ON Semiconductor ADT7461 Technical data

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±1°C Temperature Monitor with

FEATURES

On-chip and remote temperature sensor

0.25°C resolution/1°C accuracy on remote channel

1°C resolution/3°C accuracy on local channel

Automatically cancels up to 3 kΩ (typ) of resistance in series

with remote diode to allow noise filtering

Extended, switchable temperature measurement range

0°C to +127°C (default) or –55°C to +150°C

Pin- and register-compatible with the ADM1032

2-wire SMBus serial interface with SMBus alert support

Two SMBus address versions available:

ADT7461 SMBus address is 0x4C

ADT7461-2 SMBus address is 0x4D

Programmable over/under temperature limits

Offset registers for system calibration

Up to two over temperature fail-safe

Small 8-lead SOIC or8-lead MSOP packages

170 μA operating current, 5.5 μA standby current

APPLICATIONS

Desktop and notebook computers

Industrial controllers

Smart batteries

Embedded systems

Instrumentation

THERM

outputs

Series Resistance Cancellation

ADT7461

GENERAL DESCRIPTION

The ADT74611 is a dual-channel digital thermometer and under/over temperature alarm intended for use in PCs and thermal management systems. It is pin- and register-compatible with the ADM1032. The ADT7461 has three additional features: series resistance cancellation (where up to 3 kΩ (typical) of resistance in series with the temperature monitoring diode may be automatically cancelled from the temperature result, allowing noise filtering); configurable and an extended, switchable temperature measurement range. The ADT7461 can accurately measure the temperature of a remote thermal diode to ±1°C and the ambient temperature to ±3°C. The temperature measurement range defaults to 0°C to +127°C, compatible with the ADM1032, but can be switched to a wider measurement range of −55°C to +150°C. The ADT7461 communicates over a 2-wire serial interface compatible with system management bus (SMBus) standards. An

signals when the on-chip or remote temperature is out of range.

THERM

The

control of a cooling fan. The

as a second

output is a comparator output that allows on/off

ALERT

output can be reconfigured

THERM

output, if required.

The SMBus address of the ADT7461 is 0x4C. An ADT7461-2 is also available, which uses SMBus Address 0x4D.

Protected by U.S. Patents 5,195,827; 5,867,012; 5,982,221; 6,097,239;

6,133,753; 6,169,442; other patents pending.

ALERT

output;

output

CONVERSION RATE

ON-CHIP

TEMPERATURE

SENSOR

ANALOG

MUX

D+

SRC

BLOCK

D–

EXTERNAL DIODE OPEN CIRCUIT

ADT7461

LOCAL TEMPERATURE

VALUE REGISTER

ADC

RUN/STANDBYBUSY

REMOTE TE MPERATURE

VALUE REGISTER

REMOTE OFFSET

STATUS REGIST ER

SMBus INTERFACE

751 8

DIGITAL MUX

SCLKSDATAGNDV

LIMIT

COMPARATOR

ADDRESS POINTER

LOCAL TEMPERATURE

LOW LIMIT REGIST ER

LOCAL TEMPERATURE

HIGH LIMIT REGIST ER

REMOTE TE MPERATURE

LOW LIMIT REGIST ER

REMOTE TE MPERATURE

HIGH LIMIT REGIST ER

DIGITAL MUX

LOCAL THERM LIMIT

EXTERNAL THERM LIMIT

CONFIGURATION

THERM ALERT /

INTERRUPT

MASKING

THERM2

04110-0-012

Figure 1. Functional Block Diagram

ADT7461

TABLE OF CONTENTS

Specifications ..................................................................................... 3

SMBus Timing Specifications..........................................................4

Absolute Maximum Ratings ............................................................5

Thermal Characteristics...............................................................5

ESD Caution .................................................................................. 5

Pin Configuration and Function Descriptions .............................6

Typical Performance Characteristics..............................................7

Functional Description.....................................................................9

Series Resistance Cancellation ....................................................9

Temperature Measurement Method...........................................9

Temperature Measurement Results..........................................10

Temperature Measurement Range............................................10

Temperature Data Format..........................................................10

ADT7461 Registers..................................................................... 11

Serial Bus Interface.....................................................................15

Addressing the Device................................................................15

ALERT

Output............................................................................17

Low Power Standby Mode .........................................................17

Sensor Fault Detection...............................................................17

The ADT7461 Interrupt System ...............................................17

Application Information............................................................19

Thermal Inertia and Self-Heating ............................................19

Layout Considerations ............................................................... 20

Application Circuit .....................................................................21

Outline Dimensions........................................................................22

Ordering Guide...........................................................................23

REVISION HISTORY

12/07- Rev 1: Conversion to ON Semiconductor

7/05—Rev. A to Rev. B

Changes to Features Section .............................................................1

Changes to Applications Section...................................................... 1

Changes to General Description Section ........................................ 1

Changes to Addressing the Device Section ...................................14

Updated Outline Dimensions..........................................................21

Changes to the Ordering Guide......................................................22

10/04—Rev. 0 to Rev. A

Change to SMBus Specifications.....................................................4

Changes to Figure 6 and Figure 10 .................................................7

Added Figure 9 and Figure 13 ......................................................... 7

Changes to Temperature Measurement Section .........................10

Changes to Figure 19 and Figure 25 .............................................16

Changes to Serial Bus Interface Section.......................................23

10/03—Revision 0: Initial Version

Rev. 3 | Page 2 of 23 | www.onsemi.com

ADT7461

SPECIFICATIONS

TA = −40°C to +120°C, VDD = 3 V to 5.5 V, unless otherwise noted.

Table 1.

Parameter Min Typ Max Unit Test Conditions

POWER SUPPLY

Supply Voltage, VDD 3.0 3.30 5.5 V Average Operating Supply Current, IDD 170 215 μA 0.0625 conversions/sec rate1

5.5 10 μA Standby mode, –40°C ≤ TA ≤ +85°C

5.5 20 μA Standby mode, +85°C ≤ TA ≤ +120°C Undervoltage Lockout Threshold 2.2 2.55 2.8 V VDD input, disables ADC, rising edge Power-On-Reset Threshold 1 2.5 V

TEMPERATURE-TO-DIGITAL CONVERTER

Local Sensor Accuracy ±1 ±3 °C −40°C ≤ TA ≤ +100°C, 3 V ≤ VDD ≤ 3.6 V Resolution 1 °C Remote Diode Sensor Accuracy ±1 °C +60°C ≤ TA ≤ +100°C, −55°C ≤ T ±3 °C −40°C ≤ TA ≤ +120°C, −55°C ≤ T Resolution 0.25 °C Remote Sensor Source Current 96 μA High level3 36 μA Middle level3 6 μA Low level3 Conversion Time 32.13 114.6 ms From stop bit to conversion complete (both channels),

one-shot mode with averaging switched on

3.2 12.56 ms One-shot mode with averaging off (that is, conversion rate = 16, 32, or 64 conversions per second)

Maximum Series Resistance Cancelled 3 kΩ Resistance split evenly on both the D+ and D– inputs

OPEN-DRAIN DIGITAL OUTPUTS

THERM, ALERT/THERM2

(

) Output Low Voltage, VOL 0.4 V I High Level Output Leakage Current, IOH 0.1 1 μA V ALERT

Output Low Sink Current

= −6.0 mA3

OUT

= V

OUT

1 mA

ALERT

forced to 0.4 V

SMBus INTERFACE3, 4

Logic Input High Voltage, VIH 2.1 V 3 V ≤ VDD ≤ 3.6 V

SCLK, SDATA

Logic Input Low Voltage, VIL 0.8 V 3 V ≤ VDD ≤ 3.6 V

SCLK, SDATA Hysteresis 500 mV SMBus Output Low Sink Current 6 mA SDATA forced to 0.6 V Logic Input Current, IIH, IIL −1 +1 μA SMBus Input Capacitance, SCLK, SDATA 5 pF SMBus Clock Frequency 400 kHz SMBus Timeout5 25 64 ms User programmable SCLK Falling Edge to SDATA Valid Time 1 μs Master clocking in data

See Table 8 for information on other conversion rates.

