MAX31790ATI+ - Maxim Integrated

General Description

The MAX31790 controls the speeds of up to six fans

using six independent PWM outputs. The desired fan

speeds (or PWM duty cycles) are written through the I2C

interface. The outputs drive “4-wire” fans directly, or can

be used to modulate the fan’s power terminals using an

external pass transistor.

Tachometer inputs monitor fan tachometer logic outputs

for precise (Q1%) monitoring and control of fan RPM as

well as detection of fan failure. Six pins are dedicated

tachometer inputs. Any of the six PWM outputs can also

be configured to serve as tachometer inputs.

The PWM_START inputs select the PWM output status

at startup to ensure appropriate fan drive when power is

first applied.

To ensure low acoustic impact of fan control, all changes

in PWM duty cycle take place at a controlled, program-

mable rate.

The MAX31790’s 3.0V to 5.5V supply voltage range and

I2C-compatible interface make it ideal for fan control in

a wide range of cooling applications. The MAX31790 is

available in a 28-pin TQFN package and operates over

the -40NC to +125NC temperature range.

Benets and Features

●Provides Precise Fan Control and Monitoring for

Large Fan Count Applications

• Controls Up to Six Independent Fans with PWM

Drive

• Up to 12 Tachometer Inputs

• Controlled Duty Cycle Rate-of-Change for Best

Acoustics

●Improves System Reliability

• Versatile Startup Congurations Provide Reliable

Fan Control at Power-up without Having to Wait for

System µC Direction

• I2C Bus Interface with Timeout and Watchdog

Applications

●Servers

●Networking

●Telecom

19-6540; Rev 2; 1/15

Ordering Information appears at end of data sheet.

Typical Operating Circuit

MAX31790

FAN1

V4-WIRE FAN

VCC

RPU

VCC

3.0V TO 5.5V

CRYSTAL

(OPTIONAL)

0.1µF12pF

12pF

PWMOUT2

TACH2

PWMOUT3

TACH3

PWM OUTPUT

DRIVER,

TACH MONITOR

I2C INTERFACE,

REGISTERS,

AND

CONTROL LOGIC PWMOUT4

TACH4

PWMOUT5

TACH5

PWMOUT1

SDA

VCC

XTAL2XTAL1

TACH1

SCL

FAN_FAIL

HOST

INTERFACE

SET

POWER-UP

VALUES

CLKOUT

FULL_SPEED

PWMOUT6

TACH6

GND 6 CHANNELS

PWM_START1

PWM_START0

FREQ_START

SPIN_START

WD_START

ADD1

ADD0

For related parts and recommended products to use with this part,

refer to: www.maximintegrated.com/MAX31790.related

MAX31790 6-Channel PWM-Output Fan RPM Controller

EVALUATION KIT AVAILABLE

Voltage Range on VCC, SDA, SCL, ADD0, ADD1,

FAN_FAIL, PWMOUTn Relative to GND ...........-0.3V to +6.0V

Voltage Range on TACHn, WD_START, SPIN_START,

FREQ_START, CLKOUT, FULL_SPEED,

PWM_STARTn Relative to GND ..............-0.3V to VCC + 0.3V

(not to exceed +6.0V)

Input Current at Any Pin .................................................... +5mA

Package Input Current .................................................... +20mA

Continuous Power Dissipation (TA = +70NC)

TDFN (derate 20.8 mW/NC above +70NC) ..............1666.7mW

Operating Temperature Range ........................ -40NC to +125NC

Storage Temperature Range ............................ -65NC to +150NC

Junction Temperature .....................................................+150NC

ESD Protection (All Pins, HBM) (Note 1) ........................ Q2000V

Lead Temperature (soldering, 10s) ................................+300NC

Soldering Temperature (reflow) ......................................+260NC

Absolute Maximum Ratings

Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional opera-

tion of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute

maximum rating conditions for extended periods may affect device reliability.

Recommended Operating Conditions

(TA = -40°C to +125°C, unless otherwise noted.) (Notes 2, 3)

Electrical Characteristics

(TA = -40°C to +125°C, typical values are VCC = 3.3V, TA = +25°C, unless otherwise noted.) (Notes 2, 3)

Note 1: Human Body Model, 100pF discharged through a 1.5kω resistor.

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MAX31790 6-Channel PWM-Output Fan RPM Controller

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

Operating Supply Voltage VCC 3.0 3.3 5.5 V

Input High Voltage VIH VCC x

0.7 V

Input Low Voltage VIL VCC x

0.3 V

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

Quiescent Supply Current

(Note 4) ICC

3.0V < VCC < 3.6V 1.5 3 mA

4.5V < VCC < 5.5V 2.5 8

POR Threshold VPOR 2 V

Watchdog Timer Accuracy fTOSC = 32.768kHz (Note 5) -0.5 +0.5 s

Output Low Voltage (SDA, FAN_

FAIL, PWMOUTn, CLKOUT) IOL = 3mA 0.4 V

Output High Voltage (CLKOUT) IOH = 1mA, VCC = 3.0V 2.7 V

XTAL1 Input Threshold 0.85 V

Input Leakage IL(Note 6) -1 +1 FA

Input Capacitance All digital inputs 5 pF

Fan Control Characteristics

(TA = -40°C to +125°C, typical values are VCC = 3.3V, TA = +25°C, unless otherwise noted.) (Note 3)

I2C AC Electrical Characteristics

(VCC = +3.0V to +5.5V, TA = -40°C to +125°C, timing referenced to VIL(MAX) and VIH(MIN), unless otherwise noted.) (Notes 3, 8)

(Figure 1)

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MAX31790 6-Channel PWM-Output Fan RPM Controller

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

Serial Clock Frequency fSCL DC 400 kHz

Bus Free Time Between STOP

and START Conditions tBUF 1.3 Fs

START Condition Hold Time tHD:STA 0.6 Fs

STOP Condition Setup Time tSU:STO 90% of SCL to 10% of SDA 600 ns

Clock Low Period tLOW 1.3 Fs

Clock High Period tHIGH 0.6 Fs

START Condition Setup Time tSU:STA 90% of SCL to 90% of SDA 100 ns

Data Setup Time tSU:DAT 10% of SDA to 10% of SCL 100 ns

Data In Hold Time tHD:DAT 10% of SCL to 10% of SDA (Note 9) 0 0.9 Fs

Maximum Receive SCL/SDA Rise

Time tR(Note 10) 300 ns

Minimum Receive SCL/SDA Rise

Time tR(Note 10) 20 + 0.1 x CBns

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

PWM Resolution 9 9 Bits

PWM Frequency Accuracy -6 +6 %

TACH Count Resolution 11 11 Bits

TACH Count Oscillator and

CLKOUT Clock fTOSC (Note 7) 32.768 kHz

TACH Count Oscillator and

CLKOUT Accuracy (Note 7) fERR:TOSC

Using internal oscillator:

TA = +25°C, VCC = 3.3V -0.5 +0.5

Using internal oscillator:

0°C < TA < +70°C, 3.0V < VCC < 3.6V -2.5 +2.5

Using internal oscillator:

-40°C < TA < +125°C, 3.0V < VCC < 3.6V -4.0 +4.0

Using internal oscillator:

-40°C < TA < +125°C, 3.0V < VCC < 5.5V -7.0 +7.0

Using external crystal -0.1 +0.1

TACH Minimum Input Pulse

Width tTACHMIN Pulse width must be greater than this value

to be detected 25 75 Fs

I2C AC Electrical Characteristics (continued)

(VCC = +3.0V to +5.5V, TA = -40°C to +125°C, timing referenced to VIL(MAX) and VIH(MIN), unless otherwise noted.) (Notes 3, 8)

(Figure 1)

Note 2: All voltages referenced to ground. Currents entering the IC are specified as positive.

Note 3: Limits are 100% production tested at TA = +25°C and/or TA = +85°C. Limits over the operating temperature range and

relevant supply voltage range are guaranteed by design and characterization. Typical values are not guaranteed.

Note 4: SDA = SCL = VCC, PWM active with PWM_FREQUENCY = 25kHz.

Note 5: The watchdog timer is derived from fTOSC and the watchdog timer accuracy specifications do not include the oscillator’s

associated error fERR:TOSC.

Note 6: Applies to pins SDA, SCL, PWM_STARTn, WD_START, FREQ_START, SPIN_START, ADDn, TACHn, PWMOUTn, FULL_SPEED.

Note 7: fTOSC is used to measure fan speed by counting the number of 8192Hz (fTOSC/4) clock cycles that take place during a

selectable number of tachometer periods. For internal oscillator only, typical frequency shift due to aging is within ±0.5%.

Aging stressing includes level 1 moisture reflow preconditioning (24hr +125°C bake, 168hr +85°C/85%RH moisture soak,

and three solder reflow passes +260°C +0°C/-5°C peak) followed by 192hr (max) VCC biased.

Note 8: All timing specifications are guaranteed by design.

Note 9: A master device must provide a hold time of at least 300ns for the SDA signal to bridge the undefined region of SCL’s fall-

ing edge.

Note 10: CB—total capacitance of one bus line in pF.

Note 11: Holding the SDA line low for a time greater than tTIMEOUT causes the device to reset SDA to the idle state of the serial

bus communication (SDA set high).

External Crystal Parameters

(Note 3)

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MAX31790 6-Channel PWM-Output Fan RPM Controller

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

Maximum Receive SCL/SDA Fall

Time tF(Note 10) 300 ns

Minimum Receive SCL/SDA Fall

Time tF(Note 10) 20 + 0.1 x CBns

Transmit SDA Fall Time tF10pF P CB P 400pF (Note 10)

20 +

0.1 x

250 ns

Pulse Width of Suppressed Spike tSP 35 ns

SDA Time Low for Reset of Serial

Interface tTIMEOUT (Note 11) 25 45 ms

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

Crystal Oscillator Startup Time 1 s

Nominal Frequency fO32.768 kHz

Series Resistance ESR 50 kI

Load Capacitance CL12 pF

Typical Operating Characteristics

(TA = +25°C, unless otherwise noted.)

Figure 1. I2C Timing Diagram

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MAX31790 6-Channel PWM-Output Fan RPM Controller

SCL

NOTE: TIMING IS REFERENCED TO VIL(MAX) AND VIH(MIN).

SDA

STOP START REPEATED

START

tBUF

tHD:STA

tHD:DAT tSU:DAT

tSU:STO

tHD:STA tSP

tSU:STA

tHIGH

tLOW

POWER-SUPPLY CURRENT

vs. POWER-SUPPLY VOLTAGE

MAX31790 toc01

SUPPLY VOLTAGE (V)

SUPPLY CURRENT (mA)

5.54.53.5

2.5 6.5

PWM FREQUENCY = 25kHz 1.47kHz

PWM FREQUENCY = 25Hz

POWER-SUPPLY CURRENT

vs. TEMPERATURE

MAX31790 toc02

TEMPERATURE(°C)

SUPPLY CURRENT (mA)

100500

-50 150

PWM FREQUENCY = 25kHz

VDD = 3.3V

PWM FREQUENCY = 25Hz

Pin Conguration

Pin Description

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MAX31790 6-Channel PWM-Output Fan RPM Controller

PIN NAME FUNCTION

1 FREQ_START This input is sampled at power-up and sets the power-up value for the PWM output frequency. See

the Register Map for values.

