chip registers. Out-of-limit comparisons

generate flags that are stored in the status register. A result that

exceeds the high temperature limit, the low temperature limit,

or remote diode open circuit will cause the ALERT

output to

assert low. Exceeding THERM

temperature limits causes the

THERM

output to assert low. The ALERT output can be

reprogrammed as a second THERM

output.

The limit registers can be programmed, and the device

controlled and configured via the serial SMBus. The

contents of any register can also be read back via the SMBus.

Control and configuration functions consist of switching

the device between normal operation and standby mode,

selecting the temperature measurement scale, masking or

enabling the ALERT

output, switching Pin 8 between

ALERT

and THERM2, and selecting the conversion rate.

Temperature Measurement Method

A simple method of measuring temperature is to exploit

the negative temperature coefficient of a diode, measuring

the base-emitter voltage (V

) of a transistor operated at

constant current.

This technique requires calibration to null the effect of the

absolute value of V

, which varies from device to device.

The technique used in the ADT7481 measures the change

in V

when the device is operated at two different currents.

Figure 14 shows the input signal conditioning used to

measure the output of a remote temperature sensor. This

figure shows the remote sensor as a substrate transistor, but

it could equally be a discrete transistor. If a discrete

transistor is used, the collector is not grounded and is linked

to the base. To prevent ground noise interfering with the

measurement, the more negative terminal of the sensor is not

referenced to ground, but is biased above ground by an

internal diode at the D− input. C1 may optionally be added

as a noise filter with a recommended maximum value of

1,000 pF.

To measure DV

, the operating current through the

sensor is switched among two related currents. The currents

through the temperature diode are switched between I, and

N × I, giving DV

. The temperature can then be calculated

using the DV

measurement.

The resulting DV

waveforms pass through a 65 kHz

low-pass filter to remove noise and then to a

chopper-stabilized amplifier. This amplifies and rectifies the

waveform to produce a dc voltage proportional to DV

The ADC digitizes this voltage producing a temperature

measurement. To reduce the effects of noise, digital filtering

is performed by averaging the results of 16 measurement

cycles for low conversion rates. At rates of 16, 32, and

64 conversions/second, no digital averaging takes place.

Signal conditioning and measurement of the local

temperature sensor is performed in the same manner.

Figure 14. Input Signal Conditioning

LOW-PASS FILTER

= 65 kHz

REMOTE

SENSING

TRANSISTOR

BIAS

DIODE

D+

D−

BIAS

IN × I

OUT+

OUT−

To ADC

C1*

*CAPACITOR C1 IS OPTIONAL. IT IS ONLY NECESSARY IN NOISY ENVIRONMENTS. C1 = 1000 pF MAX

Temperature Measurement Results

The results of the local and remote temperature

measurements are stored in the local and remote temperature

value registers and are compared with limits programmed

into the local and remote high and low limit registers.

The local temperature measurement is an 8-bit

measurement with 1°C resolution. The remote temperature

measurements are 10-bit measurements, with the 8 MSBs

stored in one register and the 2 LSBs stored in another

registers.

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Table 6. REGISTER ADDRESS FOR THE

TEMPERATURE VALUES

Temperature

Channel

Address, MSBs

Address, LSBs

Local 0x00 N/A

Remote 1 0x01 0x10 (2 MSBs)

Remote 2 0x30 0x33 (2 MSBs)

If Bit 3 of the Configuration 1 register is set to 1, then the

Remote 2 temperature values can be read from the following

Remote 2, MSBs = 0x01

Remote 2, LSBs = 0x10

The above is true only when Bit 3 of the Configuration 1

needs to be switched back to 0.

Only the two MSBs in the remote temperature low byte

are used. This gives the remote temperature measurement a

resolution of 0.25°C. Table 7 shows the data format for the

remote temperature low byte.

