Analog Devices dBCool ADT7475 Manual

Remote thermal monitor and fan controller

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FEATURES

Controls and monitors up to 4 fans

High and low frequency fan drive signal

1 on-chip and 2 remote temperature sensors

Extended temperature measurement range, up to 191°C

Automatic fan speed control mode controls system cooling

based on measured temperature

Enhanced acoustic mode dramatically reduces user

perception of changing fan speeds

Thermal protection feature via THERM output

Monitors performance impact of Intel® Pentium® 4 processor

Thermal control circuit via THERM input

3-wire and 4-wire fan speed measurement

Limit comparison of all monitored values

Meets SMBus 2.0 electrical specifications

(fully SMBus 1.1 compliant)

Fully ROHS compliant

Rev. 0

Information furnished by Analog Devices is believed to be accurate and reliable. However, no

responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other

rights of third parties that may result from its use. Specifications subject to change without notice. No

license is granted by implication or otherwise under any patent or patent rights of Analog Devices.

Trademarks and registered trademarks are the property of their respective owners.

FUNCTIONAL BLOCK DIAGRAM

ADT7475

PWM

PWM1

REGISTERS

PWM2

AND

ENHANCEMENT

CONTROLLERS

PWM3

(HF AND LF)

TACH1

TACH2

TACH3

TACH4

PERFORMANCE

THERM

TO ADT7475

D1+

D1–

CONDITIONING

D2+

D2–

MULTIPLEXER

CCP

BAND GAP

TEMP. SENSOR

dBCool

Monitor and Fan Controller

GENERAL DESCRIPTION

The ADT7475 dBCool controller is a thermal monitor and

multiple PWM fan controller for noise-sensitive or power-

sensitive applications requiring active system cooling. The

ADT7475 can drive a fan using either a low or high frequency

drive signal, monitor the temperature of up to two remote

sensor diodes plus its own internal temperature, and measure

and control the speed of up to four fans, so they operate at the

lowest possible speed for minimum acoustic noise.

The automatic fan speed control loop optimizes fan speed

for a given temperature. The effectiveness of the system's

thermal solution can be monitored using the THERM input.

The ADT7475 also provides critical thermal protection to the

system using the bidirectional THERM pin as an output to

prevent system or component overheating.

SCL SDA SMBALERT

SERIAL BUS

INTERFACE

ACOUSTIC

AUTOMATIC

FAN SPEED

CONTROL

FAN

SPEED

COUNTER

MONITORING

THERMAL

PROTECTION

INPUT

10-BIT

SIGNAL

ADC

AND

ANALOG

BAND GAP

REFERENCE

GND

Figure 1.

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.

Tel: 781.329.4700

Fax: 781.461.3113

Remote Thermal

ADT7475

ADDRESS

POINTER

PWM

CONFIGURATION

REGISTERS

INTERRUPT

MASKING

INTERRUPT

STATUS

REGISTERS

LIMIT

COMPARATORS

VALUE AND

LIMIT

REGISTERS

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Summary of Contents for Analog Devices dBCool ADT7475