2 SPIN_START This input is sampled at power-up and sets the initial spin-up behavior. See the Register Map for

values.

3 ADD1 Address Select Inputs. Sampled at the start of every I2C transaction. One of 16 possible addresses

can be selected by connecting ADD0 and ADD1 to GND, VCC, SDA, or SCL.

4 ADD0

5 WD_START This input is sampled at power-up and sets the initial I2C watchdog behavior. See the Register Map

for values.

6 XTAL2 Pins for Connecting to Optional 32,768Hz Crystal. The crystal can be used when the best RPM

precision is required. At POR the internal oscillator is used, and a nominal 32,768Hz clock is pro-

duced at CLKOUT. If a crystal is connected between XTAL1 and XTAL2, the crystal oscillator can

be enabled by writing to a register. If no crystal is present, ground XTAL1 and leave XTAL2 uncon-

nected.

7 XTAL1

8 GND Ground

SPIN_START

ADD0

WD_START

XTAL2

XTAL1

FREQ_START

PWMOUT1

PWMOUT2

TACH2

VCC

PWMOUT3

TACH3

PWM_START0

4567

2021 19 17 16 15

PWM_START1

FULL_SPEED

PWMOUT5

TACH5

PWMOUT6

TACH6

ADD1 TACH1

28 8

CLKOUT GND

FAN_FAIL

23 13 TACH4

SCL

22 14 PWMOUT4

SDA

TQFN

(4mm x 4mm)

TOP VIEW

MAX31790

Pin Description (continued)

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MAX31790 6-Channel PWM-Output Fan RPM Controller

PIN NAME FUNCTION

9 TACH6

Logic/Analog Inputs for Tach Signals. If a fan has a logic tach output, it can be used for RPM con-

trol. For a 2-wire fan, analog input can be used for fan-failure detection. Also functions as a “locked

rotor” input.

11 TACH5

13 TACH4

15 TACH3

17 TACH2

19 TACH1

10 PWMOUT6

Open-Drain Output to 4-Wire Fan’s PWM Input or (Less Frequently) to Power Transistor Modulating

Fan Power Supply. Can also be used as a tachometer signal input. Can be pulled up as high as

5.5V.

12 PWMOUT5

14 PWMOUT4

16 PWMOUT3

18 PWMOUT2

20 PWMOUT1

21 VCC Power-Supply Input. 3.3V nominal. Bypass VCC to GND with a 0.1FF capacitor.

22 SDA I2C Serial-Data Input/Output, Open Drain. Can be pulled up to 5.5V regardless of VCC.

23 SCL I2C Serial-Clock Input. Can be pulled up to 5.5V regardless of VCC.

24 FAN_FAIL Active-Low, Open-Drain Fan-Failure Output. Active only when fault is present.

25 PWM_START0 These inputs are sampled at power-up and set the power-up value for all PWMOUT duty cycles.

See the Register Map for values.

26 PWM_START1

27 FULL_SPEED When low, this input forces all PWM outputs to 100%. Exception: If a fan has failed and “Duty Cycle

= 0 on Failure” has been selected for that fan.

28 CLKOUT

CMOS Push-Pull 32,768Hz Clock Output. Signal generated from internal oscillator when external

crystal is not used. If a crystal is connected between XTAL1 and XTAL2 and enabled, the crystal

oscillator generates the output. Output is always active.

— EP Exposed Pad. Connect to GND.

Block Diagram

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MAX31790 6-Channel PWM-Output Fan RPM Controller

MAX31790

PWMOUT6

TACHn ENABLE

GND

VCCXTAL2XTAL1

PWMOUT5

PWMOUT4

PWMOUT3

PWMOUT2

PWMOUT1

TACH6

TACH5

TACH4

TACH3

TACH2

TACH1

FULL_SPEED

TACH12

TACH11

TACH10

TACH9

TACH8

TACH7

PULSE-WIDTH

MODULATOR

FAN CONTROLLER

LOGIC

FAN

TACHOMETER

INPUTS

FAN

FAIL LOGIC

POWER

CONTROL

DIVIDER

USER MEMORY

POR PIN SAMPLE LOGIC

WATCHDOG

TIMER

SLAVE

ADDRESS

DECODE

LOGIC

I2C

INTERFACE

BLOCK

I2C BUS

TIMEOUT

INTERNAL

OSCILLATOR

OSC BIT

CLKOUT

OSCILLATOR

CIRCUIT

FAN_FAIL

SDA

SCL

ADD0

ADD1

WD_STARTSPIN_STARTFREQ_STARTPWM_START1PWM_START0

Detailed Description

The MAX31790 controls the speeds of up to six fans

using six independent PWM outputs. The desired fan

speeds (or PWM duty cycles) are written through the I2C

interface. The outputs drive “4-wire” fans directly or can

be used to control 2-wire or 3-wire fans by modulating

the fan’s power supply voltage. Modulating the power

supply voltage can be achieved by various techniques

and are described in the Controlling 2-Wire and 3-Wire

Fans section.

The MAX31790 has two main methods for controlling fan

speeds: PWM mode and RPM mode. Additional level of

control is achieved by the incorporated rate-of-change

control that allows the device to control the max rate at

which the PWM duty cycle is incremented/decremented.

Tachometer inputs monitor fan tachometer logic outputs

for precise (±1%) monitoring and control of fan RPM as

well as detection of fan failure. Six pins are dedicated

tachometer inputs. Any of the six PWM outputs can also

be configured to serve as tachometer inputs, allowing for

up to 12 fans to be monitored.

The device can monitor the TACHn inputs and determine

when a fan has failed. Failure is detected in various ways

depending on the fan control mode. Once a selectable

number of fault detections has occurred, the FAN_FAIL

output asserts (if fault detection is not masked for the

fan).

Power-on values for PWM duty cycles, PWM frequencies,

fan spin-up, and the watchdog are achieved by five pin

inputs.

Fan Control

The device has two main methods for controlling fan

speeds: PWM mode and RPM mode.

PWM Mode

In PWM mode, the device produces a PWM waveform

that drives the fan’s PWM speed control input. The fan’s

speed is proportional to the PWM duty cycle delivered to

its PWM input terminal. The duty cycle is set by the fan’s

associated PWMOUT Target Duty Cycle registers and

the actual duty cycle can be read from the correspond-

ing PWMOUT Duty Cycle register. Because the duty

cycle ramps to new values at a controlled rate, the values

in the two registers can be different. See the Register

Descriptions section for details.

RPM Mode

In RPM mode, the device monitors tachometer output

pulses from the fan and adjusts the PWM duty cycle to

force the fan’s speed to the desired value. Fan speed

is measured by counting the number of internal 8192Hz

(fTOSC/4) clock cycles that take place during a select-

able number of tachometer periods. The number of clock

cycles counted (11-bit value) is stored in the associated

TACH Count registers and the desired number of cycles

is stored in the TACH Target Count registers. See the

Rate-of-Change Control

Sudden changes in fan speed can be easily heard by

users. The device helps reduce the audibility of fan-

speed changes by controlling the rate at which the PWM

duty cycle is incremented. Three bits in the associated

Fan Dynamics register sets the rate at which the duty

cycle is incremented/decremented. This allows the time

required for an LSB of change in the PWM duty cycle to

vary from 0ms to 125ms.

The selected rate of change also applies when the

FULL_SPEED input is asserted or when the fans are

forced to 100% due to a fan failure. See the Register

Descriptions section for details.

In RPM mode when the fan’s speed is near the target

speed, that is, when the TACH count is near the TACH

target count, the control loop dynamics can often be

improved by slowing the rate of change of the PWM duty

cycle. This operates as follows: First, set a value for the

count “window” and store it in the appropriate Window

the current TACH count and the target TACH count. If the

absolute value of this difference is less than the value in

the appropriate Window register, the update rate of the

PWM duty cycle is slowed to 1 LSB per second. When

the current TACH count falls outside of the window, the

duty cycle rate of change reverts to the selected value.

Spin-Up

When a fan is not spinning, and a low duty cycle

waveform is applied to its PWM terminal, it can fail to

overcome inertia and start spinning. To overcome this

potential problem, a 100% duty cycle waveform can be

applied to the fan’s PWM input for a short time before

a lower duty cycle waveform is applied. This “spin-up”

period allows the fan to overcome inertia and begin oper-

ating. Spin-up is controlled using the corresponding Fan

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MAX31790 6-Channel PWM-Output Fan RPM Controller

Configuration register. Spin-up can be disabled, or it can

cause the fan to be driven with a 100% duty cycle until

it produces two tachometer pulses, up to a maximum of

0.5s, 1s, or 2s. When spin-up is enabled and the duty

cycle is making a transition from 0% to a value that is less

than 100% (from 0% to 50%, for example), the duty cycle

first goes to 100%. When two tachometer pulses have

been detected, or when the maximum spin-up period has

elapsed, the duty cycle drops to the target value (50%

in this example). The SPIN_START pin sets the spin-up

value at power-up.

Sequential Fan Activation

When multiple high-current fans are activated simultane-

ously, the startup current can stress the system’s power

supply. To minimize this effect, the device includes a

selectable sequential fan activation feature. When select-

ed, this feature inserts a short minimum delay between

the activation times of fans.

The bits for controlling sequential fan activation are

located in the Failed Fan Options/Sequential Start regis-

ter. They select the time delay between fan activations to

be one of the following: 0, 250ms, 500ms, 1s, 2s, or 4s.

The default time is 500ms per channel.

Sequential fan activation applies to POR, fan failure

forcing the fans to full speed, and assertion of the

FULL_SPEED input, which forces all the fans to full

speed. In all these cases, all fans are forced to full

speed. The sequence operates as follows:

• PWM1 activates. The PWM duty cycle begins to

increase at the selected rate of change.

• After the selected delay time has elapsed, PWM2 acti-

vates. Again, the PWM duty cycle begins to increase

at the selected rate of change.

• The other PWM channels activate in sequence, each

delayed by the selected delay time relative to the

previous channel. Note that the time delay applies to

unused or disabled channels.

FULL_SPEED Input

Driving this input low forces all fans to full speed with the

exception of any failed fans (if 0% on failure has been

selected). This input allows an external temperature

switch to provide fail-safe overtemperature protection. In

systems with multiple MAX31790s, all FAN_FAIL outputs

can be connected to all FULL_SPEED inputs, thereby

providing full-speed operation if any fan fails, regardless

of which MAX31790 controls it. This input is active even

in standby mode.

