ADT7482ARMZ-RL7 Datasheet ADT7482ARMZ, ADT7482ARMZ-REEL, ADT7482ARMZ-REEL7 | P7-P9

ADT7482

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Theory of Operation

The ADT7482 is a local and 2 remote temperature sensor

and overtemperature/undertemperature alarm. When the

ADT7482 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 a remote diode open circuit causes 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.

Series Resistance Cancellation

Parasitic resistance to the D+ and D− inputs to the

ADT7482, seen in series with the remote diode, is caused by

a variety of factors, including PCB track resistance and track

length. This series resistance appears as a temperature offset

in the remote sensor temperature measurement. This error

typically causes a 0.5C offset per ohm of parasitic resistance

in series with the remote diode.

The ADT7482 automatically cancels out the effect of this

series resistance on the temperature reading, providing a more

accurate result, without the need for user characterization of

this resistance. The ADT7482 is designed to automatically

cancel typically up to 1.5 kW of resistance. By using an

advanced temperature measurement method, this is

transparent to the user. This feature allows resistances to be

added to the sensor path to produce a filter, allowing the part

to be used in noisy environments. See the Noise Filtering

section for more details.

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. However, this technique requires

calibration to null out the effect of the absolute value of V

which varies from device to device.

The technique used in the ADT7482 is to measure the

change in V

when the device is operated at three different

currents. Previous devices have used only two operating

currents. The use of a third current allows automatic

cancellation of resistances in series with the external

temperature sensor.

Figure 15 shows the input signal conditioning used to

measure the output of an external temperature sensor. This

figure shows the external 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 should

be linked to the base. To prevent ground noise from

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.

Capacitor C1 can be added as a noise filter (a recommended

maximum value of 1,000 pF). However, a better option in

noisy environments is to add a filter, as described in the

Noise Filtering section. See the Layout Considerations

section for more information.

To measure DV

, the operating current through the

sensor is switched among three related currents. Shown in

Figure 15, N1  I and N2  I are different multiples of the

current, I. The currents through the temperature diode are

switched between I and N1  I, giving DV

BE1

, and then

between I and N2  I, giving DV

BE2

. The temperature can

then be calculated using the two DV

measurements. This

method can also be shown to cancel the effect of any series

resistance on the temperature measurement.

The resulting DV

waveforms are passed 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 and a temperature measurement is

produced. 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 internal

temperature sensor are performed in the same manner.

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Figure 15. Input Signal Conditioning

LOW-PASS FILTER

= 65 kHz

REMOTE

SENSING

TRANSISTOR

BIAS

DIODE

D+

D−

BIAS

I N1  I

OUT+

OUT−

To ADC

N2  I

C1*

*CAPACITOR C1 IS OPTIONAL.

IT SHOULD ONLY BE USED IN

NOISY ENVIRONMENTS.

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

list of the temperature measurement registers.

Table 6. REGISTER ADDRESS FOR THE

TEMPERATURE VALUES

Temperature

Channel

MSBs

LSBs

Local 0x00 N/A

Remote 1 0x01 0x10 (2 MSBs)

Remote 2 0x30 0x33 (2 MSBs)

Set Bit 3 of the Configuration 1 register to 1, to read the

Remote 2 temperature values from the following register

addresses:

Remote 2, MSBs = 0x01

Remote 2, LSBs = 0x10

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

this bit 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, both the

high and low byte, the two registers should be read LSB first

and then MSB. Reading the LSB causes the MSB to be

locked until it is read. This guarantees that the two values are

read as a result of the same temperature measurement.

Temperature Measurement Range

The temperature measurement range for both local and

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

However, the ADT7482 can be operated using an extended

temperature range. It can measure the full temperature range

of a remote thermal diode, from −55C to +150C. 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.

In extended temperature mode, the upper and lower

temperature measured by the ADT7482 is limited by the

remote diode selection. The temperature registers

themselves can have values from −64C to +191C.

However, 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 ADT7482 itself should not be

exposed to temperatures greater than those specified in the

Absolute Maximum Ratings section. Further, the device is

only guaranteed to operate as specified at ambient

temperatures from −40C to +120C.

Temperature Data Format

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.

Switching between measurement ranges can be done at

any time. Switching the range also switches the data format.

The next temperature result following the switching is

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

contents of the limit registers do not change. Ensure that

when the data format changes, the limit registers are

reprogrammed as necessary. For more information, refer to

the Limit Registers section.

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The ADT7482 has two temperature data formats. When

the temperature measurement range is from 0C to 127C

(default), the temperature data format for both local and

remote temperature results is binary.

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.

Registers

The registers in the ADT7482 are 8-bits wide. These

registers are used to store the results of remote and local

temperature measurements and 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 itself does not have, or

require, an address, as 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 register on the

ADT7482, which is stored in the address 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. Therefore, if a read operation is performed

immediately after power-on, without first writing to the

address pointer, the value of the local temperature is

returned, since its register address is 0x00.

Configuration Registers

There are two configuration registers used to control the

operation of the ADT7482. Configuration 1 register is at

Address 0x03 for reads and Address 0x09 for writes. See

Table 9 for details regarding the operation of this register.

Configuration 2 Register is at Address 0x24 for both reads

and writes. Setting Bit 7 of this register locks all lockable

registers. The affected registers can only be modified if the

ADT7482 is powered down and powered up again. See

Table 16 for a list of the registers affected by the lock bit.

Temperature Value Registers

The ADT7482 has five registers to store the results of

local and remote temperature measurements. These

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

over the SMBus.

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

 The Remote 1 temperature value high byte register is at

Address 0x01; the Remote 1 low byte register is at

Address 0x10.

 The Remote 2 temperature value high byte register is at

Address 0x30; the Remote 2 low byte register is at

Address 0x33.

 The Remote 2 temperature values can be read from

Addresses 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 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 ADT7482 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. THERM

and ALERT

are also active in standby mode. Changes made to the limit registers in standby mode that effect the THERM or

ALERT outputs cause these signals to be updated. 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

/Remote2

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 Te mp

Range

Setting this bit to 1 enables the extended temperature measurement range of −50C to +150C.

Default = 0 = 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.

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P20

ADT7482ARMZ-RL7

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SENSOR DIGITAL 0C-127C 10MSOP

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ADT7482ARMZ-RL7

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