Guaranteed by characterization, but not production tested.

Guaranteed by design, but not production tested.

See the SMBUS Timing Specifications section for more information.

Disabled by default; see the Serial Bus Interface section for details on enabling it.

≤ +150°C, 3 V ≤ VDD ≤ 3.6 V

≤ +150°C, 3 V ≤ VDD ≤ 5.5 V

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ADT7461

SMBus TIMING SPECIFICATIONS

Table 2. SMBus Timing Specifications

Parameter Limit at T

400 kHz max

SCLK

1.3 μs min Clock low period, between 10% points

LOW

0.6 μs min Clock high period, between 90% points

HIGH

MIN

and T

tR 300 ns max Clock/data rise time tF 300 ns max Clock/data fall time t

600 ns min Start condition setup time

SU; STA

600 ns min Start condition hold time

HD; STA

100 ns min Data setup time

SU; DAT

300 ns min Data hold time

HD; DAT

600 ns min Stop condition setup time

SU; STO

1.3 μs min Bus free time between stop and start conditions

BUF

Guaranteed by design, but not production tested.

Time from 10% of SDATA to 90% of SCLK.

Time for 10% or 90% of SDATA to 10% of SCLK.

Time for 90% of SCLK to 10% of SDATA.

Unit Description

MAX

SCLK

SDATA

HD;DAT

LOW

HD;STA

BUF

STOP START STOPSTART

HIGH

SU;DAT

Figure 2. Serial Bus Timing

SU;STA

HD;STA

SU;STO

04110-0-001

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ADT7461

ABSOLUTE MAXIMUM RATINGS

Table 3.

Parameter Rating

Positive Supply Voltage (VDD) to GND −0.3 V, +5.5 V D+ −0.3 V to VDD + 0.3 V D− to GND −0.3 V to +0.6 V

SCLK, SDATA, THERM

Input Current, SDATA, Input Current, D− ±1 mA ESD Rating, All Pins (Human Body Model) 2000 V Maximum Junction Temperature (TJ max) 150°C Storage Temperature Range −65°C to +150°C IR Reflow Peak Temperature 220°C

Pb-Free Parts Only 260°C (±0.5°C)

Lead Temperature (Soldering 10 sec) 300°C

ALERT

THERM

−0.3 V to +5.5 V

−0.3 V to VDD + 0.3 V

−1 mA, +50 mA

THERMAL CHARACTERISTICS

8-lead SOIC package

= 121°C/W

8-lead MSOP package

= 142°C/W

Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the device at these or any other conditions above those indicated in the operational section of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.

ESD CAUTION

ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on the human body and test equipment and can discharge without detection. Although this product features proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance degradation or loss of functionality.

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ADT7461

PIN CONFIGURATION AND FUNCTION DESCRIPTIONS

THERM

D+

D–

ADT7461

TOP VIEW

(Not to Scale)

SCLK

SDATA

ALERT/THERM2

GND

04110-0-013

Figure 3. Pin Configuration

Table 4. Pin Function Descriptions

Pin No. Mnemonic Description

1 VDD Positive Supply, 3 V to 5.5 V. 2 D+ Positive Connection to Remote Temperature Sensor. 3 D− Negative Connection to Remote Temperature Sensor.

THERM

Open-drain output that can be used to turn a fan on/off or throttle a CPU clock in the event of an overtemperature condition. Requires pull-up to V

5 GND Supply Ground Connection.

THERM2

ALERT

Open-Drain Logic Output Used as Interrupt or SMBus Alert. This may also be configured as a second

output. Requires pull-up resistor. 7 SDATA Logic Input/Output, SMBus Serial Data. Open-Drain Output. Requires pull-up resistor. 8 SCLK Logic Input, SMBus Serial Clock. Requires pull-up resistor.

THERM

Rev. 3 | Page 6 of 23 | www.onsemi.com

ADT7461

TYPICAL PERFORMANCE CHARACTERISTICS

D+ TO GND

250mV EXTERNAL

100mV INTERNAL

–20

D+ TO V

–40

TEMPERATURE ERROR (°C)

–60

–80

0 20406080

100

LEAKAGE RESISTANCE (MΩ)

Figure 4. Temperature Error vs. Leakage Resistance

–0.1

–0.2

–0.3

–0.4

–0.5

–0.6

TEMPERATURE ERROR (°C)

–0.7

–0.8

–3 –10 10 30 50 70 90 110 130 150

TEMPERATURE (°C)

Figure 5. Temperature Error vs. Actual Temperature Using 2N3906

–5

TEMPERATURE ERROR (°C)

–10

04110-0-017

–15

020

250mV INTERNAL

FREQUENCY (MHz)

100mV EXTERNAL

04110-0-015

Figure 7. Temperature Error vs. Power Supply Noise Frequency

–10

–20

–30

–40

–50

TEMPERATURE ERROR (°C)

–60

04110-0-022

–70

0 5 10 15 20

CAPACITANCE (nF)

04110-0-018

Figure 8. Temperature Error vs. Capacitance Between D+ and D−

TEMPERATURE ERROR (°C)

–1

–2

0 100 200 300

40mV NO FILTER 60mV NO FILTER 40mV WITH FILTER 60mV WITH FILTER

FREQUENCY (MHz)

400 500 600

Figure 6. Temperature Error vs. Differential Mode Noise Frequency

(With and Without R-C-R Filter of 100 Ω–2.2 nF–100 Ω)

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180

160

140

120

100

TEMPERATURE ERROR (°C)

04110-0-027

–20

0 100 200 300

FREQUENCY (MHz)

100mV NO FILTER

100mV WITH FILTER

400 500 600

04110-0-024

Figure 9. Temperature Error vs. 100 mV Differential Mode Noise Frequency

(With and Without R-C-R Filter of 100 Ω–2.2 nF–100 Ω)

ADT7461

40mV NO FILTER 60mV NO FILTER 40mV WITH FILTER 60mV WITH FILTER

100mV NO FILTER

TEMPERATURE ERROR (°C)

–1

0 100 200 300

FREQUENCY (MHz)

400 500 600

Figure 10. Temperature Error vs. Common-Mode Noise Frequency

(With and Without R-C-R Filter of 100 Ω–2.2 nF–100 Ω)

(μA)

0 100 200 300

SCL CLOCK FREQUENCY (kHz)

5.5V

40050 150 250 350

Figure 11. Standby Supply Current vs. Clock Frequency

TEMPERATURE ERROR (°C)

04110-0-025

–5

0 100 200 300

FREQUENCY (MHz)

100mV WITH FILTER

400 500 600

04110-0-026

Figure 13. Temperature Error vs. 100 mV Common-Mode Noise Frequency

(With and Without R-C-R Filter of 100 Ω–2.2 nF–100 Ω)