POR Options

Five inputs allow setup of the device’s behavior at power-

up. The following inputs are sampled when power is first

applied to the device:

WD_START: At power-up the watchdog operation is

controlled by the WD_START pin. Connect WD_START to

VCC to enable, or to GND to disable the watchdog func-

tion. When enabled using WD_START, the timeout period

is 30s. After power is applied, the watchdog function can

be enabled or disabled, and the timeout period can be

changed by editing the Global Configuration register.

SPIN_START: At power-up, spin-up operation is controlled

by the SPIN_START pin. Connect SPIN_START to GND to

disable, VCC to enable spin-up for a maximum of 1s, or

unconnected to enable spin-up for a maximum of 0.5s.

After power is applied, the spin-up function can be enabled

or disabled, and the spin-up period can be changed by

editing the associated Fan Configuration register.

PWM_START 0, PWM_START1: At power-up, the PWM

output duty cycles are controlled by the PWM_START0

and PWM_START1 pins. Connect PWM_START0/

PWM_START1 to GND, VCC, or leave unconnected

to achieve different duty cycles for all PWM outputs.

See the PWMOUT Target Duty Cycle register for the

corresponding values and connections. After power is

applied, the PWM duty cycles can be changed, by edit-

ing that PWM’s associated PWMOUT Target Duty Cycle

FREQ_START: At power-up, all PWM output frequen-

cies are controlled by the FREQ_START pin. Connect

FREQ_START to GND for 30Hz, VCC for 25kHz, or uncon-

nected for 1.47kHz. After power is applied, the PWM

output frequencies can be changed by editing the PWM

Frequency register.

Watchdog

The device includes an optional I2C watchdog function

that monitors the I2C bus for transactions. When the

watchdog function is enabled, all fans (with the excep-

tion of failed fans “0% on fail” selected) are forced to

full speed if no I2C transactions occur within a selected

period (5s, 10s, or 30s). Watchdog timing is selected

using the Global Configuration register.

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MAX31790 6-Channel PWM-Output Fan RPM Controller

Fan Monitoring

Monitoring Tachometer Signals

The TACH inputs accept either tachometer or “locked

rotor” output signals from 3-wire or 4-wire fans. When

measuring fan speed, the device counts the number of

internal 8192Hz (fTOSC/4)clock cycles that occur during

1, 2, 4, 8, 16, or 32 tachometer periods. (The speed of

each fan is measured once per second.) The number of

tachometer periods is selectable for each fan by using

the appropriate Fan Dynamics register. Tachometer

pulses less than tTACHMIN in duration are ignored to

minimize the effect of noise on the tachometer lines. The

TACH count for a given RPM can be obtained from the

following equation:

TACH Count SR 8192

NP RPM

= ××

where:

NP = number of tachometer pulses per revolution.

Most general-purpose brushless DC fans produce two

tachometer pulses per revolution.

SR = 1, 2, 4, 8, 16, or 32. This is the number of tachom-

eter periods over which the tachometer clock is counted.

See the Speed Range bit information described in the

corresponding Fan Dynamics register description.

The tachometer count consists of 11 bits in the TACH

Count registers and is available in RPM and PWM

modes. In RPM mode, the desired fan count is written

to the associated TACH Target Count register. In PWM

mode, the desired fan duty cycle is written to the associ-

ated PWMOUT Target Duty Cycle register.

Note that the device is intended to be used with 4-wire

fans. Modulating a fan’s power supply with a PWM

waveform, as is sometimes done with 2-wire and 3-wire

fans, results in incorrect tachometer counts due to the

periodic removal of power from the fan’s internal circuitry.

Therefore, it is suggested to use PWM mode when inter-

facing with 2-wire or 3-wire fans.

Using PWM Outputs as Tachometer Inputs

Each Fan Configuration register includes a PWM/TACH

bit that allows the PWMOUT to be configured as a TACH

input. In TACH mode, the settings for TACH input enable,

locked rotor operation, and TACH pulses counted that

have been selected for a given fan channel apply to that

channel’s TACH input and also to TACH signals sensed

by that channel’s PWM output.

Figure 2 to Figure 6 show some examples of TACH-PWM

connections for various fan configurations

Figure 2. 12 Fans, 12 TACH Monitors, No PWM

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MAX31790 6-Channel PWM-Output Fan RPM Controller

TACH1

PWMOUT1 (TACH7)

TACH2

PWMOUT2 (TACH8)

TACH3

PWMOUT3 (TACH9)

TACH4

PWMOUT4 (TACH10)

TACH5

PWMOUT5 (TACH11)

TACH6

PWMOUT6 (TACH12)

MAX31790

Figure 3. 11 Fans, 11 TACH Monitors,1 PWM Figure 4. 10 Fans, 10 TACH Monitors, 2 PWMs

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MAX31790 6-Channel PWM-Output Fan RPM Controller

TACH1

PWMOUT1

TACH2

PWMOUT2 (TACH8)

TACH3

PWMOUT3 (TACH9)

TACH4

PWMOUT4 (TACH10)

TACH5

PWMOUT5 (TACH11)

TACH6

PWMOUT6 (TACH12)

MAX31790

TACH1

PWMOUT1

TACH2

PWMOUT2 (TACH8)

TACH3

PWMOUT3 (TACH9)

TACH4

PWMOUT4

TACH5

PWMOUT5 (TACH11)

TACH6

PWMOUT6 (TACH12)

MAX31790

Fan Failure

When enabled, the device monitors the TACH inputs to

determine when a fan has failed. For fans with tachom-

eter outputs, failure is detected in various ways depend-

ing on the fan control mode. In every case, from one to

six consecutive fault detections (selected by the Fan

Fault Queue bits) are required to decide that the fan has

failed. When the selected number of fault detections has

occurred, the FAN_FAIL output asserts (if fault detection

is not masked for the fan).

PWM Mode Failure Detection

In PWM mode, the TACH Target Count register holds the

upper limit for tachometer count values. A potential fault

condition is identified when the TACH count exceeds the

value written to the TACH Target Count register. If the

Fan Fault Queue bit value is 1 and the following tachom-

eter count (1 second later) also exceeds the limit value,

the fan is considered to have failed. A higher Fan Fault

Queue bit value requires a larger number of consecutive

values in excess of the limit value. When a PWM output

is used as a TACH input, the PWM-mode failure criteria

apply. In PWM mode, fan-failure detection is masked

when the target duty cycle is set to zero.

Figure 5. 9 Fans, 9 TACH Monitors, 3 PWMs Figure 6. 8 Fans, 8 TACH Monitors, 4 PWMs

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MAX31790 6-Channel PWM-Output Fan RPM Controller

TACH1

PWMOUT1

TACH2

PWMOUT2 (TACH8)

TACH3

PWMOUT3

TACH4

PWMOUT4

TACH5

PWMOUT5 (TACH11)

TACH6

PWMOUT6

MAX31790

TACH1

PWMOUT1

TACH2

PWMOUT2 (TACH8)

TACH3

PWMOUT3

TACH4

PWMOUT4 (TACH10)

TACH5

PWMOUT5

TACH6

PWMOUT6 (TACH12)

MAX31790

RPM Mode Failure Detection

In RPM mode, a potential fault condition is identified

when any of the following three conditions occur: 1) the

TACH count exceeds the associated value stored in the

TACH Target Count register while the PWM duty cycle is

100%, 2) the TACH count exceeds two times the TACH

Target Count register value while the duty cycle is less

than 100%, or 3) the TACH count reaches its maximum

value of 7FFh. If the Fan Fault Queue bit value is 1 and

the TACH Count value also exceeds the limit value,

the fan is considered to have failed. A higher Fan Fault

Queue value requires a larger number of consecutive

values in excess of the limit value. In RPM mode, fan-

failure detection is masked when the TACH Target Count

Locked Rotor Mode Failure Detection

Some fans have a locked rotor output that produces a

logic-level output to indicate that the fan has stopped spin-

ning. Locked rotor signals can be monitored by setting the

TACH/Locked Rotor bit in the associated Fan Configuration

adjusted in this same register. A fan fault is detected when

a locked rotor signal has been present for 1 second.

Failure Indication

Fan failure is indicated in the Fan Fault register and also

with the open-drain FAN_FAIL output after the number of

consecutive faults selected by the Fan Fault Queue bits

have occurred. The FAN_FAIL output can be masked

using the mask bits in the Fan Fault Mask register.

When a fan has failed, PWM to the affected fan can

continue as though the fan is still operational, or the duty

cycle can be automatically set to 0 or 100% as deter-

mined by the Failed Fan Options bits. See the Register

Descriptions section for full details.

The failed condition can be cancelled by writing PWM

Target duty cycle or TACH target count to the fan’s

control registers. The new value can be the same as the

value already in the register. After writing to the register,

the fan-failure detection process begins again. If the fan

is still in a failed state, fan failure again is detected.

Slave Address Byte and Address Pins

The slave address byte consists of a 7-bit slave address

plus an R/W bit (Figure 7). The device’s slave address is

determined by the state of the ADD0 and ADD1 address

pins during a START condition of an I2C transaction. The

ADD0 and ADD1 pins can be connected to GND, VCC,

SDA, or SCL. These pins allow up to 16 MAX31790s to

reside on the same I2C bus. See Table 1 for a complete

list of all 16 possibilities and the corresponding ADD0 and

ADD1 pin connections.

For example, the device’s slave address byte is 40h

when ADD0 and ADD1 pins are grounded during a

START condition. I2C communication is described in

detail in the I2C Serial Interface Description section.

Note: If the state of the ADD0 and ADD1 pins is changing

during normal operation, the slave address of the device

dynamically changes to reflect the pins states at every

START condition. The ADD0 and ADD1 pins cannot

change during an I2C transaction.

Table 1. Slave Address Table

Figure 7. MAX31790 Slave Address Byte Example

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MAX31790 6-Channel PWM-Output Fan RPM Controller

ADD1

CONNECTION

ADD0

CONNECTION

SLAVE

ADDRESS BYTE

(HEX)

GND GND 40

GND SCL 42

GND SDA 44

GND VCC 46

SCL GND 48

SCL SCL 4A

SCL SDA 4C

SCL VCC 4E

SDA GND 50

SDA SCL 52

SDA SDA 54

SDA VCC 56

VCC GND 58

VCC SCL 5A

VCC SDA 5C

VCC VCC 5E

01 0

/W00

MSB LSB

SLAVE ADDRESS*

SEE TABLE 1

*THE SLAVE ADDRESS IS DETERMINED BY ADDRESS PINS ADD0 AND ADD1.

READ/WRITE BIT

Memory Description

The device’s control registers are organized in rows of 8 bytes. The I2C master can read or write individual bytes, or can

read or write multiple bytes. When writing consecutive bytes, all writes are to the same row. When the final byte in the

row is reached, the next byte written is the row’s first byte. For example, a write that starts with 02h (Fan 1 Configuration)

can write to 02h, 03h, 04h, 05h, 06h, and 07h. If writes continue, the next byte written is 00h, and so on.