Page 1 Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Page 2: Table Of Contents
ADT7475 TABLE OF CONTENTS Specifications..................3 Limits, Status Registers, and Interrupts........20 Absolute Maximum Ratings............5 Limit Values ................20 Thermal Characteristics .............. 5 Status Registers ................21 ESD Caution.................. 5 THERM Timer ................23 Pin Configuration and Function Descriptions......6 Fan Drive Using PWM Control ..........
Page 3: Specifications
ADT7475 SPECIFICATIONS to T to V , unless otherwise noted. All voltages are measured with respect to GND, unless otherwise specified. Typicals are at T = 25°C and represent most likely parametric norm. Logic inputs accept input high voltages up to V even when device is operating down to V .
Page 4: Input High Voltage, V
ADT7475 Parameter Unit Test Conditions/Comments DIGITAL INPUT LOGIC LEVELS (TACH INPUTS) Input High Voltage, V Maximum input voltage Input Low Voltage, V −0.3 Minimum input voltage Hysteresis V p-p DIGITAL INPUT LOGIC LEVELS (THERM) ADTL+ Input High Voltage, V 0.75 × V Input Low Voltage, V DIGITAL INPUT CURRENT Input High Current, I...
Page 5: Absolute Maximum Ratings
ADT7475 ABSOLUTE MAXIMUM RATINGS Table 2. Stresses above those listed under Absolute Maximum Ratings Parameter Rating may cause permanent damage to the device. This is a stress Positive Supply Voltage (V 3.6 V rating only; functional operation of the device at these or any Voltage on Any Input or Output Pin −0.3 V to +3.6 V other conditions above those indicated in the operational...
Page 6: Pin Configuration And Function Descriptions
ADT7475 PIN CONFIGURATION AND FUNCTION DESCRIPTIONS PWM1/XTO ADT7475 TACH3 TOP VIEW PWM2/SMBALERT D1– (Not to Scale) TACH1 TACH2 D2– PWM3 TACH4/THERM/GPIO/ SMBALERT Figure 3. Pin Configuration Table 3. Pin Function Descriptions Pin No. Mnemonic Description Digital Input (Open Drain). SMBus serial clock input. Requires SMBus pull-up. Ground Pin for the ADT7475.
Page 7: Typical Performance Characteristics
ADT7475 TYPICAL PERFORMANCE CHARACTERISTICS –10 –20 –30 100mV –40 60mV 40mV –50 –60 –10 100M 200M 300M 400M 500M 600M NOISE FREQUENCY (Hz) CAPACITANCE (nF) Figure 4. Temperature Error vs. Capacitance between D+ and D− Figure 7. Remote Temperature Error vs. Differential Mode Noise Frequency 1.20 1.18 1.16...
Page 8 ADT7475 250mV –2 100mV –4 –0.5 –6 –1.0 –8 –1.5 –10 –2.0 –12 –40 –20 100M 200M 300M 400M 500M 600M OIL BATH TEMPERATURE ( ° C) FREQUENCY (Hz) Figure 12. Remote Temperature Error vs. ADT7475 Temperature Figure 10. Remote Temperature Error vs. Power Supply Noise Frequency –0.5 –1.0 –1.5...
Page 9: Product Description
ADT7475 PRODUCT DESCRIPTION RECOMMENDED IMPLEMENTATION The ADT7475 is a complete thermal monitor and multiple fan controller for any system requiring thermal monitoring and • Configuring the ADT7475 as in Figure 13 allows the cooling. The device communicates with the system via a serial system designer to use the following features: system management bus.
Page 10: Serial Bus Interface
ADT7475 SERIAL BUS INTERFACE In the ADT7475, write operations contain either one or two bytes, and read operations contain one byte. To write data to On PCs and servers, control of the ADT7475 is carried out one of the device data registers or read data from it, the address using the SMBus.
Page 11: Write Operations
ADT7475 START BY ACK. BY ACK. BY STOP BY ADT7475 MASTER ADT7475 MASTER FRAME 1 FRAME 2 SERIAL BUS ADDRESS BYTE ADDRESS POINTER REGISTER BYTE Figure 15. Writing to the Address Pointer Register Only START BY ACK. BY NO ACK. BY STOP BY MASTER ADT7475...
Page 12: Read Operations