800

700

600

500

(μA)

400

300

200

100

04110-0-020

0.01 0.1 1 10

CONVERSION RATE (Hz)

5.5V

04110-0-019

100

Figure 14. Operating Supply Current vs. Conversion Rate

(μA)

3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.4

(V)

Figure 12. Standby Current vs. Supply Voltage

04110-0-021

TEMPERATURE ERROR (°C)

–5

0 2 10 200 1k 2k 3k 4k

SERIES RESISTANCE (Ω)

3.3V T = –30

3.3V T = +25

3.3V T = +120

5.5V T = –30

5.5V T = +25

5.5V T = +120

Figure 15. Temperature Error vs. Series Resistance

04110-0-023

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ADT7461

FUNCTIONAL DESCRIPTION

The ADT7461 is a local and remote temperature sensor and over/under temperature alarm, with the added ability to automatically cancel the effect of 3 kΩ (typical) of resistance in series with the temperature monitoring diode. When the ADT7461 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 the remote temperature sensor. The ADC digitizes these signals and the results are stored in the local and remote temperature value registers.

The local and remote measurement results are compared with the corresponding high, low, and

stored in eight 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 an external diode fault causes the

low. Exceeding

THERM

output to assert low. The

as a second

THERM

temperature limits causes the

ALERT

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 6 between

and selecting the conversion rate.

SERIES RESISTANCE CANCELLATION

Parasitic resistance to the D+ and D− inputs to the ADT7461, 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’s temperature measurement. This error typically causes a 0.5°C offset per ohm of parasitic resistance in series with the remote diode.

The ADT7461 automatically cancels out the effect of this series resistance on the temperature reading, giving a more accurate result, without the need for user characterization of this resistance. The ADT7461 is designed to automatically cancel typically up to 3 kΩ 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.

THERM

temperature limits,

ALERT

output to assert

THERM

output can be reprogrammed

ALERT

and

THERM2

TEMPERATURE MEASUREMENT METHOD

A simple method of measuring temperature is to exploit the negative temperature coefficient of a diode by measuring the base-emitter voltage (V current. However, this technique requires calibration to null out the effect of the absolute value of V to device.

The technique used in the ADT7461 is to measure the change

when the device is operated at three different currents.

in V

Previous devices have used only two operating currents, but it is the use of a third current that allows automatic cancellation of resistances in series with the external temperature sensor.

Figure 16 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 will not be grounded and should be 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 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 on C1.

To m e as u re Δ V

switched among three related currents. Figure 16 shows N1 × I and N2 × I as different multiples of the current, I. The currents through the temperature diode are switched between I and N1 × I, giving ΔV giving ΔV two ΔV

. The temperature may then be calculated using the

BE2

measurements. This method can also be shown to

cancel the effect of any series resistance on the temperature measurement.

The resulting ΔV 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 ΔV age 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 per second, no digital averaging takes place.

Signal conditioning and measurement of the internal temperature sensor is performed in the same manner.

) of a transistor operated at constant

, which varies from device

, the operating current through the sensor is

, and then between I and N2 × I,

BE1

waveforms are passed through a 65 kHz

. The ADC digitizes this volt-

Rev. 3 | Page 9 of 23 | www.onsemi.com

ADT7461

×IN2×I

D+

REMOTE SENSING

TRANSISTOR

C1*

D–

*CAPACITOR C1 IS OPTIONAL. IT SHOULD ONLY BE USED IN NOISY ENVIRONMENTS.

BIAS

DIODE

Figure 16. Input Signal Conditioning

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 value is in Register 0x00 and has a resolution of 1°C. The external temperature value is stored in two registers, with the upper byte in Register 0x01 and the lower byte in Register 0x10. Only the two MSBs in the external temperature low byte are used. This gives the external temperature measurement a resolution of 0.25°C. Table 5 shows the data format for the external temperature low byte.

Table 5. 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 external temperature value, both the high and low byte, the two registers should be read in succession. Reading one register does not lock the other, so both should be read before the next conversion finishes. In practice, there is more than enough time to read both registers, as transactions over the SMBus are significantly faster than a conversion time.

TEMPERATURE MEASUREMENT RANGE

The temperature measurement range for both internal and external measurements is, by default, 0°C to +127°C. However, the ADT7461 can be operated using an extended temperature range. It can measure the full temperature range of an external diode, from −55°C to +150°C. The user can switch between these two temperature ranges by setting or clearing Bit 2 in the configuration 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 that can be measured by the ADT7461 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

BIAS

LOW-PASS FILTER

= 65kHz

range of −55°C to +150°C.

Above 150°C, they may lose their semiconductor characteristics and approximate conductors instead. This results in a diode short. In this case, a read of the temperature result register gives the last good temperature measurement. The user should be aware that the temperature measurement on the external channel may not be accurate for temperatures that are outside the operating range of the remote sensor.

While both local and remote temperature measurements can be made while the part is in extended temperature mode, the ADT7461 itself should not be exposed to temperatures greater than those specified in the Absolute Maximum Ratings section. Also, the device is guaranteed to operate only as specified at ambient temperatures from −40°C to +120°C.

TEMPERATURE DATA FORMAT

The ADT7461 has two temperature data formats. When the temperature measurement range is from 0°C to +127°C (default), the temperature data format for both internal and external temperature results is binary. When the measurement range is in extended mode, an offset binary data format is used for both internal and external results. Temperature values in the offset binary data format are offset by 64°C. Examples of temperatures in both data formats are shown in Table 6.

Table 6. Temperature Data Format (Local and Remote Temperature High Byte)

Temperature Binary Offset Binary1

–55°C 0 000 00002 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 11113 1 101 0110

Offset binary scale temperature values are offset by 64°C.

Binary scale temperature measurement retur ns 0°C for all temperatures < 0°C.

Binary scale temperature measurement returns 127°C for all temperatures > 127°C.

OUT+

TO ADC

OUT–

04110-0-002

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ADT7461

The user can switch between measurement ranges 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 are not changed. The user must ensure that the limit registers are reprogrammed, as necessary, when the data format changes. See the Limit Registers section for more information.

ADT7461 REGISTERS

The ADT7461 contains a total of 22 8-bit registers. 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. Additional details are provided in Table 7 to Table 11.

Address Pointer Register

The address pointer register 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 ADT7461, which is stored in the address pointer register. This register address is written to by the second byte of a write operation or is used for a subsequent read operation.

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.

Temperature Value Registers

The ADT7461 has three 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 by the user over the SMBus. The local temperature value register is at Address 0x00.

The external temperature value high byte register is at Address 0x01, with the low byte register at Address 0x10. The power-on default for all three registers is 0x00.

Configuration Register

The configuration register is Address 0x03 at read and Address 0x09 at write. Its power-on default is 0x00. Only four bits of the configuration register are used. Bits 0, 1, 3, and 4 are reserved and should not be written to by the user.

Bit 7 of the configuration register is used to mask the

ALERT

output. If Bit 7 is 0, the

power-on default. If Bit 7 is set to 1, the

output is enabled. This is the

ALERT

disabled. This only applies if Pin 6 is configured as

Pin 6 is configured as

THERM2

, the value of Bit 7 has no effect.