Consecutive reads are not subject to the single-row limitation. A read can start at any address and can continue through

FFh. If reads continue past FFh, they wrap around to 00h.

“User Bytes” are general-purpose R/W bytes. X denotes the input state at POR.

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MAX31790 6-Channel PWM-Output Fan RPM Controller

R/W REGISTER POR

STATE FUNCTION D7 D6 D5 D4 D3 D2 D1 D0

R/W 00h 0010 0XX0b Global

Configuration

Run/Standby

0 = Run

1 = Standby

Reset:

0 = Normal

1 = Reset

Bus

Timeout

0 =

Enabled

RESERVED

OSC:

0 = Internal

oscillator

1 = External

crystal

I2C Watchdog:

00b = Disabled

01b = 5s

10b = 10s

11b = 30s

I2C

Watchdog

Status

1 = Watchdog

fault

detected

R/W 01h FREQ_START PWM

Frequency

PWM4-PWM6 Frequency:

0000b = 25Hz

0001b = 30Hz

0010b = 35Hz

0011b = 100Hz

0100b = 125Hz

0101b = 149.7Hz

0110b = 1.25kHz

0111b = 1.47kHz

1000b = 3.57kHz

1001b = 5kHz

1010b = 12.5kHz

1011b = 25kHz

PWM1-PWM3 Frequency:

0000b = 25Hz

0001b = 30Hz

0010b = 35Hz

0011b = 100Hz

0100b = 125Hz

0101b = 149.7Hz

0110b = 1.25kHz

0111b = 1.47kHz

1000b = 3.57kHz

1001b = 5kHz

1010b = 12.5kHz

1011b = 25kHz

R/W 02h 0XX0 0000b Fan 1

Configuration

Mode:

0 = PWM

1 = RPM

Spin-Up

00b = No Spin-up

01b = 2 TACH counts

or 0.5s

10b = 2 TACH counts

or 1s

11b = 2 TACH counts

or 2s

Control/Monitor

0 = Control

1 = Monitor

only

TACH

Input

Enable

1 = Enabled

TACH/

Locked

Rotor

0 = TACH

1 = Locked

Rotor

Locked

Rotor

Polarity

0 = Low

1 = High

PWM/TACH

0 = PWM

1 = TACH

R/W 03h 0XX0 0000b Fan 2

Configuration Same as Fan 1 Configuration

R/W 04h 0XX0 0000b Fan 3

Configuration Same as Fan 1 Configuration

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MAX31790 6-Channel PWM-Output Fan RPM Controller

R/W REGISTER POR

STATE FUNCTION D7 D6 D5 D4 D3 D2 D1 D0

R/W 05h 0XX0 0000b Fan 4

Configuration Same as Fan 1 Configuration

R/W 06h 0XX0 0000b Fan 5

Configuration Same as Fan 1 Configuration

R/W 07h 0XX0 0000b Fan 6

Configuration Same as Fan 1 Configuration

R/W 08h 0100 1100b Fan 1

Dynamics

Speed Range (TACH Periods

Counted)

000b = 1

001b = 2

010b = 4 (default)

011b = 8

100b = 16

101b = 32

110b = 32

111b = 32

PWM Rate-of-Change:

000b = 0ms per LSB (PWM)

000b = 0.9765ms per LSB (RPM)

001b = 1.953125ms per LSB

010b = 3.90625ms per LSB

011b = 7.8125ms per LSB (default)

100b = 15.625ms per LSB

101b = 31.25ms per LSB

110b = 62.5ms per LSB

111b = 125ms per LSB

Asymmetric

Rate of

Change

1 = Enabled

RESERVED

R/W 09h 0100 1100b Fan 2

Dynamics Same as Fan 1 Dynamics

R/W 0Ah 0100 1100b Fan 3

Dynamics Same as Fan 1 Dynamics

R/W 0Bh 0100 1100b Fan 4

Dynamics Same as Fan 1 Dynamics

R/W 0Ch 0100 1100b Fan 5

Dynamics Same as Fan 1 Dynamics

R/W 0Dh 0100 1100b Fan 6

Dynamics Same as Fan 1 Dynamics

R/W 0Eh 0000 0000b User Byte —

R/W 0Fh 0000 0000b User Byte —

R/W 10h 0000 0000b Fan Fault

Status 2 RESERVED RESERVED Fan 12 Fault

1 = Fault

Fan 11 Fault

1 = Fault

Fan 10 Fault

1 = Fault

Fan 9 Fault

1 = Fault

Fan 8 Fault

1 = Fault

Fan 7 Fault

1 = Fault

R/W 11h 0000 0000b Fan Fault

Status 1 RESERVED RESERVED Fan 6 Fault

1 = Fault

Fan 5 Fault

1 = Fault

Fan 4 Fault

1 = Fault

Fan 3 Fault

1 = Fault

Fan 2 Fault

1 = Fault

Fan 1 Fault

1 = Fault

R/W 12h 0011 1111b Fan Fault

Mask 2 RESERVED RESERVED Fan 12 Mask

1 = Masked

Fan 11 Mask

1 = Masked

Fan 10 Mask

1 = Masked

Fan 9 Mask

1 = Masked

Fan 8 Mask

1 = Masked

Fan 7 Mask

1 = Masked

R/W 13h 0011 1111b Fan Fault

Mask 1 RESERVED RESERVED Fan 6 Mask

1 = Masked

Fan 5 Mask

1 = Masked

Fan 4 Mask

1 = Masked

Fan 3 Mask

1 = Masked

Fan 2 Mask

1 = Masked

Fan 1 Mask

1 = Masked

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MAX31790 6-Channel PWM-Output Fan RPM Controller

R/W REGISTER POR

STATE FUNCTION D7 D6 D5 D4 D3 D2 D1 D0

R/W 14h 0100 0101b

Failed Fan

Options/

Sequential

Start

Sequential Start Delay

000b = 0s

001b = 250ms

010b = 500ms

011b = 1s

100b = 2s

101b, 110b, 111b = 4s

RESERVED

Failed Fan Options

00b = duty cycle =

0% on fail

01b = Continue

PWM or RPM mode

operation on fail.

10b = duty cycle =

100% on fail

11b = All fans to 100%

on any unmasked fan

failure.

Fan Faut Queue

00b = 1 fault

01b = 2 faults

10b = 4 faults

11b = 6 faults

R/W 15h 0000 0000b User Byte —

R/W 16h 0000 0000b User Byte —

R/W 17h 0000 0000b User Byte —

R 18h 1111 1111b TACH 1

Count MSB 210 29282726252423

R 19h 1110 0000b TACH 1

Count LSB 2221200 0 0 0 0

R 1Ah 1111 1111b TACH 2

Count MSB Same as TACH 1 Count

R 1Bh 1110 0000b TACH 2

Count LSB

R 1Ch 1111 1111b TACH 3

Count MSB Same as TACH 1 Count

R 1Dh 1110 0000b TACH 3

Count LSB

R 1Eh 1111 1111b TACH 4

Count MSB Same as TACH 1 Count

R 1Fh 1110 0000b TACH 4

Count LSB

R 20h 1111 1111b TACH 5

Count MSB Same as TACH 1 Count

R 21h 1110 0000b TACH 5

Count LSB

R 22h 1111 1111b TACH 6

Count MSB Same as TACH 1 Count

R 23h 1110 0000b TACH 6

Count LSB

R 24h 1111 1111b TACH 7

Count MSB Same as TACH 1 Count

R 25h 1110 0000b TACH 7

Count LSB

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MAX31790 6-Channel PWM-Output Fan RPM Controller

R/W REGISTER POR

STATE FUNCTION D7 D6 D5 D4 D3 D2 D1 D0

R 26h 1111 1111b TACH 8

Count MSB Same as TACH 1 Count

R 27h 1110 0000b TACH 8

Count LSB

R 28h 1111 1111b TACH 9

Count MSB Same as TACH 1 Count

R 29h 1110 0000b TACH 9

Count LSB

R 2Ah 1111 1111b TACH 10

Count MSB Same as TACH 1 Count

R 2Bh 1110 0000b TACH 10

Count LSB

R 2Ch 1111 1111b TACH 11

Count MSB Same as TACH 1 Count

R 2Dh 1110 0000b TACH 11

Count LSB

R 2Eh 1111 1111b TACH 12

Count MSB Same as TACH 1 Count

R 2Fh 1110 0000b TACH 12

Count LSB

R 30h 0000 0000b

PWMOUT 1

Duty Cycle

MSB

2827262524232221

R 31h 0000 0000b

PWMOUT 1

Duty Cycle

LSB

200 0 0 0 0 0 0

R 32h 0000 0000b

PWMOUT 2

Duty Cycle

MSB

Same as PWMOUT 1 Duty Cycle

R 33h 0000 0000b

PWMOUT 2

Duty Cycle

LSB

R 34h 0000 0000b

PWMOUT 3

Duty Cycle

MSB

Same as PWMOUT 1 Duty Cycle

R 35h 0000 0000b

PWMOUT 3

Duty Cycle

LSB

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MAX31790 6-Channel PWM-Output Fan RPM Controller

R/W REGISTER POR

STATE FUNCTION D7 D6 D5 D4 D3 D2 D1 D0

R 36h 0000 0000b

PWMOUT 4

Duty Cycle

MSB

Same as PWMOUT 1 Duty Cycle

R 37h 0000 0000b

PWMOUT 4

Duty Cycle

LSB

R 38h 0000 0000b

PWMOUT 5

Duty Cycle

MSB

Same as PWMOUT 1 Duty Cycle

R 39h 0000 0000b

PWMOUT 5

Duty Cycle

LSB

R 3Ah 0000 0000b

PWMOUT 6

Duty Cycle

MSB

Same as PWMOUT 1 Duty Cycle

R 3Bh 0000 0000b

PWMOUT 6

Duty Cycle

LSB

R 3Ch 0000 0000b RESERVED 0 0 0 0 0 0 0 0

R 3Dh 0000 0000b RESERVED 0 0 0 0 0 0 0 0

R 3Eh 0000 0000b RESERVED 0 0 0 0 0 0 0 0

R 3Fh 0000 0000b RESERVED 0 0 0 0 0 0 0 0

R/W 40h PWM_START

PWMOUT1

Target Duty

Cycle MSB

2827262524232221

R/W 41h PWM_START

PWMOUT1

Target Duty

Cycle LSB

20RESERVED RESERVED RESERVED RESERVEDRESERVED RESERVED RESERVED

R/W 42h PWM_START

PWMOUT2

Target Duty

Cycle MSB

Same as PWMOUT 1 Target Duty Cycle

R/W 43h PWM_START

PWMOUT2

Target Duty

Cycle LSB

R/W 44h PWM_START

PWMOUT3

Target Duty

Cycle MSB

Same as PWMOUT 1 Target Duty Cycle

R/W 45h PWM_START

PWMOUT3

Target Duty

Cycle LSB

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MAX31790 6-Channel PWM-Output Fan RPM Controller

