Rainbow Electronics MAX6616 User Manual

General Description

The MAX6615/MAX6616 monitor two temperature chan­nels, either the internal die temperature and the temper­ature of an external thermistor, or the temperatures of
two external thermistors. The temperature data controls
a PWM output signal to adjust the speed of a cooling
fan, thereby minimizing noise when the system is run­ning cool, but providing maximum cooling when power
dissipation increases. The fans’ tachometer output sig­nals are monitored by the MAX6615/MAX6616 to detect
fan failure. If a fan failure is detected, the FAN_FAIL
output is asserted.

The 2-wire serial interface accepts standard system
management bus (SMBusTM) write byte, read byte,
send byte, and receive byte commands to read the
temperature data and program the alarm thresholds.
The programmable alarm output can be used to gener­ate interrupts, throttle signals, or overtemperature shut­down signals.

The MAX6616 features six GPIOs to provide additional
flexibility. All of the GPIOs power-up as inputs, with the
exception of GPIO0, which powers up as either an input
or an output as determined by connecting the PRESET
pin to ground or VCC.

The MAX6616 is available in a 24-pin QSOP package,
while the MAX6615 is available in a 16-pin QSOP pack­age. Both devices operate from a single-supply voltage
range of 3.0V to 5.5V, have operating temperature
ranges of -40°C to +125°C, and consume just 500µA of
supply current.

Applications

Desktop Computers

Servers

Power Supplies

Networking Equipment

Workstations

Features

♦ Two Thermistor Inputs

♦ Two Open-Drain PWM Outputs for Fan-Speed

Control

♦ Local Temperature Sensor

♦ Six GPIOs (MAX6616)

♦ Programmable Fan-Control Characteristics

♦ Controlled PWM Rate-of-Change Ensures

Unobtrusive Fan-Speed Adjustments

♦ Fail-Safe System Protection
♦ OT Output for Throttling or Shutdown

♦ Nine Different Pin-Programmable SMBus

Addresses

♦ 16-Pin and 24-Pin QSOP Packages

MAX6615/MAX6616

Dual-Channel Temperature Monitors and

Fan-Speed Controllers with Thermistor Inputs

________________________________________________________________ Maxim Integrated Products 1

19-3713; Rev 1; 7/05

For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.

EVALUATION KIT

AVAILABLE

Ordering Information

SMBus is a trademark of Intel Corp.

Typical Application Circuits and Pin Configurations appear at
end of data sheet.

THERMISTORS

AND LOCAL

TEMP SENSOR

PWM

GENERATOR

AND TACH

COUNTER

SMBus

INTERFACE

AND

REGISTERS

LOGIC

ADD0 ADD1

*MAX6616 ONLY

FAN_FAIL

MAX6615
MAX6616

GND

V

CC

SDA

SCL

REF

TH1

TH2

PWM1

PWM2

TACH1

TACH2

OT

GPIO0*

GPIO5*

PRESET*

Functional Diagram

PART TEMP RANGE PIN-PACKAGE

MAX6615AEE -40°C to +125°C 16 QSOP

MAX6616AEG -40°C to +125°C 24 QSOP

MAX6615/MAX6616

Dual-Channel Temperature Monitors and
Fan-Speed Controllers with Thermistor Inputs

2 _______________________________________________________________________________________

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
operation 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.

All Voltages Are Referenced to GND
Supply Voltage (V

CC

) ...............................................-0.3V to +6V

PWM_, TACH_, OT, FAN_FAIL ............................-0.3V to +13.5V

ADD0, ADD1, SDA, SCL ..........................................-0.3V to +6V

All Other Pins..............................................-0.3V to (V

CC

+ 0.3V)

SDA, OT, FAN_FAIL, PWM_, GPIO_ Current....................±50mA

TH_ Current ........................................................................±1mA

REF Current ......................................................................±20mA

Continuous Power Dissipation (T

A

= +70°C)
16-Pin QSOP (derated at 8.3mW/°C

above +70°C)............................................................666.7mW

24-Pin QSOP (derated at 9.5mW/°C

above +70°C)...........................................................761.9 mW

ESD Protection (all pins, Human Body Model) ....................±2kV

Operating Temperature Range .........................-40°C to +125°C

Junction Temperature......................................................+150°C

Storage Temperature Range .............................-65°C to +150°C

Lead Temperature (soldering, 10s) .................................+300°C

ELECTRICAL CHARACTERISTICS

(VCC= +3.0V to +5.5V, TA= 0°C to +125°C, unless otherwise noted. Typical values are at VCC= +3.3V, TA= +25°C.)