ALERT

output is

ALERT

. If

If Bit 6 is set to 0 (the power-on default), the device is in operating mode with the ADC converting. If Bit 6 is set to 1, the device is in standby mode and the ADC does not convert. The SMBus does, however, remain active in standby mode, so values can be read from or written to the ADT7461 via the SMBus in this mode. The

ALERT

and

THERM

outputs are also active in standby mode. Changes made to the registers in standby mode that affect the

THERM

ALERT

outputs cause these signals to

be updated.

Bit 5 determines the configuration of Pin 6 on the ADT7461. If Bit 5 is 0 (default), then Pin 6 is configured as an

output. If Bit 5 is 1, then Pin 6 is configured as a

ALERT

output. Bit 7, the

configured as an

mask bit, is only active when Pin 6 is

ALERT

output. If Pin 6 is set up as a

ALERT

THERM2

output, then Bit 7 has no effect.

Bit 2 sets the temperature measurement range. If Bit 2 is 0 (default), the temperature measurement range is set between 0°C to +127°C. Setting Bit 2 to 1 means that the measurement range is set to the extended temperature range.

Table 7. Configuration Register Bit Assignments

Power-On

Bit Name Function

ALERT

7 MASK1

6 RUN/STOP

ALERT/THERM2

4, 3 Reserved 0

Temperature

Range Select

1, 0 Reserved 0

0 = 1 =

0 = Run 1 = Standby

0 = 1 =

0 = 0°C to 127°C 1 = Extended range

enabled

ALERT

masked

ALERT THERM2

Default

Conversion Rate Register

The conversion rate register is Address 0x04 at read and Address 0x0A at write. The lowest four bits of this register are used to program the conversion rate by dividing the internal oscillator clock by 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, or 1024 to give conversion times from 15.5 ms (Code 0x0A) to 16 seconds (Code 0x00). For example, a conversion rate of 8 conversions per second means that beginning at 125 ms intervals, the device performs a conversion on the internal and external temperature channels.

This register can be written to and read back over the SMBus. The higher four bits of this register are unused and must be set to 0. The default value of this register is 0x08, giving a rate of 16 conversions per second. Use of slower conversion times greatly reduces the device power consumption, as shown in

Table 8.

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ADT7461

Table 8. Conversion Rate Register Codes

Average Supply Current

Code Conversion/Second

0x00 0.0625 121.33 0x01 0.125 128.54 0x02 0.25 131.59 0x03 0.5 146.15 0x04 1 169.14 0x05 2 233.12 0x06 4 347.42 0x07 8 638.07 0x08 16 252.44 0x09 32 417.58 0x0A 64 816.87 0x0B to 0xFF Reserved

Limit Registers

The ADT7461 has eight limit registers: high, low, and temperature limits for both local and remote temperature measurements. The remote temperature high and low limits span two registers each to contain an upper and lower byte for each limit. There is also a

THERM

registers can be written to and read back over the SMBus. See Table 12 for address details of the limit registers and their power-on default values.

When Pin 6 is configured as an

ALERT

registers perform a > comparison while the low limit registers perform a ≤ comparison. For example, if the high limit register is programmed with 80°C, then measuring 81°C results in an out-of-limit condition, setting a flag in the status register. If the low limit register is programmed with 0°C, measuring 0°C or lower results in an out-of-limit condition.

Exceeding either the local or remote

THERM

low. When Pin 6 is configured as

either the local or remote high limit asserts default hysteresis value of 10°C is provided that applies to both THERM

channels. This hysteresis value may be reprogrammed to any value after power-up (Register Address 0x21).

It is important to remember that the temperature limits data format is the same as the temperature measurement data format. So, if the temperature measurement uses default binary, the temperature limits also use the binary scale. If the temperature measurement scale is switched, however, the temperature limits do not switch automatically. The user must reprogram the limit registers to the desired value in the correct data format. For example, if the remote low limit is set at 10°C and the default binary scale is being used, the limit register value should be 0000 1010b. If the scale is switched to offset binary, the value in the low temperature limit register should be reprogrammed to be 0100 1010b.

Status Register

The status register is a read-only register at Address 0x02. It

μA Typ at VDD = 5.5 V

THERM

hysteresis register. All limit

output, the high limit

THERM

limit asserts

THERM2

, exceeding

low. A

contains status information for the ADT7461.

Bit 7 of the status register indicates the ADC is busy converting when it is high. The other bits in this register flag the out-oflimit temperature measurements (Bits 6 to 3 and Bits 1 to 0) and the remote sensor open circuit (Bit 2).

If Pin 6 is configured as an

ALERT

output, the following applies. If the local temperature measurement exceeds its limits, Bit 6 (high limit) or Bit 5 (low limit) of the status register asserts to flag this condition. If the remote temperature measurement exceeds its limits, then Bit 4 (high limit) or Bit 3 (low limit) asserts. Bit 2 asserts to flag an open-circuit condition on the remote sensor. These five flags are NOR’d together so if any of them is high, the ALERT

interrupt latch is set and the

ALERT

output goes low.

Reading the status register clears the five flags, Bits 6 to 2, provided the error conditions causing the flags to be set have gone away. A flag bit can be reset only if the corresponding value register contains an in-limit measurement or if the sensor is good.

ALERT

The

interrupt latch is not reset by reading the status

ALERT

output has been serviced by the master reading the device address, provided the error condition has gone away and the status register flag bits are reset.

When Flag 1 and/or Flag 0 are set, the

THERM

output goes low to indicate the temperature measurements are outside the programmed limits. The

reset, unlike the

ALERT

THERM

output does not need to be

output. Once the measurements are within the limits, the corresponding status register bits are reset automatically and the

add hysteresis by programming Register 0x21. The

THERM

output goes high. The user may

THERM

output is reset only when the temperature falls to limit value minus hysteresis value.

When Pin 6 is configured as

THERM2

, only the high temperature limits are relevant. If Flag 6 and/or Flag 4 are set, the THERM2

output goes low to indicate the temperature measurements are outside the programmed limits. Flag 5 and Flag 3 have no effect on

otherwise the same as

THERM2

THERM

. The behavior of

THERM2

Table 9. Status Register Bit Assignments

Bit Name Function

7 BUSY 1 when ADC is converting 6 LHIGH1 1 when local high temperature limit is tripped 5 LLOW1 1 when local low temperature limit is tripped 4 RHIGH1 1 when remote high temperature limit is tripped 3 RLOW1 1 when remote low temperature limit is tripped 2 OPEN1 1 when remote sensor is an open circuit

1 RTHRM

0 LTHRM

These flags stay high until the status register is read or they are reset by POR.

1 when remote

1 when local

THERM

limit is tripped

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ADT7461

Offset Register

Offset errors may be introduced into the remote temperature measurement by clock noise or by the thermal diode being located away from the hot spot. To achieve the specified accuracy on this channel, these offsets must be removed.

The offset value is stored as a 10-bit, twos complement value in Registers 0x11 (high byte) and 0x12 (low byte, left justified). Only the upper 2 bits of Register 0x12 are used. The MSB of Register 0x11 is the sign bit. The minimum offset that can be programmed is −128°C, and the maximum is +127.75°C. The value in the offset register is added to the measured value of the remote temperature.

The offset register powers up with a default value of 0°C and has no effect unless the user writes a different value to it.