R/W REGISTER POR

STATE FUNCTION D7 D6 D5 D4 D3 D2 D1 D0

R/W 46h PWM_START

PWMOUT4

Target Duty

Cycle MSB

Same as PWMOUT 1 Target Duty Cycle

R/W 47h PWM_START

PWMOUT4

Target Duty

Cycle LSB

R/W 48h PWM_START

PWMOUT5

Target Duty

Cycle MSB

Same as PWMOUT 1 Target Duty Cycle

R/W 49h PWM_START

PWMOUT5

Target Duty

Cycle LSB

R/W 4Ah PWM_START

PWMOUT6

Target Duty

Cycle MSB

Same as PWMOUT 1 Target Duty Cycle

R/W 4Bh PWM_START

PWMOUT6

Target Duty

Cycle LSB

R/W 4Ch 0000 0000b User Byte —

R/W 4Dh 0000 0000b User Byte —

R/W 4Eh 0000 0000b User Byte —

R/W 4Fh 0000 0000b User Byte —

R/W 50h 0011 1100b

TACH 1

Target Count

MSB

210 29282726252423

R/W 51h 0000 0000b

TACH 1

Target Count

LSB

222120RESERVED RESERVEDRESERVED RESERVED RESERVED

R/W 52h 0011 1100b

TACH 2

Target Count

MSB

Same as TACH 1 Target Count

R/W 53h 0000 0000b

TACH 2

Target Count

LSB

R/W 54h 0011 1100b

TACH 3

Target Count

MSB

Same as TACH 1 Target Count

R/W 55h 0000 0000b

TACH 3

Target Count

LSB

X = Input state at POR.

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MAX31790 6-Channel PWM-Output Fan RPM Controller

R/W REGISTER POR

STATE FUNCTION D7 D6 D5 D4 D3 D2 D1 D0

R/W 56h 0011 1100b

TACH 4

Target Count

MSB

Same as TACH 1 Target Count

R/W 57h 0000 0000b

TACH 4

Target Count

LSB

R/W 58h 0011 1100b

TACH 5

Target Count

MSB

Same as TACH 1 Target Count

R/W 59h 0000 0000b

TACH 5

Target Count

LSB

R/W 5Ah 0011 1100b

TACH 6

Target Count

MSB

Same as TACH 1 Target Count

R/W 5Bh 0000 0000b

TACH 6

Target Count

LSB

R/W 5Ch 0000 0000b User Byte —

R/W 5Dh 0000 0000b User Byte —

R/W 5Eh 0000 0000b User Byte —

R/W 5Fh 0000 0000b User Byte —

R/W 60h 0000 0000b Window 1 2726252423222120

R/W 61h 0000 0000b Window 2 Same as Window 1

R/W 62h 0000 0000b Window 3 Same as Window 1

R/W 63h 0000 0000b Window 4 Same as Window 1

R/W 64h 0000 0000b Window 5 Same as Window 1

R/W 65h 0000 0000b Window 6 Same as Window 1

R/W 66h 0000 0000b User Byte —

R/W 67h 0000 0000b User Byte —

X = Input state at POR.

Global Conguration Register (00h)

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MAX31790 6-Channel PWM-Output Fan RPM Controller

Power-On Value 0010 0XX0b

Read Access All

Write Access All

Memory Type Volatile

00h Run/

Standby Reset Bus

Timeout RESERVED OSC I2C Watchdog

I2C

Watchdog

Status

BIT 7 BIT 0

BIT 7

Run/Standby: Places the device in standby mode.

0 = Run

1 = Standby

Entering standby mode sets all PWM duty cycles to 0 and stops fan failure detection. However,

driving the FULL_SPEED input low forces all enabled PWMOUT outputs high (100% duty cycle)

regardless of the state of the Run bit.

BIT 6

Reset:

0 = Normal operation

1 = Reset all registers to POR value

This bit automatically resets itself and always returns a 0 when read.

BIT 5

Bus Timeout: I2C bus timeout.

0 = Enabled

1 = Disabled

The I2C interface resets if SDA is low for more than 35ms.

BIT 4 RESERVED

BIT 3

Oscillator Selection:

0 = Internal oscillator (default at power-on)

1 = External 32.768kHz crystal

Selects on-chip oscillator or external 32.768kHz crystal/ceramic resonator for TACH count and

source for CLKOUT pin. Use crystal or ceramic resonator if higher accuracy is required. When

switching from the internal oscillator to an external crystal, the device operates from the internal

oscillator until the crystal oscillator has started up. If the crystal is damaged or the oscillator fails

to start, the device continues to operate from the internal oscillator.

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MAX31790 6-Channel PWM-Output Fan RPM Controller

BITS 2:1

I2C Watchdog: When active, the watchdog monitors SDA and SCL for valid I2C transactions. If

there are no valid transactions between the master and the device within the watchdog period,

all fan PWM outputs go to 100%. If the watchdog times out and valid I2C transactions begin to

occur again, operation resumes with the previous PWM value. The master can then program the

PWM outputs, target TACH counts, or other functions in the normal manner. When the watchdog

function is active, ensure that the master communicates to the device periodically, for example,

reading a status register. The POR state is set by the state of the WD_START pin at power-up.

BITS 2:1 I2C WATCHDOG PERIOD (s) POR CONDITION

00b Inactive (no watchdog) WD_START = GND

01b 5 —

10b 10 —

11b 30 WD_START = VCC

BIT 0

I2C Watchdog Status:

0 = I2C transactions occurred within watchdog period

1 = Time between I2C transaction exceeds watchdog period

Clear this bit by writing a 0 to it.

PWM Frequency Register (01h)

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MAX31790 6-Channel PWM-Output Fan RPM Controller

Power-On Value XXXX XXXXb

Read Access All

Write Access All

Memory Type Volatile

01h PWM4-PWM6 Frequency PWM1-PWM3 Frequency

BIT 7 BIT 0

BITS 7:4

PWM4-PWM6 Frequency: These bits select the PWM OUT frequency for PWMOUT4, PWMOUT5,

and PWMOUT6 according to the following:

BITS 7:4 PWM OUPUT FREQUENCY POR CONDITION

0000b 25Hz —

0001b 30Hz FREQ_START = GND

0010b 35Hz —

0011b 100Hz —

0100b 125Hz —

0101b 149.7Hz —

0110b 1.25kHz —

0111b 1.47kHz FREQ_START = Unconnected

1000b 3.57kHz —

1001b 5kHz —

1010b 12.5kHz —

1011b 25kHz FREQ_START = VCC

The POR state is set by the state of the FREQ_START pin at power-up as shown.

BITS 3:0

PWM1-PWM3 Frequency: These bits select the PWM OUT frequency for PWMOUT1, PWMOUT2,

and PWMOUT3 according to the following:

BITS 7:4 PWM OUPUT FREQUENCY POR CONDITION

0000b 25Hz —

0001b 30Hz FREQ_START = GND

0010b 35Hz —

0011b 100Hz —

0100b 125Hz —

0101b 149.7Hz —

0110b 1.25kHz —

0111b 1.47kHz FREQ_START = Unconnected

1000b 3.57kHz —

1001b 5kHz —

1010b 12.5kHz —

1011b 25kHz FREQ_START = VCC

The POR state is set by the state of the FREQ_START pin at power-up as shown.

Fan 1 Conguration Register (02h)

Fan 2 Conguration Register (03h)

Fan 3 Conguration Register (04h)

Fan 4 Conguration Register (05h)

Fan 5 Conguration Register (06h)

Fan 6 Conguration Register (07h)

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MAX31790 6-Channel PWM-Output Fan RPM Controller

Power-On Value 0XX0 0000b

Read Access All

Write Access All

Memory Type Volatile

02h, 03h,

04h, 05h,

06h, 07h

Mode Spin-Up Control/

Monitor

TACH Input

Enable

TACH/

Locked

Rotor

Locked

Rotor

Polarity

PWM/

TACH

BIT 7 BIT 0

BIT 7

Mode: RPM/PWM mode select.

0 = PWM mode. PWM duty cycle is set by the value in the associated PWMOUT Target Duty Cycle

1 = RPM mode. The PWM duty cycle is adjusted to produce the TACH count value in the

associated TACH Target Count register.

When changing from PWM to RPM mode, if the current RPM value is different from the value

selected in the TACH Target Count register, the PWM duty cycle starts from the current value and

increment/decrements toward the desired value at the selected duty cycle rate-of-change.

BITS 6:5

Spin-Up: When spin-up is selected and the fan is started with a target PWM duty cycle less than

100%, the device produces 100% duty cycle until two tachometer pulses have been detected. A

maximum spin-up time is also selectable to ensure that the spin-up time is not excessive. After

two tachometer pulses have been detected, or the spin-up has timed out, the duty cycle goes

to the value in the PWMOUT Target Duty Cycle register. The POR state is set by the state of the

SPIN_START pin at power-up.

BITS 6:5 SPIN-UP BEHAVIOR POR CONDITION

00b No spin-up SPIN_START = GND

01b Spin-up until two tachometer pulses or

0.5s (max) SPIN_START = Unconnected

10b Spin-up until two tachometer pulses or

1s (max) SPIN_START = VCC

11b Spin-up until two tachometer pulses or

2s (max) —

BIT 4

Control/Monitor:

0 = Control fan speed.

1 = Monitor only. Associated duty cycle = 0% regardless of other settings; monitor associated

TACH or locked rotor if enabled by bit 3.

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MAX31790 6-Channel PWM-Output Fan RPM Controller

BIT 3

TACH Input Enable: Enables associated TACH input function and fan fault detection

(automatically enabled in RPM mode).

0 = Disabled. When disabled and TACH input is not used, bits 1 and 2 are ignored.

1 = Enabled

BIT 2

TACH/Locked Rotor: Selects TACH input function as TACH count or locked rotor. In locked rotor

mode, assertion of the associated TACH input indicates that the fan has stopped.

0 = TACH count

1 = Locked rotor

BIT 1

Locked Rotor Polarity:

0 = Low locked rotor. Associated TACH input low in locked rotor mode indicates fan is stopped.

1 = High locked rotor. Associated TACH input high in locked rotor mode indicates fan is stopped.

BIT 0

PWM/TACH: When set to 0, the associated PWMOUT produces a PWM waveform for control of

fan speed. When set to 1, PWMOUT becomes a TACH input whose channel number is equal to

the number of the PWMOUT channel plus six.