ADT7475 If more than one device’s SMBALERT output is low, the The byte write operation is shown in Figure 18. one with the lowest device address has priority in accor- dance with normal SMBus arbitration. SLAVE REGISTER ADDRESS W A DATA ADDRESS Once the ADT7475 has responded to the alert response...
Page 13: Input Circuitry
ADT7475 ADDITIONAL ADC FUNCTIONS FOR VOLTAGE INPUT CIRCUITRY MEASUREMENTS The internal structure for the V analog input is shown in A number of other functions are available on the ADT7475 to Figure 20. The input circuit consists of an input protection offer the system designer increased flexibility.
Page 14 ADT7475 Table 5. 10-Bit A/D Output Code vs. V A/D Output (3.3 V Decimal Binary (10 Bits) <0.0042 <0.00293 00000000 00 0.0042–0.0085 0.0293–0.0058 00000000 01 0.0085–0.0128 0.0058–0.0087 00000000 10 0.0128–0.0171 0.0087–0.0117 00000000 11 0.0171–0.0214 0.0117–0.0146 00000001 00 0.0214–0.0257 0.0146–0.0175 00000001 01 0.0257–0.0300 0.0175–0.0205 00000001 10...
Page 15: Temperature Measurement Method
ADT7475 TEMPERATURE MEASUREMENT METHOD Remote Temperature Measurement Local Temperature Measurement The ADT7475 can measure the temperature of two remote diode sensors or diode-connected transistors connected to Pin 10 and The ADT7475 contains an on-chip band gap temperature sensor Pin 11, or Pin 12 and Pin 13. whose output is digitized by the on-chip, 10-bit ADC.
Page 16 ADT7475 The technique used in the ADT7475 is to measure the change A remote temperature measurement takes nominally 38 ms. The in V when the device is operated at two different currents. results of remote temperature measurements are stored in 10-bit, twos complement format, as shown in Table 6.
Page 17: Factors Affecting Diode Accuracy
ADT7475 Transistors, such as 2N3904, 2N3906, or equivalents in SOT-23 FACTORS AFFECTING DIODE ACCURACY packages, are suitable devices to use. Remote Sensing Diode Table 6. Twos Complement Temperature Data Format The ADT7475 is designed to work with either substrate Temperature Digital Output (10-Bit) transistors built into processors or with discrete transistors.
Page 18: Additional Adc Functions For Temperature Measurement
ADT7475 Reading Temperature from the ADT7475 Changing Bit 1 of Configuration Register 5 (0x7C) changes the resolution and therefore the range of the temperature offset as It is important to note that temperature can be read from the either having a range of –63°C to +127°C, with a resolution of ADT7475 as an 8-bit value (with 1°C resolution) or as a 10-bit 1°C, or having a range of -63°C to +64°C, with a resolution of value (with 0.25°C resolution).
Page 19 ADT7475 PWM duty cycle (Reg. 0x38, Reg. 0x39, and Reg. 0x3A). Single Channel ADC Conversions This effectively runs the fans at the fastest allowed speed. Setting Bit 6 of Configuration Register 2 (Reg. 0x73) places the The fans run at this speed until the temperature drops below ADT7475 into single channel ADC conversion mode.
Page 20: Limits, Status Registers, And Interrupts
ADT7475 LIMITS, STATUS REGISTERS, AND INTERRUPTS LIMIT VALUES Reg. 0x57 TACH2 Minimum High Byte = 0x00 default Associated with each measurement channel on the ADT7475 Reg. 0x58 TACH3 Minimum Low Byte = 0x00 default are high and low limits. These can form the basis of system status monitoring;...
Page 21: Status Registers
ADT7475 Status Register 2 (Reg. 0x42) As mentioned previously, the ADC performs round-robin Bit 7 (D2) = 1, indicates an open or short on D2+/D2– inputs. conversions. The total monitoring cycle time for averaged voltage and temperature monitoring is 146 ms. The total Bit 6 (D1) = 1, indicates an open or short on D1+/D1–...