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ADT7461

Table 10. Sample Offset Register Codes

Offset Value 0x11 0x12

−128°C 1000 0000 00 00 0000

−4°C 1111 1100 00 00 0000

−1°C 1111 1111 00 000000

−0.25°C 1111 1111 10 00 0000 0°C 0000 0000 00 00 0000 +0.25°C 0000 0000 01 00 0000 +1°C 0000 0001 00 00 0000 +4°C 0000 0100 00 00 0000 +127.75°C 0111 1111 11 00 0000

One-Shot Register

The one-shot register is used to initiate a conversion and comparison cycle when the ADT7461 is in standby mode, after which the device returns to standby. Writing to the one-shot register address (0x0F) causes the ADT7461 to perform a conversion and comparison on both the internal and the external temperature channels. This is not a data register as such; the write operation to Address 0x0F causes the one-shot conversion. The data written to this address is irrelevant and is not stored.

Table 12. List of Registers

Read Address (Hex) Write Address (Hex) Name Power-On Default

Not applicable Not applicable Address Pointer Undefined 0x00 Not applicable Local Temperature Value 0000 0000 (0x00) 0x01 Not applicable External Temperature Value High Byte 0000 0000 (0x00) 0x02 Not applicable Status Undefined 0x03 0x09 Configuration 0000 0000 (0x00) 0x04 0x0A Conversion Rate 0000 1000 (0x08) 0x05 0x0B Local Temperature High Limit 0101 0101 (0x55) (85°C) 0x06 0x0C Local Temperature Low Limit 0000 0000 (0x00) (0°C) 0x07 0x0D External Temperature High Limit High Byte 0101 0101 (0x55) (85°C) 0x08 0x0E External Temperature Low Limit High Byte 0000 0000 (0x00) (0°C) Not applicable 0x0F1 One-Shot 0x10 Not applicable External Temperature Value Low Byte 0000 0000 0x11 0x11 External Temperature Offset High Byte 0000 0000 0x12 0x12 External Temperature Offset Low Byte 0000 0000 0x13 0x13 External Temperature High Limit Low Byte 0000 0000 0x14 0x14 External Temperature Low Limit Low Byte 0000 0000

0x19 0x19

0x20 0x20

0x21 0x21

0x22 0x22

0xFE Not applicable Manufacturer ID 0100 0001 (0x41) 0xFF Not applicable Die Revision Code 0101 0001 (0x51)

Writing to Address 0x0F causes the ADT7461 to perform a single measurement. It is not a data register; therefore, data written to it is irrelevant.

THERM

External

THERM

Local THERM

Hysteresis

Consecutive

Consecutive ALERT Register

The value written to this register determines how many out-oflimit measurements must occur before an

ALERT

is generated. The default value is that one out-of-limit measurement generates an

ALERT

. The maximum value that can be chosen is

4. The purpose of this register is to allow the user to perform some filtering of the output. This is particularly useful at the fastest three conversion rates, where no averaging takes place. This register is at Address 0x22.

Table 11. Consecutive ALERT

Number of Out-of-Limit

Measurements Required

yxxx 000x 1 yxxx 001x 2 yxxx 011x 3 yxxx 111x 4

x = don’t care bit.

y = SMBus timeout bit; default = 0. See the Serial Bus Interface section.

Limit

ALERT

0110 1100 (0x55) (85°C)

0101 0101 (0x55) (85°C)

0000 1010 (0x0A) (10°C)

0000 0001 (0x01)

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ADT7461

SERIAL BUS INTERFACE

Control of the ADT7461 is carried out via the serial bus. The ADT7461 is connected to this bus as a slave device, under the control of a master device.

After a conversion sequence completes, there should be no SMBus transactions to the ADT7461 for at least one conversion time, to allow the next conversion to complete. The conversion time depends on the value programmed in the conversion rate register.

The ADT7461 has an SMBus timeout feature. When this is enabled, the SMBus times out typically after 25 ms of inactivity. However, this feature is not enabled by default. Bit 7 of the consecutive alert register (Address = 0x22) should be set to enable it.

Consult the SMBus 1.1 specification for more information (www.smbus.org).

ADDRESSING THE DEVICE

In general, every SMBus device has a 7-bit device address, except for some devices that have extended 10-bit addresses. When the master device sends a device address over the bus, the slave device with that address responds. The ADT7461 is available with one device address, 0x4C (1001 100b). The ADT7461-2 is also available with one device address, 0x4D (1001 101b)

The serial bus protocol operates as follows:

1. The master initiates data transfer by establishing a start

condition, defined as a high-to-low transition on the serial data line SDATA, while the serial clock line SCLK remains high. This indicates that an address/data stream will follow. All slave peripherals connected to the serial bus respond to the start condition and shift in the next eight bits, consisting of a 7-bit address (MSB first) plus an R/ determines the direction of the data transfer, that is, whether data will be written to or read from the slave device. The peripheral whose address corresponds to the transmitted address responds by pulling the data line low during the low period before the ninth clock pulse, known as the acknowledge bit. All other devices on the bus now remain idle while the selected device waits for data to be read from or written to it. If the R/

writes to the slave device. If the R/

reads from the slave device.

bit, which

bit is a 0, the master

bit is a 1, the master

2. Data is sent over the serial bus in a sequence of nine clock

pulses, eight bits of data followed by an acknowledge bit from the slave device. Transitions on the data line must occur during the low period of the clock signal and remain stable during the high period, since a low-to-high transition when the clock is high may be interpreted as a stop signal. The number of data bytes that can be transmitted over the serial bus in a single read or write operation is limited only by what the master and slave devices can handle.

3. When all data bytes have been read or written, stop condi-

tions are established. In write mode, the master pulls the data line high during the tenth clock pulse to assert a stop condition. In read mode, the master device overrides the acknowledge bit by pulling the data line high during the low period before the ninth clock pulse. This is known as a no acknowledge. The master then takes the data line low during the low period before the tenth clock pulse, then high during the tenth clock pulse to assert a stop condition.

Any number of bytes of data may be transferred over the serial bus in one operation, but it is not possible to mix read and write in one operation because the type of operation is determined at the beginning and cannot subsequently be changed without starting a new operation. With the ADT7461, write operations contain either one or two bytes, while read operations contain one byte.

To write data to one of the device data registers or to read data from it, the address pointer register must be set so that the correct data register is addressed. The first byte of a write operation always contains a valid address that is stored in the address pointer register. If data is to be written to the device, the write operation contains a second data byte that is written to the register selected by the address pointer register.

This is illustrated in Figure 17. The device address is sent over the bus followed by R/

bytes. The first data byte is the address of the internal data register to be written to, which is stored in the address pointer register. The second data byte is the data to be written to the internal data register. The examples shown in Figure 17 to Figure 19 use the ADT7461 SMBus Address 0x4C.

set to 0. This is followed by two data

Rev. 3 | Page 15 of 23 | www.onsemi.com

ADT7461

1919

SCLK

SDAT

START BY

MASTER

A5 A4 A3 A2 A1 A0 R/W D7

FRAME 1

SERIAL BUS ADDRESS BYTE

SCLK (CONTINUED)

SDATA (CONTINUED)

ACK. BY

ADT7461

D7 D6 D5 D4 D3

D6 D5 D4 D3 D2 D1 D0

ADDRESS POINTER REGISTER BYTE

FRAME 2

FRAME 3

DATA BYTE

D1 D0

ACK. BY ADT7461

ACK. BY

ADT7461

STOP BY

MASTER

04110-0-003

Figure 17. Writing a Register Address to the Address Pointer Register, then Writing Data to the Selected Register

1919

SCLK

DAT

A6 A5 A4 A3 A2 A1 A0 R/W D7 D6 D5 D4 D3 D2 D1 D0

START BY

MASTER

FRAME 1

SERIAL BUS ADDRESS BYTE

ACK. BY

ADT7461

ADDRESS POINTER REGISTER BYTE

FRAME 2

ACK. BY ADT7461

STOP BY

MASTER

04110-0-004

Figure 18. Writing to the Address Pointer Register Only

191 9

SCLK

DAT

START BY

MASTER

A6 A5 A4 A3 A2 A1 A0 R/W D7 D6 D5 D4 D3 D2 D1 D0

ACK. BY

FRAME 1

SERIAL BUS ADDRESS BYTE

ADT7461

Figure 19. Reading from a Previously Selected Register

When reading data from a register there are two possibilities.