Fan 1 Dynamics Register (08h)

Fan 2 Dynamics Register (09h)

Fan 3 Dynamics Register (0Ah)

Fan 4 Dynamics Register (0Bh)

Fan 5 Dynamics Register (0Ch)

Fan 6 Dynamics Register (0Dh)

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MAX31790 6-Channel PWM-Output Fan RPM Controller

Power-On Value 0100 1100b

Read Access All

Write Access All

Memory Type Volatile

08h, 09h,

0Ah, 0Bh,

0Ch, 0Dh

Speed Range PWM Rate-of-Change

Asymmetric

Rate of

Change

Reserved

BIT 7 BIT 0

BITS

7:5

Speed Range: The device determines fan speed by counting the number of internal 8192Hz (fTOSC/4) clock

cycles (using an 11-bit counter) during one or more fan tachometer periods. Three bits set the nominal RPM

range for the fan, as shown in the table below. For example, a setting of 010b causes the device to count the

number of 8192Hz (fTOSC/4) clock cycles that occur during four complete tachometer periods. If the fan has

a nominal speed of 2000 RPM and two tachometer pulses per revolution, one tachometer period is nominally

15ms, and four tachometer periods are 60ms. With an 8192Hz (fTOSC/4) clock, the TACH count is therefore

equal to 491. With a fan speed of 1/3 the nominal value, the count is 1474. If the fan’s nominal speed is

1000 RPM, the full-speed TACH count is 983. At 1/3 the nominal speed, there are 2948 clock cycles in four

tachometer periods. This is greater than the maximum 11-bit count of 2047, so four tachometer periods is too

many for this fan; a setting of 001 (two clock cycles) is recommended instead.

The table below shows the full-speed tachometer counts for several combinations of nominal fan speeds and

bits 7:5 settings. The shaded combinations provide the best results. Nonshaded combinations should generally

be avoided. When setting bits 7:5, the goal is to obtain the highest tachometer count without exceeding the

maximum count of 2047 when the fan is at the minimum speed of interest. For example, if the minimum speed of

interest is 1/3 of full speed, the maximum tachometer count is three times the value shown in the table.

BITS 7:5

NUMBER

OF TACH

PERIODS

COUNTED

RPM

500 1000 2000 4000 8000 16000

000b 1 491 (60ms) 245

(30ms)

122

(15ms)

(7.5ms)

(3.75ms)

(1.875ms)

001b 2 983

(120ms)

491

(60ms)

245

(30ms)

122

(15ms)

(7.5ms)

(3.75ms)

010b

(default) 41966

(240ms)

983

(120ms)

491

(60ms)

245

(30ms)

122

(15ms)

(7.5ms)

011b 8 2047

(480ms)

1966

(240ms)

983

(120ms)

491

(60ms)

245

(30ms)

122

(15ms)

100b 16 2047

(960ms)

2047

(480ms)

1966

(240ms)

983

(120ms)

491

(60ms)

245

(30ms)

101b, 110b,

111b 32 2047

(1920ms)

2047

(960ms)

2047

(480ms)

1966

(240ms)

983

(120ms)

491

(60ms)

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MAX31790 6-Channel PWM-Output Fan RPM Controller

BITS

4:2

PWM Rate-of-Change: PWM duty cycle rate of change. The PWM duty cycle at the associated PWMOUT

outputs varies from 0 to full scale in 512 increments. The rate-of-change bits determine the time interval between

duty cycle output increments/decrements. Regardless of the settings, there are a few cases for which the rate-

of-change is always 0:

In RPM mode, when a TACH target count of 2047 (7FFh) is selected, the duty cycle immediately goes to 0%.

A full-scale target count is assumed to mean that the intent is to shut down the fan, and going directly to 0%

avoids the possibility of loss of control-loop feedback at high TACH counts. If a slow-speed decrease toward

0% is desired, select a TACH target count at the slowest practical value for the fan. Once that count has been

reached, selecting a count of 2047 (7FF) then takes the drive immediately to 0%.

In PWM mode, when a target duty cycle of 0% is selected, the duty cycle goes to 0%. Again, it is assumed

that the intent is to shut down the fan. If a slow-speed decrease toward 0% is desired, a target duty cycle of

the slowest practical value for the fan in question should be chosen. Once that duty cycle has been reached,

selecting a target value of 0% then takes the drive immediately to 0%.

When the current duty cycle is 0% in PWM mode, selecting a new target duty cycle immediately takes the duty

cycle to that value. The fan spins up first if spin-up is enabled. When the current duty cycle is 0% in RPM mode,

selecting a new TACH target count that is less than 2047 (7FFh) immediately takes the duty cycle to the value in

the PWMOUT Target Duty Cycle register. From this value, the duty cycle increments as needed to achieve the

desired TACH target count. The fan spins up first if spin-up is enabled.

BITS 4:2

TIME BETWEEN DUTY CYCLE

INCREMENTS (ms) RPM

TIME FROM 33% TO 100%

(s)

PWM RPM PWM RPM

000b 0 0.9765 0 0.33

001b 1.953125 0.67

010b 3.90625 1.34

011b (default) 7.8125 2.7

100b 15.625 5.3

101b 31.25 10.7

110b 62.5 21.4

111b 125 42.8

BIT 1

Asymmetric Rate of Change:

0 = Same rate of change whether duty cycle is increasing or decreasing.

1 = Rate of change when duty cycle is decreasing is half the rate when increasing.

BIT 0 RESERVED

User Byte Register (0Eh–0Fh)

User Byte Register (15h–17h)

User Byte Register (4Ch–4Fh)

User Byte Register (5Ch–5Fh)

User Byte Register (66h–67h)

Fan Fault Status 2 Register (10h)

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MAX31790 6-Channel PWM-Output Fan RPM Controller

Power-On Value 0000 0000b

Read Access All

Write Access All

Memory Type Volatile

0Eh, 0Fh,

15h, 16h,

17h, 4Ch,

4Dh, 4Eh,

4Fh, 5Ch,

5Dh, 5Eh,

5Fh, 66h,

67h

— — — — — — — —

BIT 7 BIT 0

General-purpose volatile bits. These bits have no affect on the device operation.

Power-On Value 0000 0000b

Read Access All

Write Access All

Memory Type Volatile

10h RESERVED RESERVED Fan 12

Fault

Fan 11

Fault

Fan 10

Fault Fan 9 Fault Fan 8 Fault Fan 7 Fault

BIT 7 BIT 0

BITS 7:6 RESERVED

BITS 5:0

Fan [12:7] Fault: This register applies only to PWMOUTs that are being used as TACH inputs.

The associated fan experienced faults as defined in the Fan Failure section. When a fan fault is

detected, the associated fault bit is set to 1. The fault bits latch until they are cleared by writing a

PWM target duty cycle or TACH target count to the associate fan’s control register, thus allowing

short-term faults to be identified.

Fan Fault Status 1 Register (11h)

Fan Fault Mask 2 Register (12h)

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MAX31790 6-Channel PWM-Output Fan RPM Controller

Power-On Value 0000 0000b

Read Access All

Write Access All

Memory Type Volatile

11h RESERVED RESERVED Fan 6 Fault Fan 5 Fault Fan 4 Fault Fan 3 Fault Fan 2 Fault Fan 1 Fault

BIT 7 BIT 0

BITS 7:6 RESERVED

BITS 5:0

Fan [6:1] Fault: The associated fan experienced faults as defined in the Fan Failure section.

When a fan fault is detected, the associated fault bit is set to 1. The fault bits latch until they are

cleared by writing a PWM target duty cycle or TACH target count to the fan’s control register,

thus allowing short-term faults to be identified.

Power-On Value 0011 1111 b

Read Access All

Write Access All

Memory Type Volatile

12h RESERVED RESERVED Fan 12

Mask

Fan 11

Mask

Fan 10

Mask Fan 9 Mask Fan 8 Mask Fan 7 Mask

BIT 7 BIT0

BITS 7:6 RESERVED

BITS 5:0

Fan [12:7] Mask: This register applies only to PWMOUTs that are being used as TACH inputs.

These bits mask faults on selected fans from asserting the FAN_FAIL output. Faults are still

indicated by the fault status bits:

0 = Not masked

1 = Masked

When a fan fails, the PWM behavior is controlled by the Failed Fan Options register.

Fan Fault Mask 1 Register (13h)

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MAX31790 6-Channel PWM-Output Fan RPM Controller

Power-On Value 0011 1111b

Read Access All

Write Access All

Memory Type Volatile

13h RESERVED RESERVED Fan 6 Mask Fan 5 Mask Fan 4 Mask Fan 3 Mask Fan 2 Mask Fan 1 Mask

BIT 7 BIT0

BITS 7:6 RESERVED

BITS 5:0

Fan [6:1] Mask: These bits mask faults on selected fans from asserting the FAN_FAIL output.

Faults are still indicated by the fault status bits:

0 = Not masked

1 = Masked

When a fan fails, the PWM behavior is controlled by the Failed Fan Options register.

Failed Fan Options/Sequential Start Register (14h)

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MAX31790 6-Channel PWM-Output Fan RPM Controller

Power-On Value 0100 0101b

Read Access All

Write Access All

Memory Type Volatile

14h Sequential Start Delay RESERVED Failed Fan Options Fan Fault Queue

BIT 7 BIT 0

BITS 7:5

Sequential Start Delay: Note that faults are not monitored until the channel is activated after its

associated start delay. These bits select the time delay between fan activations according to the

following:

BITS 7:5 TIME BETWEEN SEQUENTIAL FAN STARTUP

000b 0s

001b 250ms

010b

(default) 500ms

011b 1s

100b 2s

101b

110b

111b

BIT 4 RESERVED

BITS 3:2

Failed Fan Options: These bits select the behavior following a fan failure according to the

following:

BITS 3:2 SET DUTY CYCLE ON FAILURE

00b 0%

01b Continue current PWM or RPM mode operation

10b 100% on failed fan

11b All fans to 100% on any unmasked fan failure

BITS 1:0

Fan Fault Queue: These bits select the number of consecutive fault detections that are required

to decide that the fan has failed according to the following:

BITS 1:0 NUMBER OF QUALIFIED FAULTS

00b 1

01b 2

10b 4

11b 6

When the selected number of fault detections has occurred, the FAN_FAIL output asserts (if fault

detection is not masked for the fan). Note: Fan fault queue has no effect on the locked rotor fan-

failure detection.