Page 22 ADT7475 Bit 2 (V ) = 1, masks SMBALERT for V channel. Handling SMBALERT Interrupts To prevent the system from being tied up servicing interrupts, it Bit 0 (V ) = 1, masks SMBALERT for V channel. is recommended to handle the SMBALERT interrupt as follows: Interrupt Mask Register 2 (Reg.
Page 23: Therm Timer
ADT7475 The user can also set up the ADT7475 so when the THERM pin The 8-bit THERM timer register (Reg. 0x79) is designed so is driven low externally, the fans run at 100%. The fans run at that the Bit 0 is set to 1 on the first THERM assertion. Once the 100% for the duration of the time that the THERM pin is cumulative THERM assertion time has exceeded 45.52 ms, Bit 1 pulled low.
Page 24 ADT7475 Generating SMBALERT Interrupts from THERM Timer If the THERM timer value exceeds the THERM timer limit Events value, then the F4P bit (Bit 5) of Status Register 2 is set, and an SMBALERT is generated. Note the F4P bit (Bit 5) of Mask The ADT7475 can generate SMBALERT s when a programma- Register 2 (Reg.
Page 25 ADT7475 Configuring the THERM Behavior Alternatively, OS- or BIOS-level software can timestamp when the system is powered on. If an SMBALERT is gen- Configure the relevant pin as the THERM timer input. erated due to the THERM timer limit being exceeded, Setting Bit 1 ( THERM timer enable) of Configuration another timestamp can be taken.
Page 26: Fan Drive Using Pwm Control
ADT7475 The only other stipulation is that the MOSFET should have a Enabling and Disabling THERM on Individual Channels gate voltage drive, V < 3.3 V, for direct interfacing to the THERM can be enabled/disabled for individual or combina- PWM output pin. The MOSFET should also have a low on tions of temperature channels using bits <7:5>...
Page 27 ADT7475 Because the MOSFET can handle up to 3.5 A, it is simply a Because 4-wire fans are powered continuously, the fan speed is matter of connecting another fan directly in parallel with the not switched on or off as with previous PWM driven/powered first.
Page 28 ADT7475 Fan Speed Measurement If the fan output has a resistive pull-up to 12 V, or other voltage greater than 3.6 V, the fan output can be clamped with a Zener The fan counter does not count the fan TACH output pulses diode, as shown in Figure 37.
Page 29 ADT7475 Reading Fan Speed from the ADT7475 Calculating Fan Speed The measurement of fan speeds involves a 2-register read for Assuming a fan with a two pulses per revolution (and two each measurement. The low byte should be read first. This pulses per revolution being measured) fan speed is calculated causes the high byte to be frozen until both high and low byte by the following:...
Page 30 ADT7475 PWM2 Configuration (Reg. 0x5D) Fan Spin-Up <4> INV The ADT7475 has a unique fan spin-up function. It spins the fan 0 = Logic high for 100% PWM duty cycle. at 100% PWM duty cycle until two TACH pulses are detected on 1 = Logic low for 100% PWM duty cycle.
Page 31: Operating From 3.3 V Standby
ADT7475 When the V voltage drops below the V low limit, the PWM Configuration Register (Reg. 0x5C to Reg. 0x5E) following occurs: <7:5> BHVR Status Bit 1 (V ) in Status Register 1 is set. 111 = manual mode. SMBALERT is generated, if enabled. Once under manual control, each PWM output can be manu- ally updated by writing to Reg.
Page 32: Programming The Automatic Fan Speed Control Loop
ADT7475 PROGRAMMING THE AUTOMATIC FAN SPEED CONTROL LOOP To more efficiently understand the automatic fan speed control The automatic fan speed control mode is very flexible owing to loop, it is strongly recommended to use the ADT7475 evalua- the number of programmable parameters, including T tion board and software while reading this section.