1. If the ADT7461’s address pointer register value is unknown

or not the desired value, it is necessary to set it to the correct value before data can be read from the desired data register. This is done by writing to the ADT7461 as before, but only the data byte containing the register read address is sent, since data is not to be written to the register. This is shown in Figure 18.

A read operation is then performed consisting of the serial bus address, R/

bit set to 1, followed by the data byte

read from the data register. This is shown in Figure 19.

2. If the address pointer register is known to be at the desired

address, data can be read from the corresponding data register without first writing to the address pointer register

FRAME 2

DATA BYTE FROM ADT7461

NACK. BY

MASTER

STOP BY MASTER

04110-0-005

and the bus transaction shown in Figure 18 can be omitted.

Although it is possible to read a data byte from a data register without first writing to the address pointer register, if the address pointer register is already at the correct value, it is not possible to write data to a register without writing to the address pointer register because the first data byte of a write is always written to the address pointer register.

Also, some of the registers have different addresses for read and write operations. The write address of a register must be written to the address pointer if data is to be written to that register, but it may not be possible to read data from that address. The read address of a register must be written to the address pointer before data can be read from that register.

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ADT7461

ALERT

This is applicable when Pin 6 is configured as an

output. The

OUTPUT

ALERT

output goes low whenever an out-of-limit measurement is detected, or if the remote temperature sensor is open circuit. It is an open-drain output and requires a pull-up to V

. Several

the common line goes low if one or more of the

ALERT

outputs can be wire-ORed together, so

ALERT

outputs

goes low.

ALERT

The

processor, or it may be used as an

output can be used as an interrupt signal to a

SMBALERT

. Slave devices on the SMBus cannot normally signal to the bus master that they want to talk, but the

One or more

SMBALERT

line that is connected to the master. When the

line is pulled low by one of the devices, the

SMBALERT

ALERT

outputs can be connected to a common

function allows them to do so.

procedure shown in Figure 20 occurs.

MASTER RECEIVES SMBALERT

SMBALERT

ALERT RESPONSE

ADDRESS

MASTER SENDS ARA AND READ

COMMAND

Figure 20. Use of SMBALERT

is pulled low.

RDSTART ACK

DEVICE

ADDRESSNOACK

DEVICE SENDS

ITS ADDRESS

STOP

2. Master initiates a read operation and sends the alert

response address (ARA = 0001 100). This is a general call address that must not be used as a specific device address.

3. The device whose

ALERT

output is low responds to the

alert response address and the master reads its device address. As the device address is seven bits, an LSB of 1 is added. The address of the device is now known and can be interrogated in the usual way.

ALERT

4. If the

output is low on more than one device, the

one with the lowest device address has priority, in accordance with normal SMBus arbitration.

5. Once the ADT7461 has responded to the alert response

ALERT

address, it resets its

condition that caused the

SMBALERT

line remains low, the master sends the ARA

output, provided the error

ALERT

no longer exists. If the

again; this sequence continues until all devices whose ALERT

out-puts were low have responded.

04110-0-006

LOW POWER STANDBY MODE

The ADT7461 can be put into low power standby mode by setting Bit 6 of the configuration register. When Bit 6 is low, the ADT7461 operates normally. When Bit 6 is high, the ADC is inhibited, and any conversion in progress is terminated without writing the result to the corresponding value register. The SMBus is still enabled. Power consumption in the standby mode is reduced to less than 10 μA if there is no SMBus activity or 100 μA if there are clock and data signals on the bus. When the device is in standby mode, it is still possible to initiate a one-shot conversion of both channels by writing to the oneshot register (Address 0x0F), after which the device returns to standby. It does not matter what is written to the one-shot register, as all data written to it is ignored. It is also possible to write new values to the limit register while in standby mode. If the values stored in the temperature value registers are now outside the new limits, an

ALERT

is generated even though the

ADT7461 is still in standby.

SENSOR FAULT DETECTION

At its D+ input, the ADT7461 contains internal sensor fault detection circuitry. This circuit can detect situations where an external remote diode is either not connected or incorrectly connected to the ADT7461. A simple voltage comparator trips if the voltage at D+ exceeds V

−1 V (typical), signifying an

open circuit between D+ and D−. The output of this comparator is checked when a conversion is initiated. Bit 2 of the status register (open flag) is set if a fault is detected. If the ALERT

pin is enabled, setting this flag causes

ALERT

to assert low. If the user does not wish to use an external sensor with the ADT7461, then to prevent continuous setting of the OPEN flag, the user should tie the D+ and D− inputs together.

THE ADT7461 INTERRUPT SYSTEM

The ADT7461 has two interrupt outputs,

Both have different functions and behavior.

ALERT

and responds to violations of software-programmed temperature limits or an open-circuit fault on the external diode. THERM

is intended as a fail-safe interrupt output that cannot be masked. If the external or local temperature exceeds the programmed high temperature limits or equals or exceeds the low temperature limits, the

fault on the external diode also causes

ALERT

output is asserted low. An open-circuit

ALERT

to assert. is reset when serviced by a master reading its device address, provided the error condition has gone away and the status register has been reset.

THERM

The

ture exceeds the programmed

output asserts low if the external or local tempera-

THERM

limits.

temperature limits should normally be equal to or greater than

and

is maskable

THERM

ALERT

Rev. 3 | Page 17 of 23 | www.onsemi.com

ADT7461

the high temperature limits.

when the temperature falls back within the

external limit is set by default to 85°C, as is the local

limit. A hysteresis value can be programmed so that resets when the temperature falls to the limit value minus the hysteresis value. This applies to both local and remote measurement channels. The power-on hysteresis default value is 10°C, but this may be reprogrammed to any value after powerup. The hysteresis loop on the

THERM

is used for on/off control of a fan. The user’s system

can be set up so that when

switched on to cool the system. When the fan can be switched off. Programming an hysteresis value protects from fan jitter where the temperature hovers around

THERM

the

Table 13. THERM

THERM

limit, and the fan is constantly being switched.

Hysteresis

0°C 0 000 0000 1°C 0 000 0001 10°C 0 000 1010

Figure 21 shows how the

A user may choose to use the to signal to the host via the SMBus that the temperature has risen. The user could use the cool the system, if the temperature continues to increase. This method would ensure there is a fail-safe mechanism to cool the system without the need for host intervention.

EMPERATURE

100°C

90°C

80°C

70°C

60°C

50°C

40°C

ALERT

THERM

Figure 21. Operation of the ALERT

1. If the measured temperature exceeds the high temperature

limit, the

ALERT

2. If the temperature continues to increase and exceeds the

THERM

limit, the

used to throttle the CPU clock or switch on a fan.