TACH 1 Count Registers (18h–19h)

TACH 2 Count Registers (1Ah–1Bh)

TACH 3 Count Registers (1Ch–1Dh)

TACH 4 Count Registers (1Eh–1Fh)

TACH 5 Count Registers (20h–21h)

TACH 6 Count Registers (22h–23h)

TACH 7 Count Registers (24h–25h)

TACH 8 Count Registers (26h–27h)

TACH 9 Count Registers (28h–29h)

TACH 10 Count Registers (2Ah–2Bh)

TACH 11 Count Registers (2Ch–2Dh)

TACH 12 Count Registers (2Eh–2Fh)

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MAX31790 6-Channel PWM-Output Fan RPM Controller

Power-On Value 1111 1111b (MSB)

1110 0000b (LSB)

Read Access All

Write Access N/A

Memory Type Volatile

18h, 1Ah, 1Ch,

1Eh, 20h, 22h,

24h, 26h, 28h,

2Ah, 2Ch, 2Eh

210 29282726252423

19h, 1Bh, 1Dh,

1Fh, 21h, 23h,

25h, 27h, 29h,

2Bh, 2Dh, 2Fh

2221200 0 0 0 0

BIT 7 BIT 0

Indicates the associated number of 8192Hz (fTOSC/4) clock pulses counted during the counting period. The

TACH Count register consists of 11 bits, left-justified, contained in 2 bytes. The lower 5 bits always return

zeros. To minimize noise from spurious tachometer transitions, pulses less that tTACHMIN are ignored. The

TACH 7 Count to TACH 12 Count registers apply only to PWMOUTs that are being used as TACH inputs.

PWMOUT 1 Duty Cycle (30h–31h)

PWMOUT 2 Duty Cycle (32h–33h)

PWMOUT 3 Duty Cycle (34h–35h)

PWMOUT 4 Duty Cycle (36h–37h)

PWMOUT 5 Duty Cycle (38h–39h)

PWMOUT 6 Duty Cycle (3Ah–3Bh)

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MAX31790 6-Channel PWM-Output Fan RPM Controller

Power-On Value 0000 0000b (MSB)

0000 0000b (LSB)

Read Access All

Write Access N/A

Memory Type Volatile

30h, 32h,

34h, 36h,

38h, 3Ah

2827262524232221

31h, 33h,

35h, 37h,

39h, 3Bh

200 0 0 0 0 0 FS

BIT 7 BIT 0

This is a 9-bit left-justified value that ranges from 0 to 511 and is contained in 2 bytes. This register shows the

actual PWM duty cycle for the associate PWM output. When the value is 511 (decimal), the duty cycle is 100%.

The 6:1 bits always return zeros. Register does not apply when associated PWMOUTs are being used as TACH

inputs. The register value is converted to the duty cycle at the fan as follows:

BITS 15:7 PWM DUTY CYCLE (%)

DECIMAL HEX

0 000h 0

200 0C8h 39

300 12Ch 59

400 190h 78

480 1E0h 94

511 1FFh 100

When duty cycle reaches 100%, bit 0 is set to a 1.

PWMOUT 1 Target Duty Cycle (40h–41h)

PWMOUT 2 Target Duty Cycle (42h–43h)

PWMOUT 3 Target Duty Cycle (44h–45h)

PWMOUT 4 Target Duty Cycle (46h–47h)

PWMOUT 5 Target Duty Cycle (48h–49h)

PWMOUT 6 Target Duty Cycle (4Ah–4Bh)

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MAX31790 6-Channel PWM-Output Fan RPM Controller

Power-On Value XXXX XXXXb (MSB)

X000 0000b (LSB)

Read Access All

Write Access All

Memory Type Volatile

40h, 42h, 44h,

46h, 48h, 4Ah 2827262524232221

41h, 43h, 45h,

47h, 49h, 4Bh 20RESERVED RESERVED RESERVED RESERVED RESERVED RESERVED RESERVED

BIT 7 BIT 0

This is a 9-bit left-justified value that ranges from 0 to 511 and is contained in 2 bytes.

In PWM mode, write the desired PWM duty cycle to these two registers. The device then increments the duty

cycle to this value at a rate determined by the PWM duty cycle rate-of-change bits.

In RPM mode, the value contained in this register is the duty cycle at PWMOUT immediately after spin-up or after

changing the TACH target count from 2047 (7FF) to any value lower than 2047 (7FF). For example, if the fan is

currently stopped with spin-up disabled, and a new TACH target count corresponding to 60% of the full-scale

fan speed is to be selected, the duty cycle can be programmed to immediately go to 60% when the new TACH

target count is selected, and then close the RPM control loop starting from that duty cycle. The register value is

converted to the duty cycle at the fan as follows:

BITS 15:7 PWM DUTY CYCLE (%)

DECIMAL HEX

0 000h 0

200 0C8h 39

300 12Ch 59

400 190h 78

480 1E0h 94

511 1FFh 100

The value of the PWMOUT target duty cycle at POR depends on the state of the PWM_START0 and PWM_

START1 inputs as follows:

POR CONDITION

PWM_START0 PWM_START1 PWM DUTY CYCLE (%)

GND GND 0

GND Unconnected 30

GND VCC 40

Unconnected GND 50

Unconnected VCC 60

VCC GND 75

VCC VCC 100

TACH 1 Target Count (50h–51h)

TACH 2 Target Count (52h–53h)

TACH 3 Target Count (54h–55h)

TACH 4 Target Count (56h–57h)

TACH 5 Target Count (58h–59h)

TACH 6 Target Count (5Ah–5Bh)

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MAX31790 6-Channel PWM-Output Fan RPM Controller

Power-On Value 0011 1100b (MSB)

0000 0000b (LSB)

Read Access All

Write Access All

Memory Type Volatile

50h, 52h,

54h, 56h,

58h, 5Ah

210 29282726252423

51h, 53h,

55h, 57h,

59h, 5Bh

222120RESERVED RESERVED RESERVED RESERVED RESERVED

BIT 7 BIT 0

This is an 11-bit left-justified value that is contained in 2 bytes. It indicates the desired number of 8192Hz

(fTOSC/4) clock pulses counted during the counting period.

In RPM mode, write the desired tachometer count to this register. The device then adjusts the associated PWM

duty cycle to achieve this tachometer count.

In PWM mode, this register is not used as part of the fan control algorithm. In both PWM and RPM modes, this

When changing from PWM mode to RPM mode, best results are obtained by loading this register with the

current TACH count before changing to RPM mode. The TACH target count for a given RPM is obtained by the

following equation: 60

TACH Count SR 8192

NP RPM

= ××

where:

NP = number of TACH pulse per revolution

SR = 1, 2, 4, 8, 16, 32 (see the Speed Range bit information in the associated Fan Dynamics register)

Window 1 Register (60h)

Window 2 Register (61h)

Window 3 Register (62h)

Window 4 Register (63h)

Window 5 Register (64h)

Window 6 Register (65h)

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MAX31790 6-Channel PWM-Output Fan RPM Controller

Power-On Value 0000 0000b

Read Access All

Write Access All

Memory Type Volatile

60h, 61h,

62h, 63h,

64h, 65h

2726252423222120

BIT 7 BIT 0

When the fan’s speed is near the target speed in RPM mode—that is, when the TACH count is near the

corresponding TACH target count—the control loop dynamics can often be improved by slowing the rate of

change of the PWM duty cycle. This operates as follows: First, set a value for the count “window” and store it

in the appropriate window register. In RPM mode, calculate the difference between the current TACH count

and the target TACH count. If the absolute value of this difference is less than the value in the window register,

then the update rate of the PWM duty cycle is slowed to 1 LSB per second. When the current TACH count falls

outside of the window, the duty cycle rate of change reverts to the selected value. Note: When operating in

PWM mode, the window value is typically set to 0.

I2C Serial Interface Description

I2C Denitions

The following terminology is commonly used to describe

I2C data transfers. See the timing diagram (Figure 1) and

the I2C AC Electrical Characteristics table for additional

information.

Master Device: The master device controls the slave

devices on the bus. The master device generates SCL

clock pulses and START and STOP conditions.

Slave Devices: Slave devices send and receive data

at the master’s request.

Bus Idle or Not Busy: Time between STOP and

START conditions when both SDA and SCL are inac-

tive and in their logic-high states.

START Condition: A START condition is generated

by the master to initiate a new data transfer with a

slave. Transitioning SDA from high to low while SCL

remains high generates a START condition.

STOP Condition: A STOP condition is generated

by the master to end a data transfer with a slave.

Transitioning SDA from low to high while SCL remains

high generates a STOP condition.

Repeated START condition: The master can use

a repeated START condition at the end of one data

transfer to indicate that it will immediately initiate a

new data transfer following the current one. Repeated

STARTs are commonly used during read operations

to identify a specific memory address to begin a data

transfer. A repeated START condition is issued identi-

cally to a normal START condition.

Bit Write: Transitions of SDA must occur during the

low state of SCL. The data on SDA must remain valid

and unchanged during the entire high pulse of SCL

plus the setup and hold time requirements. Data is

shifted into the device during the rising edge of the

SCL.

Bit Read: At the end of a write operation, the master

must release the SDA bus line for the proper amount

of setup time before the next rising edge of SCL dur-

ing a bit read. The device shifts out each bit of data

on SDA at the falling edge of the previous SCL pulse

and the data bit is valid at the rising edge of the cur-

rent SCL pulse. Remember that the master generates

all SCL clock pulses including when it is reading bits

from the slave.

Acknowledgement (ACK and NACK): An acknowl-

edgement (ACK) or not-acknowledge (NACK) is

always the 9th bit transmitted during a byte trans-

fer. The device receiving data (the master during a

read or the slave during a write operation) performs

an ACK by transmitting a zero during the 9th bit. A

device performs a NACK by transmitting a one (done

by releasing SDA) during the 9th bit. The timing

(Figure 1) for the ACK and NACK is identical to all

other bit writes. An ACK is the acknowledgment that

the device is properly receiving data. A NACK is used

to terminate a read sequence or as an indication that

the device is not receiving data.

Byte Write: A byte write consists of 8 bits of informa-

tion transferred from the master to the slave (most

significant bit first) plus a 1-bit acknowledgement from

the slave to the master. The 8 bits transmitted by the

master are done according to the bit write definition

and the acknowledgement is read using the bit read

definition.

Byte Read: A byte read is an 8-bit information transfer

from the slave to the master plus a 1-bit ACK or NACK

from the master to the slave. The 8 bits of information

that are transferred (most significant bit first) from the

slave to the master are read by the master using the

bit read definition, and the master transmits an ACK

using the bit write definition to receive additional data

bytes. The master must NACK the last byte read to

terminate communication so the slave returns control

of SDA to the master.

Slave Address Byte: Each slave on the I2C bus

responds to a slave address byte sent immediately

following a START condition. The slave address byte

contains the slave address in the most significant 7

bits and the R/W bit in the least significant bit. See

the Slave Address Byte and Address Pins section for

details on the MAX31790’s slave address.

If an incorrect (nonmatching) slave address is written,

the MAX31790 assumes the master is communicating

with another I2C device and ignores the communica-

tion until the next START condition is sent.

Memory Address: During an I2C write operation to

the MAX31790, the master must transmit a memory

address to identify the memory location where the

slave is to store the data. The memory address is

always the second byte transmitted during a write

operation following the slave address byte.

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MAX31790 6-Channel PWM-Output Fan RPM Controller

I2C Communication

See Figure 8 for I2C communication examples.

Writing a Single Byte to a Slave: The master must

generate a START condition, write the slave address

byte (R/W = 0), write the memory address, write the

byte of data, and generate a STOP condition. The

master must read the slave’s acknowledgement dur-

ing all byte write operations.