Table 7. EXTENDED TEMPERATURE RESOLUTION

(REMOTE TEMPERATURE LOW BYTE)

Extended Resolution Remote Temperature Low Byte

0.00°C 0 000 0000

0.25°C 0 100 0000

0.50°C 1 000 0000

0.75°C 1 100 0000

When reading the full remote temperature value,

including both the high and low byte, the two registers

should be read LSB first and then the MSB. This is because

reading the LSB will cause the MSB to be locked until it is

read. This is to guarantee that the two values read are derived

from the same temperature measurement. The MSB register

updates only after it has been read. The MSB will not lock

if a SMBus repeat start is used between reading the two

registers. There needs to be a stop between reading the LSB

and MSB.

If the LSB register is read but not the MSB register, then

fail-safe protection is provided by the THERM

and ALERT

signals which update with the latest temperature measurements

rather than the register values.

Temperature Measurement Range

The temperature measurement range for both local and

remote measurements is, by default, 0°C to +127°C.

However, the ADT7481 can be operated using an extended

temperature range. The temperature range in the extended

mode is −64°C to +191°C. The user can switch between these

two temperature ranges by setting or clearing Bit 2 in the

Configuration 1 register. A valid result is available in the next

measurement cycle after changing the temperature range.

Bit 2 Configuration Register 2 = 0 = 0°C to +127°C = default

Bit 2 Configuration Register 2 = 1 = −64°C to +191°C

In extended temperature mode, the upper and lower

temperatures that can be measured by the ADT7481 are

limited by the remote diode selection. While the temperature

registers can have values from −64°C to +191°C, most

temperature sensing diodes have a maximum temperature

range of −55°C to +150°C.

Note that while both local and remote temperature

measurements can be made while the part is in extended

temperature mode, the ADT7481 should not be exposed to

temperatures greater than those specified in the Absolute

section. Furthermore, the device is only guaranteed to operate

as specified at ambient temperatures from −40°C to +120°C.

Temperature Data Format

The ADT7481 has two temperature data formats. When

the temperature measurement range is from 0°C to +127°C

(default), the temperature data format is binary for both local

and remote temperature results. See the Temperature

Measurement Range section for information on how to

switch between the two data formats.

When the measurement range is in extended mode, an

offset binary data format is used for both local and remote

results. Temperature values in the offset binary data format

are offset by +64. Examples of temperatures in both data

formats are shown in Table 8.

Table 8. TEMPERATURE DATA FORMAT

(LOCAL AND REMOTE TEMPERATURE HIGH BYTE)

Temperature Binary

Offset Binary

(Note 1)

−55°C 0 000 0000

(Note 2)

0 000 1001

0°C 0 000 0000 0 100 0000

+1°C 0 000 0001 0 100 0001

+10°C 0 000 1010 0 100 1010

+25°C 0 001 1001 0 101 1001

+50°C 0 011 0010 0 111 0010

+75°C 0 100 1011 1 000 1011

+100°C 0 110 0100 1 010 0100

+125°C 0 111 1101 1 011 1101

+127°C 0 111 1111 1 011 1111

+150°C 0 111 1111

(Note 3)

1 101 0110

1. Offset binary scale temperature values are offset by +64.

2. Binary scale temperature measurement returns 0 for all

temperatures <0°C.

3. Binary scale temperature measurement returns 127 for all

temperatures >127°C.

The user may switch between measurement ranges at any

time. Switching the range will also switch the data format.

The next temperature result following the switching will be

reported back to the register in the new format. However, the

contents of the limit registers will not change. It is up to the

user to ensure that when the data format changes, the limit

registers are reprogrammed as necessary. More information

on this can be found in the Limit Registers section.

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Registers

The registers in the ADT7481 are eight bits wide. These

registers are used to store the results of remote and local

temperature measurements, high and low temperature limits,

and to configure and control the device. A description of these

registers follows.

Address Pointer Register

The address pointer register does not have, nor does it

require, an address because the first byte of every write

operation is automatically written to this register. The data

in this first byte always contains the address of another

pointer register. It is to this register address that the second

byte of a write operation is written to, or to which a

subsequent read operation is performed.