THERM

is reset automatically

outputs is useful when

asserts, a fan can be

THERM

Binary Representation

THERM

ALERT

THERM

ALERT

and

output as an

output to turn on a fan to

and THERM Interrupts

output asserts low.

THERM

output asserts low. This can be

THERM

limit. The

THERM

goes high again,

outputs operate.

SMBALERT

THERM LIMIT

THERM LIMIT-HYSTERESIS

HIGH TEMP LIMIT

RESET BY MASTER

04110-0-007

default hysteresis value of 10°C is shown in Figure 21.

4. The

ALERT

output deasserts only when the temperature

falls below the high temperature limit, and the master has read the device address and cleared the status register.

ALERT

Pin 6 on the ADT7461 can be configured as either an

output or as an additional

THERM

output.

THERM2

asserts low when the temperature exceeds the programmed local and/or remote high temperature limits. It is reset in the same manner as

hysteresis value applies to

Figure 22 shows how

THERM

, and it is not maskable. The programmed

THERM2

THERM

and

also.

THERM2

might operate together to implement two methods of cooling the system. In this example, the

THERM

limits. The

THERM2

limits are set lower than the

output could be used to turn on a fan. If the temperature continues to rise and exceeds the THERM

limits, the

THERM

output could provide additional

cooling by throttling the CPU.

TEMPERATURE

90°C

80°C

70°C

60°C

50°C

40°C

30°C

THERM2

THERM

1. When the

Figure 22. Operation of the THERM

THERM2

limit is exceeded, the

and THERM2 Interrupts

THERM LIMIT

THERM2 LIMIT

THERM2

signal

04110-0-008

asserts low.

2. If the temperature continues to increase and exceeds the

THERM

3. The

limit, the

THERM

temperature falls to hysteresis value is shown in Figure 22.

THERM

output asserts low.

output deasserts (goes high) when the

THERM

limit minus hysteresis. No

4. As the system continues to cool and the temperature falls

below the

THERM2

limit, the

Again, no hysteresis value is shown for

THERM2

signal resets.

THERM2

Both the external and internal temperature measurements cause THERM

and

THERM2

to operate as described.

3. The

THERM

output deasserts (goes high) when the

temperature falls to

THERM

limit minus hysteresis. The

Rev. 3 | Page 18 of 23 | www.onsemi.com

ADT7461

APPLICATION INFORMATION

Noise Filtering

For temperature sensors operating in noisy environments, the industry standard practice was to place a capacitor across the D+ and D− pins to help combat the effects of noise. However, large capacitances affect the accuracy of the temperature measurement, leading to a recommended maximum capacitor value of 1,000 pF. While this capacitor reduces the noise, it does not eliminate it, making it difficult to use the sensor in a very noisy environment.

The ADT7461 has a major advantage over other devices for eliminating the effects of noise on the external sensor. The series resistance cancellation feature allows a filter to be constructed between the external temperature sensor and the part. The effect of any filter resistance seen in series with the remote sensor is automatically cancelled from the temperature result.

The construction of a filter allows the ADT7461 and the remote temperature sensor to operate in noisy environments. Figure 23 shows a low-pass R-C-R filter with the following values: R = 100 Ω and C = 1 nF. This filtering reduces both commonmode noise and differential noise.

100Ω

REMOTE

EMPERATURE

SENSOR

Figure 23. Filter Between Remote Sensor and ADT7461

Factors Affecting Diode Accuracy

100Ω

Remote Sensing Diode

The ADT7461 is designed to work with substrate transistors built into processors or with discrete transistors. Substrate transistors are generally PNP types with the collector connected to the substrate. Discrete types can be either PNP or NPN transistor connected as a diode (base-shorted to collector). If an NPN transistor is used, the collector and base are connected to D+ and the emitter to D−. If a PNP transistor is used, the collector and base are connected to D− and the emitter to D+.

To reduce the error due to variations in both substrate and discrete transistors, several factors should be taken into consideration:

• The ideality factor, n

, of the transistor is a measure of the

deviation of the thermal diode from ideal behavior. The ADT7461 is trimmed for an n

value of 1.008. The

following equation may be used to calculate the error introduced at a temperature T (°C), when using a transistor whose n sheet for the n

does not equal 1.008. Consult the processor data

values.

ΔT = (n

− 1.008)/1.008 × (273.15 Kelvin + T)

To factor this in, the user can write the ΔT value to the

1nF

D+

D–

04110-0-009

offset register. It is then automatically added to or subtracted from the temperature measurement by the ADT7461.

• Some CPU manufacturers specify the high and low current

levels of the substrate transistors. The high current level of the ADT7461, I

, is 6 μA. If the ADT7461 current levels do not match

LOW

, is 96 μA, and the low level current,

HIGH

the current levels specified by the CPU manufacturer, it may become necessary to remove an offset. The CPUs data sheet advises whether this offset needs to be removed and how to calculate it. This offset may be programmed to the offset register. It is important to note that if more than one offset must be considered, the algebraic sum of these offsets must be programmed to the offset register.

If a discrete transistor is being used with the ADT7461, the best accuracy is obtained by choosing devices according to the following criteria:

• Base-emitter voltage greater than 0.25 V at 6 μA, at the

highest operating temperature.

• Base-emitter voltage less than 0.95 V at 100 μA, at the

lowest operating temperature.

• Base resistance less than 100 Ω.

• Small variation in h

control of V

(50 to 150) that indicates tight

characteristics.

Transistors, such as the 2N3904, 2N3906, or equivalents in SOT-23 packages are suitable devices to use.

THERMAL INERTIA AND SELF-HEATING

Accuracy depends on the temperature of the remote sensing diode and/or the internal temperature sensor being at the same temperature as the environment being measured; many factors can affect this. Ideally, the sensor should be in good thermal contact with the part of the system being measured. If it is not, the thermal inertia caused by the sensor’s mass causes a lag in the response of the sensor to a temperature change. With a remote sensor, this should not be a problem since it will be either a substrate transistor in the processor or a small package device, such as the SOT-23, placed in close proximity to it.

The on-chip sensor, however, is often remote from the processor and only monitors the general ambient temperature around the package. The thermal time constant of the SOIC-8 package in still air is about 140 seconds, and if the ambient air temperature quickly changed by 100 degrees, it would take about 12 minutes (5 time constants) for the junction temperature of the ADT7461 to settle within 1 degree of this. In practice, the ADT7461 package is in electrical, and hence

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ADT7461

thermal, contact with a PCB and may also be in a forced airflow. How accurately the temperature of the board and/or the forced airflow reflects the temperature to be measured also affects the accuracy. Self-heating due to the power dissipated in the ADT7461 or the remote sensor causes the chip temperature of the device or remote sensor to rise above ambient. However, the current forced through the remote sensor is so small that self-heating is negligible. With the ADT7461, the worst-case condition occurs when the device is converting at 64 conversions per second while sinking the maximum current of 1 mA at the

ALERT

power dissipation in the device is about 4.5 mW. The thermal resistance, θ

, of the SOIC-8 package is about 121°C/W.