Reading a Single Byte from a Slave: Unlike the write

operation that uses the specified memory address

byte to define where the data is to be written, the read

operation occurs at the present value of the memory

address counter. To read a single byte from the slave,

the master generates a START condition, writes the

slave address byte with R/W =1, reads the data byte

with a NACK to indicate the end of the transfer, and

generates a STOP condition. However, since requir-

ing the master to keep track of the memory address

counter is impractical, the following method should

be used to perform reads from a specified memory

location.

Manipulating the Address Counter for Reads: A

dummy write cycle can be used to force the address

counter to a particular value. To do this, the mas-

ter generates a START condition, writes the slave

address byte (R/W = 0), writes the memory address

where it desires to read, generates a repeated START

condition, writes the slave address byte (R/W = 1),

reads data with ACK or NACK as applicable, and

generates a STOP condition. Recall that the master

must NACK the last byte to inform the slave that no

additional bytes are to be read.

Reading Multiple Bytes from a Slave: The read

operation can be used to read multiple bytes with a

single transfer. When reading bytes from the slave,

the master simply ACKs the data byte if it desires to

read another byte before terminating the transaction.

After the master reads the last byte it must NACK to

indicate the end of the transfer and generates a STOP

condition.

Figure 8. I2C Communication Examples

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MAX31790 6-Channel PWM-Output Fan RPM Controller

START

START STOP

SLAVE

ACK

SLAVE

ACK

STOP

SINGLE-BYTE WRITE

-WRITE 00h TO REGISTER 0Eh

SINGLE-BYTE READ

-READ REGISTER 0Eh

REPEATED

START

MASTER

NACK

01000000

40h

00001110

0Eh

SLAVE

ACK

START SLAVE

ACK

01000000

40h

01000001

41h

00001110

0Eh

SLAVE

ACK

SLAVE

ACK

STOP

00000000

00h

STOP

SLAVE

ACK

STOP

01110101

75h

START SLAVE

ACK

01000000

40h

00001110

0Eh

SLAVE

ACK

SLAVE

ACK

00000001

01h

SLAVE

ACK DATA IN 0Eh

DATA

REPEATED

START

MASTER

ACK

START SLAVE

ACK

01000000

40h

01000001

41h

00001110

0Eh

SLAVE

ACK

SLAVE

ACK DATA IN 0Eh

DATA

MASTER

NACK

DATA IN 0Fh

DATA

EXAMPLE I2C TRANSACTIONS WITH 40h AS THE DEVICE ADDRESS (WHEN ADD0 AND ADD1 ARE CONNECTED TO GND)

*THE SLAVE ADDRESS IS DETERMINED BY ADDRESS PINS ADD0 AND ADD1.

TYPICAL I2C WRITE TRANSACTION

2-BYTE WRITE

-WRITE 0Eh AND 0Fh

TO 01h AND 75h

2-BYTE READ

-READ 0Eh AND 0Fh

MSB LSB

b7 b6 b5 b4 b3 b2 b1 b0

MSB LSB

b7 b6 b5 b4 b3 b2 b1 b0

DATA

SLAVE

ACK

SLAVE

ACK

SLAVE

ADDRESS*

010 R/W

MSB LSB

READ/

WRITE

SEE TABLE 1

Applications Information

Controlling 4-Wire Fans (PWM Mode)

Initial Settings:

• Begin with the POR settings.

• Write the desired number of tachometer periods to

be counted in the Speed Range bits (7:5 of the cor-

responding Fan Dynamics register).

• Write the maximum allowable tachometer count to

the corresponding TACH Target Count register.

Tachometer counts greater than this value results in a

fan fault detection. Choose a value that is not encoun-

tered during normal operation, accounting for normal

fan speed tolerances.

• Set the TACH Input Enable bit in the corresponding

Fan Configuration register to 1. Note: This bit can be

set after the fan has been started, if desired. If the bit

is set before writing a PWMOUT target duty cycle, the

duty cycle should be set immediately after enabling

the TACH input to avoid failure detection before the

fan has started spinning.

Starting the Fan:

• Write the desired duty cycle value to the correspond-

ing PWMOUT Target Duty Cycle register.

Changing Speeds:

• Write the new desired duty-cycle value to the corre-

sponding PWMOUT Target Duty Cycle register.

Stopping the Fan:

• Write a duty cycle value of 0% to the PWMOUT Target

Duty Cycle register.

• If a gradual decrease in fan speed is desired, write

the lowest duty cycle at which the fan can reliably

operate. When the duty cycle reaches that value, write

0% to the PWMOUT Target Duty Cycle register.

Controlling 4-Wire Fans (RPM Mode)

Begin as in PWM mode and start the fan.

Changing from PWM Mode to RPM Mode:

• Write the desired tachometer count to the corre-

sponding TACH Target Count registers.

• Set bit 7 of the corresponding Fan Configuration reg-

ister to 1. This selects RPM mode. The fan goes to the

selected TACH target count.

Changing Speeds:

• Write the desired tachometer count to the corre-

sponding TACH Target Count register.

Stopping the Fan:

• Set the TACH target count value to full scale. The

PWM duty cycle immediately goes to 0%.

Or:

• Write the current duty cycle into the corresponding

PWMOUT Target Duty Cycle register.

• Write a value greater than the current tachometer

count into the TACH Target Count register.

• Write a 0 to bit 7 of the corresponding Fan Configuration

• Write a duty cycle value of 0% to the PWMOUT Target

Duty Cycle register.

• If a gradual decrease in fan speed is desired, write

the lowest duty cycle at which the fan can reliably

operate. When the duty cycle reaches that value, write

0 to the PWMOUT Target Duty Cycle register.

Controlling 2-Wire and 3-Wire Fans

Although the device is optimized for use with fans that

have speed-control inputs (generally referred to as 4-wire

fans), it can also be used with fans that have no speed-

control inputs. When a fan has no speed-control input, its

speed can be controlled by modulating the fan’s power-

supply voltage. One approach is shown in Figure 9,

where the PWM output of the fan is lowpass filtered and

converted to a variable DC voltage that powers the fan.

The highest available PWM frequency should be used.

This approach works well with a wide variety of fans.

Because the fan’s supply current must pass through the

MOSFET, this approach is best with lower power fans

that use on the order of 200mA or less supply current.

For higher power fans, more efficient DC drive can

be generated using a low-cost DC-DC converter. A

DC-DC converter can be used to efficiently transform the

device’s PWMOUT signal into a variable power supply for

the fan, as shown in Figure 10.

www.maximintegrated.com Maxim Integrated

│

MAX31790 6-Channel PWM-Output Fan RPM Controller

Figure 9. High-Side Linear Drive Circuit for 2-Wire or 3-Wire Fans

Figure 10. Using a Low-Cost DC-DC Converter to Efficiently Transform PWMOUT Signal Into Variable Power Supply for the Fan

www.maximintegrated.com Maxim Integrated

│

MAX31790 6-Channel PWM-Output Fan RPM Controller

MAX31790

3.3V

PWMOUT1

0.1µF

2.2µF

10µF

3.3V

VFAN

3.3V

TACH1

TACH OUTPUT TACH

OUTPUT

3.3V

4.7kΩ

91kΩ

100kΩ

2N3904

100kΩ

33kΩ

MAX31790

33063A

3.3V

PWMOUT1

0.1µF

TACH1

3.3V

4.7kΩ

IPK

VCC

1N5819

47µH

GND

C E

0.5Ω

3.6kΩ

R1A

1.2kΩ

0.1µF

470µF

47µF

+20V

470pF

A simpler approach to driving a 2-wire or 3-wire fan,

although with important limitations, is to modulate the

fan’s power supply with the PWM signal. In this case,

a low PWM frequency (typically in the 30Hz range) is

used to drive the gate of an n-channel MOSFET, which

provides the connection between the fan and the power

supply ground, as shown in Figure 11. There are three

important disadvantages to this approach. First, the

rapid application and removal of the supply voltage when

modulated at a 30Hz rate causes an audible increase

in fan noise. Second, some fan vendors recommend

against modulating the power supply with a PWM output

because of reliability concerns. Check with your fan ven-

dor to confirm that this approach is compatible with the

fan you plan to use. Third, when using a 3-wire fan, the

TACH signal is modulated by the PWM signal, making

the TACH signal difficult to use for closed-loop speed

control.

Power-Supply Decoupling

To achieve the best results when using the MAX31790,

decouple the power supply with a 0.01FF or 0.1FF

capacitor. Use a high-quality ceramic surface-mount

capacitor if possible. Surface-mount components mini-

mize lead inductance, which improves performance,

and ceramic capacitors tend to have adequate high fre-

quency response for decoupling applications.

Layout Considerations

The crystal should be placed very close to the device

to minimize excessive loading due to parasitic capaci-

tances. Care should also be taken to minimize loading

on pins that could be left unconnected as a programming

option (SPIN_START, PWM_START0, PWM_START1,

FREQ_START). Coupling on inputs due to clocks should

be minimized.

Figure 11. Controlling the Speed of a 2-/3-Wire Fan by Pulse-

Width Modulating the Fan’s Power-Supply Voltage

+Denotes a lead(Pb)-free/RoHS-compliant package.

T = Tape and reel.

*EP = Exposed pad.

Ordering Information

Packaging Information

For the latest package outline information and land patterns (foot-

prints), go to www.maximintegrated.com/packages. Note that a

“+”, “#”, or “-” in the package code indicates RoHS status only.

Package drawings may show a different suffix character, but the

drawing pertains to the package regardless of RoHS status.

www.maximintegrated.com Maxim Integrated

│

MAX31790 6-Channel PWM-Output Fan RPM Controller

PART TEMP RANGE PIN-PACKAGE

MAX31790ATI+ -40NC to +125NC28 TQFN-EP*

MAX31790ATI+T -40NC to +125NC28 TQFN-EP*

PACKAGE

TYPE

PACKAGE

CODE

OUTLINE

NO.

LAND

PATTERN NO.

28 TQFN-EP T2844+1 21-0139 90-0035

MAX31790

3.3V

TACH1

0.1µF 3.3V

VFAN

3.3V

PWMOUT1

TACH OUTPUT

3.3V

4.7kΩ

Revision History

Maxim Integrated cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim Integrated product. No circuit patent licenses

are implied. Maxim Integrated reserves the right to change the circuitry and specications without notice at any time. The parametric values (min and max limits)

shown in the Electrical Characteristics table are guaranteed. Other parametric values quoted in this data sheet are provided for guidance.

│

MAX31790 6-Channel PWM-Output Fan RPM Controller

REVISION

NUMBER

REVISION

DATE DESCRIPTION PAGES

CHANGED

0 12/12 Initial release —

1 4/13 Internal oscillator aging specification was required for Note 7 4

2 1/15 Revised Benefits and Features section 1

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