Page 33: Step 1: Hardware Configuration
ADT7475 STEP 1: HARDWARE CONFIGURATION How many fans will be supported in system, three or four? This influences the choice of whether to use the TACH4 During system design, the motherboard sensing and control pin or to reconfigure it for the THERM function. capabilities should be addressed early in the design stages.
Page 34: Recommended Implementation 1
ADT7475 CPU temperature measured using the Remote 1 RECOMMENDED IMPLEMENTATION 1 temperature channel. Configuring the ADT7475 as in Figure 44 provides the system designer with the following features: Ambient temperature measured through the Remote 2 temperature channel. Two PWM outputs for fan control of up to three fans. (The front and rear chassis fans are connected in parallel.) Bidirectional THERM pin allows the monitoring of PROCHOT output from an Intel P4 processor,...
Page 35: Recommended Implementation 2
ADT7475 CPU temperature measured using the Remote 1 RECOMMENDED IMPLEMENTATION 2 temperature channel. Configuring the ADT7475 as in Figure 45 provides the system designer with the following features: Ambient temperature measured through the Remote 2 temperature channel. Three PWM outputs for fan control of up to three fans. (All three fans can be individually controlled.) Bidirectional THERM pin allows the monitoring of PROCHOT output from an Intel P4 processor, for...
Page 36: Step 2: Configuring The Mux
ADT7475 010 = Remote 2 temperature controls PWMx STEP 2: CONFIGURING THE MUX After the system hardware configuration is determined, 101 = Fastest speed calculated by local and Remote 2 the fans can be assigned to particular temperature channels. temperature controls PWMx Not only can fans be assigned to individual channels, but the 110 = Fastest speed calculated by all three temperature behavior of the fans is also configurable.
Page 37 ADT7475 Example Mux Settings Mux Configuration Example <7:5> (BHVR), PWM1 Configuration Register 0x5c. This is an example of how to configure the mux in a system using the ADT7475 to control three fans. The CPU fan sink 101 = Fastest speed calculated by local and Remote 2 is controlled by PWM1, the front chassis fan is controlled by temperature controls PWM1 PWM 2, and the rear chassis fan is controlled by PWM3.
Page 38: Step 3: Tmin Settings For Thermal Calibration Channels
ADT7475 Registers STEP 3: T SETTINGS FOR THERMAL CALIBRATION CHANNELS Reg. 0x67, Remote 1 Temperature T = 0x9A (90°C) is the temperature at which the fans start to turn on under Reg. 0x68, Local Temperature T = 0x9A (90°C) automatic fan control. The speed at which the fan runs at T Reg.
Page 39: Step 4: Pwm Min For Each Pwm (Fan) Output
ADT7475 STEP 4: PWM FOR EACH PWM (FAN) OUTPUT Programming the PWM Registers The PWM registers are 8-bit registers that allow the is the minimum PWM duty cycle at which each fan in minimum PWM duty cycle for each output to be configured the system runs.
Page 40: Step 6: Trange For Temperature Channels
Using the ADT7475 evaluation software, you can graphically program and visualize this functionality. Reg. 0x38, PWM1 Maximum Duty Cycle = 0xFF Ask your local Analog Devices representative for details. (100% default) As PWM is changed, the automatic fan control slope also Reg.
Page 41 ADT7475 As T is changed, the slope also changes. As T Table 13. Selecting a T Value RANGE RANGE RANGE gets smaller, the fans will reach 100% speed with a smaller Bits <7:4> (°C) RANGE temperature change. 0000 0001 0010 3.33 100% 0011...
Page 42 ADT7475 2°C 2°C 2.5°C 2.5°C 3.33°C 3.33°C 4°C 4°C 5°C 5°C 6.67°C 6.67°C 8°C 8°C 10°C 10°C 13.3°C 13.3°C 16°C 16°C 20°C 20°C 26.6°C 26.6°C 32°C 32°C 40°C 40°C 53.3°C 53.3°C 80°C 80°C TEMPERATURE ABOVE T TEMPERATURE ABOVE T 2°C 2°C 2.5°C 2.5°C...