LAYOUT CONSIDERATIONS

Digital boards can be electrically noisy environments, and the ADT7461 is measuring very small voltages from the remote sensor, so care must be taken to minimize noise induced at the sensor inputs. The following precautions should be taken:

and

THERM

output. In this case, the total

3. Try to minimize the number of copper/solder joints

that can cause thermocouple effects. Where copper/solder joints are used, make sure that they are in both the D+ and D− path and at the same temperature.

Thermocouple effects should not be a major problem as 1°C corresponds to about 200 mV, and thermocouple voltages are about 3 mV/°C of temperature difference. Unless there are two thermocouples with a big temperature differential between them, thermocouple voltages should be much less than 200 mV.

4. Place a 0.1 μF bypass capacitor close to the V

pin. In

extremely noisy environments, an input filter capacitor may be placed across D+ and D− close to the ADT7461. This capacitance can effect the temperature measurement, so care must be taken to ensure any capacitance seen at D+ and D− is a maximum of 1,000 pF. This maximum value includes the filter capacitance plus any cable or stray capacitance between the pins and the sensor diode.

1. Place the ADT7461 as close as possible to the remote

sensing diode. Provided the worst noise sources, such as clock generators, data/address buses, and CRTs, are avoided, this distance can be 4 inches to 8 inches.

2. Route the D+ and D– tracks close together, in parallel, with

grounded guard tracks on each side. To minimize inductance and reduce noise pick-up, a 5 mil track width and spacing is recommended. Provide a ground plane under the tracks if possible.

GND

D+

D–

GND

Figure 24. Typical Arrangement of Signal Tracks

5MIL

04110-0-010

5. If the distance to the remote sensor is more than 8 inches,

the use of twisted pair cable is recommended. This works up to about 6 feet to 12 feet.

For extremely long distances (up to 100 feet), use a shielded twisted pair, such as the Belden No. 8451 microphone cable. Connect the twisted pair to D+ and D− and the shield to GND close to the ADT7461. Leave the remote end of the shield unconnected to avoid ground loops.

Because the measurement technique uses switched current sources, excessive cable or filter capacitance can affect the measurement. When using long cables, the filter capacitance may be reduced or removed.

Rev. 3 | Page 20 of 23 | www.onsemi.com

ADT7461

APPLICATION CIRCUIT

Figure 25 shows a typical application circuit for the ADT7461 using a discrete sensor transistor connected via a shielded, twisted pair cable. The pull-ups on SCLK, SDATA, and

ALERT

are required only if they are not already provided elsewhere in the system.

ADT7461

D+

D–

CPU THERMAL

DIODE

SHIELD2N3906

Figure 25. Typical Application Circuit

SCLK

SDATA

ALERT/

THERM2

THERM

GND

The SCLK and SDATA pins of the ADT7461 can be interfaced directly to the SMBus of an I/O controller, such as the Intel® 820 chipset.

0.1μF

FAN

ENABLE

TYP 10kΩ

FAN

CONTROL

CIRCUIT

3V TO 3.6V

SMBUS

CONTROLLER

5V OR 12V

04110-0-011

Rev. 3 | Page 21 of 23 | www.onsemi.com

ADT7461

OUTLINE DIMENSIONS

4.00 (0.1574)

3.80 (0.1497)

5.00 (0.1968)

4.80 (0.1890)

6.20 (0.2440)

5.80 (0.2284)

1.27 (0.0500) BSC

0.25 (0.0098)

0.10 (0.0040)

COPLANAR ITY

0.10

CONTROLLING DIMENSIONS ARE IN MILLIMETERS; INCH DIMENSIONS (IN PARENTHESES) ARE ROUNDED-OFF MILLIMETER EQUIVALENTS FOR REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN

SEATING

PLANE

COMPLIANT TO JEDEC STANDARDS MS-012-AA

1.75 (0.0688)

1.35 (0.0532)

0.51 (0.0201)

0.31 (0.0122)

0.25 (0.0098)

0.17 (0.0067)

0.50 (0.0196)

0.25 (0.0099)

8°

1.27 (0.0500)

0°

0.40 (0.0157)

Figure 26. 8-Lead Standard Small Outline Package [SOIC_N]

Narrow Body

(R-8)

Dimensions shown in millimeters and (inches)

3.00

BSC

SEATING PLANE

4.90 BSC

1.10 MAX

0.23

0.08

8° 0°

0.80

0.60

0.40

3.00

BSC

PIN 1

0.65 BSC

.15 .00

0.38

0.22

COPLANAR ITY

0.10

COMPLIANT TO JEDEC STANDARDS MO-187-AA

Figure 27. 8-Lead Mini Small Outline Package [MSOP]

(RM-8)

Dimensions shown in millimeters

× 45°

Rev. 3 | Page 22 of 23 | www.onsemi.com

ADT7461

ORDERING GUIDE

Model Temperature Range Package Description Package Option Branding SMBus Address

ADT7461AR −40°C to +125°C 8-Lead SOIC_N R-8 4C ADT7461AR-REEL −40°C to +125°C 8-Lead SOIC_N R-8 4C ADT7461AR-REEL7 −40°C to +125°C 8-Lead SOIC_N R-8 4C ADT7461ARZ1 −40°C to +125°C 8-Lead SOIC_N R-8 4C ADT7461ARZ-REEL1 −40°C to +125°C 8-Lead SOIC_N R-8 4C ADT7461ARZ-REEL71 −40°C to +125°C 8-Lead SOIC_N R-8 4C ADT7461ARM −40°C to +125°C 8-Lead MSOP RM-8 T1B 4C ADT7461ARM-REEL −40°C to +125°C 8-Lead MSOP RM-8 T1B 4C ADT7461ARM-REEL7 −40°C to +125°C 8-Lead MSOP RM-8 T1B 4C ADT7461ARMZ1 −40°C to +125°C 8-Lead MSOP RM-8 T1B 4C ADT7461ARMZ-REEL1 −40°C to +125°C 8-Lead MSOP RM-8 T1B 4C ADT7461ARMZ-R71 −40°C to +125°C 8-Lead MSOP RM-8 T1B 4C ADT7461ARMZ-21 −40°C to +125°C 8-Lead MSOP RM-8 T1F 4D ADT7461ARMZ-2R1 −40°C to +125°C 8-Lead MSOP RM-8 T1F 4D ADT7461ARMZ-2RL71 −40°C to +125°C 8-Lead MSOP RM-8 T1F 4D EVAL-ADT7461EB Evaluation Board

Z = Pb-free part.

ON Semiconductor and are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes without further notice to any products herein. SCILLC makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does SCILLC assume any liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages. “Typical” parameters which may be provided in SCILLC data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All operating parameters, including “Typicals” must be validated for each customer application by customer's technical experts. SCILLC does not convey any license under its patent rights nor the rights of others. SCILLC products are not designed, intended, or authorized for use as components in systems intended for surgical implant into the body, or other applications intended to support or sustain life, or for any other application in which the failure of the SCILLC product could create a situation where personal injury or death may occur. Should Buyer purchase or use SCILLC products for any such unintended or unauthorized application, Buyer shall indemnify and hold SCILLC and its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized use, even if such claim alleges that SCILLC was negligent regarding the design or manufacture of the part. SCILLC is an Equal Opportunity/Affirmative Action Employer. This literature is subject to all applicable copyright laws and is not for resale in any manner.

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Rev. 3 | Page 23 of 23 | www.onsemi.com

ON Semiconductor ADT7461 Technical data

Specifications and Main Features

Frequently Asked Questions

User Manual