The power-on default value of the address pointer register

is 0x00, so if a read operation is performed immediately after

power-on, without first writing to the address pointer, the

value of the local temperature will be returned since its

Temperature Value Registers

The ADT7481 has five registers to store the results of

local and remote temperature measurements. These

registers can only be written to by the ADC and read by the

user over the SMBus.

• The local temperature value register is at Address 0x00.

• The Remote 1 temperature value high byte register is at

Address 0x01, with the Remote 1 low byte register at

Address 0x10.

• The Remote 2 temperature value high byte register is at

Address 0x30, with the Remote 2 low byte register at

Address 0x33.

• The Remote 2 temperature values can also be read from

Address 0x01 for the high byte, and Address 0x10 for the

low byte if Bit 3 of Configuration Register 1 is set to 1.

• To read the Remote 1 temperature values, set Bit 3 of

Configuration Register 1 to 0.

• The power-on default value for all five registers is 0x00.

Table 9. CONFIGURATION 1 REGISTER (READ ADDRESS 0x03, WRITE ADDRESS 0x09)

Bit Mnemonic Function

7 Mask Setting this bit to 1 masks all ALERTs on the ALERT pin. Default = 0 = ALERT enabled. This applies only if Pin 8 is

configured as ALERT

, otherwise it has no effect.

6 Mon/STBY Setting this bit to 1 places the ADT7481 in standby mode, that is, it suspends all temperature measurements

(ADC). The SMBus remains active and values can be written to, and read from, the registers. However THERM

and ALERT are not active in standby mode, and their states in standby mode are not reliable.

Default = 0 = temperature monitoring enabled.

5 AL/TH This bit selects the function of Pin 8. Default = 0 = ALERT. Setting this bit to 1 configures Pin 8 as the THERM2 pin.

4 Reserved Reserved for future use.

3 Remote

1/2

Setting this bit to 1 enables the user to read the Remote 2 values from the Remote 1 registers. When default = 0,

Remote 1 temperature values and limits are read from these registers.

2 Temp

Range

Setting this bit to 1 enables the extended temperature measurement range of −64°C to +191°C. When using the

default = 0, the temperature range is 0°C to +127°C.

1 Mask R1 Setting this bit to 1 masks ALERTs due to the Remote 1 temperature exceeding a programmed limit. Default = 0.

0 Mask R2 Setting this bit to 1 masks ALERTs due to the Remote 2 temperature exceeding a programmed limit. Default = 0.

Table 10. CONFIGURATION 2 REGISTER (ADDRESS 0x24)

Bit Mnemonic Function

7 Lock Bit Setting this bit to 1 locks all lockable registers to their current values. This prevents tampering with settings until

the device is powered down. Default = 0.

<6:0> Res Reserved for future use.

Conversion Rate/Channel Selector Register

The conversion rate/channel selector register for reads is

at Address 0x04, and at Address 0x0A for writes. The four

LSBs of this register are used to program the conversion

times from 15.5 ms (Code 0x0A) to 16 seconds

(Code 0x00). To program the ADT7481 to perform

continuous measurements, set the conversion rate register to

0x0B. For example, a conversion rate of eight

conversions/second means that beginning at 125 ms

intervals, the device performs a conversion on the local and

the remote temperature channels.

This register can be written to, and read back from, the

SMBus. The default value of this register is 0x08, giving a

rate of 16 conversions per second. Using slower conversion

times greatly reduces the device power consumption.

Bit 7 in this register can be used to disable averaging of the

temperature measurements. All temperature channels are

measured by default. It is possible to configure the

ADT7481 to measure the temperature of one channel only.

This can be configured using Bit 4 and Bit 5 (see Table 11).

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ADT7481ARMZ-R7

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Manufacturer:

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SENSOR DIGITAL 0C-127C 10MSOP

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ADT7481ARMZ-R7

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