Page 43: Step 7: Ttherm For Temperature Channels
ADT7475 The T limit should be considered the maximum worst-case VRM TEMPERATURE THERM operating temperature of the system. Because exceeding any limit runs all fans at 100%, it has very negative acoustic THERM effects. Ultimately, this limit should be set up as a fail-safe, and CPU TEMPERATURE one should ensure that it is not exceeded under normal system operating conditions.
Page 44: Step 8: Thyst For Temperature Channels
ADT7475 RANGE 100% THERM THERMAL CALIBRATION 100% CONFIG RAMP CONTROL PWM1 GENERATOR (ACOUSTIC ENHANCEMENT) TACHOMETER 1 RANGE TACH1 MEASUREMENT CPU FAN SINK THERMAL CALIBRATION REMOTE 2 = 100% CPU TEMP CONFIG RAMP CONTROL PWM2 GENERATOR (ACOUSTIC ENHANCEMENT) TACHOMETER 2 TACH2 MEASUREMENT RANGE LOCAL =...
Page 45 ADT7475 RANGE 100% THERM THERMAL CALIBRATION 100% CONFIG RAMP CONTROL PWM1 GENERATOR (ACOUSTIC ENHANCEMENT) TACHOMETER 1 RANGE TACH1 MEASUREMENT CPU FAN SINK THERMAL CALIBRATION REMOTE 2 = 100% CPU TEMP CONFIG RAMP CONTROL PWM2 GENERATOR (ACOUSTIC ENHANCEMENT) TACHOMETER 2 TACH2 MEASUREMENT RANGE LOCAL =...
Page 46 ADT7475 When Bit 7 of configuration register 6 (0x10) =1, then the above Enhance Acoustics Register 2 (Reg. 0x63) ramp rates change to the values below. <2:0> ACOU3, selects the ramp rate for PWM outputs 000=52.2 sec associated with the Local temperature channel. 001=26.1 sec 000 = 37.5 sec 010=17.4 sec...
Page 47: Register Tables
ADT7475 REGISTER TABLES Table 14. ADT7475 Registers Address Description Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 Default Lockable 0x10 Configuration ExtraSlow VccpLow THERM SLOW SLOW SLOW 0x00 Remote 2 Local Remote 1 Manual 0x11 Configuration...
Page 48 ADT7475 Address Description Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 Default Lockable 0x49 High Limit 0xFF 0x4E Remote 1 0x81 Temp Low Limit 0x4F Remote 1 0x7F Temp High Limit 0x50 Local Temp 0x81 Low Limit...
Page 49 ADT7475 Address Description Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 Default Lockable 0x64 PWM1 Min 0x80 Duty Cycle 0x65 PWM2 Min 0x80 Duty Cycle 0x66 PWM3 Min 0x80 Duty Cycle 0x67 Remote 1 0x5A Temp T...
Page 50 ADT7475 Address Description Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 Default Lockable 0x7C Configuration Local GPIOP GPIOD Temp TWOS 0x01 THERM THERM THERM Offset COMPL Register 5 O/P Only O/P Only Only 0x7D Configuration...
Page 51 ADT7475 1, 2 Table 19. Temperature Reading Registers (Power-On Default = 0x80) Register Address Description Read-only 3, 4 0x25 Remote 1 temperature reading (8 MSB of reading). 0x26 Read-only Local temperature reading (8 MSB of reading). 0x27 Read-only Remote 2 temperature reading (8 MSB of reading). These temperature readings can be in twos complement or Offset 64 format;...
Page 52 ADT7475 Table 23. Register 0x40—Configuration Register 1 (Power-On Default = 0x04) Name Description 1, 2 <0> STRT Read/write Logic 1 enables monitoring and PWM control outputs based on the limit settings programmed. Logic 0 disables monitoring and PWM control based on the default power-up limit settings. Note that the limit values programmed are preserved even if a Logic 0 is written to this bit and the default settings are enabled.
Page 53 ADT7475 Table 25. Register 0x42—Interrupt Status Register 2 (Power-On Default = 0x00) Name Description <1> Read-only OVT = 1 indicates that one of the THERM overtemperature limits has been exceeded. This bit is cleared on a read of the status register when the temperature drops below THERM – T HYST <2>...
Page 54 ADT7475 Table 29. Register 0x55—TACH 1 Minimum High Byte (Power-On Default = 0xFF) Bits Name Description <4:0> Reserved Read-only These bits are reserved when Bit 6 of Config 2 Register (0x73) is set (single channel ADC mode). Otherwise, these bits represent Bits <4:0> of the TACH1 minimum high byte. <7:5>...
Page 55 ADT7475 Table 31. TEMP T /PWM Frequency Registers RANGE Register Address Description Power-On Default 0x5F Read/write Remote 1 T /PWM1 frequency. 0xC4 RANGE 0x60 Read/write Local temperature T /PWM2 frequency. 0xC4 RANGE 0x61 Read/write Remote 2 T /PWM3 frequency. 0xC4 RANGE Name Description...
Page 56 ADT7475 Table 32. Register 0x62—Enhanced Acoustics Register 1 (Power-On Default = 0x00) Name Description <2:0> ACOU1 Read/write Assuming that PWMx is associated with the Remote 1 temperature channel, these bits define the maximum rate of change of the PWMx output for Remote 1 Temperature related changes. Instead of the fan speed jumping instantaneously to its newly determined speed, it ramps gracefully at the rate determined by these bits.
Page 57 ADT7475 Table 33. Register 0x63—Enhanced Acoustics Register 2 (Power-On Default = 0x00) Name Description <2:0> ACOU3 Read/write Assuming that PWMx is associated with the Local temperature channel, these bits define the maximum rate of change of the PWMx output for Local Temperature related changes. Instead of the fan speed jumping instantaneously to its newly determined speed, it ramps gracefully at the rate determined by these bits.
Page 58 ADT7475 Table 34. PWM Minimum Duty Cycle Registers Register Address Description Power-On Default 0x64 Read/write PWM1 minimum duty cycle. 0x80 (50% duty cycle) 0x65 Read/write PWM2 minimum duty cycle. 0x80 (50% duty cycle) 0x66 Read/write PWM3 minimum duty cycle. 0x80 (50% duty cycle) Name Description <7:0>...
Page 59 ADT7475 Table 38. XNOR Tree Test Enable Register Address Description Power-On Default 0x6F Read/write XNOR tree test enable register. 0x00 <0> If the XEN bit is set to 1, the device enters the XNOR tree test mode. Clearing the bit removes the device from the XNOR tree test mode.
Page 60 ADT7475 Table 42. Register 0x73—Configuration Register 2 (Power-On Default = 0x00) Name Description <0:3> Reserved. Read/write AVG = 1, averaging on the temperature and voltage measurements is turned off. This allows measurements on each channel to be made much faster. ATTN Read/write ATTN = 1, the ADT7475 removes the attenuators from the V...
Page 61 ADT7475 Table 46. Register 0x77—Extended Resolution Register 2 Name Description <3:2> TDM1 Read-only Remote 1 temperature LSBs. Holds the 2 LSBs of the 10-bit Remote 1 temperature measurement. <5:4> LTMP Read-only Local temperature LSBs. Holds the 2 LSBs of the 10-bit local temperature measurement. <7:6>...
Page 62 ADT7475 Table 50. Register 0x7B—TACH Pulses per Revolution Register (Power-On Default = 0x55) Name Description <1:0> FAN1 Read/write Sets number of pulses to be counted when measuring Fan 1 speed. Can be used to determine fan pulses per revolution for unknown fan type. Pulses Counted 00 = 1 01 = 2 (default)
Page 63 ADT7475 Name Description <6> Local Read/write Local THERM = 0, THERM temperature limit functionality enabled for local temperature channel. THERM THERM can also be disabled on any channel by the following: In Offset 64 mode, writing −64˚C to the appropriate THERM temperature limit. In twos complement mode, writing −128˚C to the appropriate THERM temperature limit.
Page 64: Outline Dimensions
16-Lead QSOP RQ-16 ADT7475ARQZ-REEL7 –40°C to +125°C 16-Lead QSOP RQ-16 Z = Pb-free part. ©2005 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. D05381-0-7/05(0) Rev. 0 | Page 64 of 64...

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