Operating Supply Voltage

Standby Current Interface inactive, ADC in idle state 10 µA

Operating Current I

External Temperature Error

Internal Temperature Error

Temperature Resolution 0.125 °C

Conversion Time 250 ms

Conversion Rate Timing Error -20 +20 %

PWM Frequency Error -20 +20 %

INPUT/OUTPUT

Output Low Voltage V

Output High Leakage Current I

Logic Low Input Voltage V

Logic High Input Voltage V

Input Leakage Current 1µA

Input Capacitance C

SMBus TIMING (Figures 2, 3) (Note 2)

Serial Clock Frequency f

Clock Low Period t

Clock High Period t

Bus Free Time Between STOP
and START Conditions

SMBus START Condition
Setup Time

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

V

CC

Interface inactive, ADC active 0.5 1 mA

S

V

OL

OH

IL

IH

IN

SCLK

LOW

HIGH

t

BUF

t

SU:STA

= +3.3V, 0.15V ≤ V

CC

thermistor errors, thermistor nonlinearity) (Note1)

VCC = +3.3V, 0°C ≤ TA ≤ +85°C, ±2.5

V

= +3.3V, 0°C ≤ TA ≤ +125°C ±4

CC

VCC = +3V, I

10% to 10% 4 µs

90% to 90% 4.7 µs

90% of SCL to 90% of SDA 4.7 µs

= 6mA 0.4 V

OUT

≤ +0.71V (excludes

TH_

3.0 5.5 V

±1 °C

1µA

0.8 V

2.1 V

5pF

10 400 kHz

4.7 µs

°C

MAX6615/MAX6616

Dual-Channel Temperature Monitors and

Fan-Speed Controllers with Thermistor Inputs

_______________________________________________________________________________________ 3

Note 1: 1°C of error corresponds to an ADC error of 7.76mV when V

REF

= 1V.

Note 2: Guaranteed by design and characterization.
Note 3: Production tested.

ELECTRICAL CHARACTERISTICS (continued)

(VCC= +3.0V to +5.5V, TA= 0°C to +125°C, unless otherwise noted. Typical values are at VCC= +3.3V, TA= +25°C.)

Typical Operating Characteristics

(V

CC

= +3.3V, TA= +25°C, unless otherwise noted.)

SUPPLY CURRENT

vs. SUPPLY VOLTAGE

MAX6615/6 toc01

SUPPLY VOLTAGE (V)

SUPPLY CURRENT (µA)

5.04.54.03.5

10

100

1000

1

3.0 5.5

LOCAL

REMOTE

SHUTDOWN

0

40

20

80

60

100

120

THERMISTOR TEMPERATURE DATA

vs. THERMISTOR TEMPERATURE

MAX6615/6 toc02

THERMISTOR TEMPERATURE (°C)

THERMISTOR TEMPERATURE DATA (°C)

0406020 80 100 120

LOCAL TEMPERATURE ERROR

vs. DIE TEMPERATURE

MAX6615/6 toc03

DIE TEMPERATURE (°C)

TEMPERATURE ERROR (°C)

755025

-1

0

1

2

-2
0 100

START Condition Hold Time t

STOP Condition Setup Time t

Data Setup Time t

Data Hold Time t

SMBus Fall Time t

SMBus Rise Time t

SMBus Timeout (Note 3) 29 37 55 ms

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

HD:STO

SU:STO

SU:DAT

HD:DAT

10% of SDA to 10% of SCL 4 µs

90% of SCL to 10% of SDA 4 µs

10% of SDA to 10% of SCL 250 ns

10% of SCL to 10% of SDA 300 ns

F

R

300 ns

1000 ns

MAX6615/MAX6616

Dual-Channel Temperature Monitors and
Fan-Speed Controllers with Thermistor Inputs

4 _______________________________________________________________________________________

Typical Operating Characteristics (continued)

(V

CC

= +3.3V, TA= +25°C, unless otherwise noted.)

GPIO SINK CURRENT

vs. SUPPLY VOLTAGE

MAX6615/6 toc04

VCC (V)

I

GPIO_

(mA)

5.04.54.03.5

20

25

30

35

40

45

50

15

3.0 5.5

V

GPIO_

= 0.4V

GPIO OUTPUT VOLTAGE

vs. GPIO SINK CURRENT

MAX6615/6 toc05

I

GPIO_

(mA)

V

GPIO_

(V)

706040 5020 3010

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0

080

VCC = 3V

VCC = 5V

-5

-2

-3

-4

-1

0

1

2

3

4

5

05025 75 100 125

PWM FREQUENCY

vs. DIE TEMPERATURE

MAX6615/6 toc06

DIE TEMPERATURE (°C)

FREQUENCY SHIFT (Hz)

NORMALIZED AT TA = +25°C

-0.04

0

-0.02

0.04

0.02

0.08

0.06

0.10

3.0 4.03.5 4.5 5.0 5.5

PWM FREQUENCY

vs. SUPPLY VOLTAGE

MAX6615/6 toc07

VCC (V)

FREQUENCY SHIFT (Hz)

NORMALIZED AT VCC = 5.0V

MAX6615/MAX6616

Dual-Channel Temperature Monitors and

Fan-Speed Controllers with Thermistor Inputs

_______________________________________________________________________________________ 5

Pin Description

PIN

MAX6616 MAX6615

1, 2, 5, 20,

23, 24

3 1 PWM1

4 2 TACH1

6 3 ADD0 SMBus Slave Address Selection

7 4 ADD1 SMBus Slave Address Selection

8 5, 10 GND Ground. Must be connected together for MAX6615.

9 6 TH1

10, 15 — N.C. No Connection

11 7 REF

12 8 TH2

13 9 FAN_FAIL

14 — PRESET Connect to GND or VCC to set POR state of the GPIO0.

16 11 OT

17 12 V

18 13 SDA

19 14 SCL

21 15 TACH2

22 16 PWM2

—

NAME

GPIO0–

GPIO5

CC

FUNCTION

Active-Low, Open-Drain GPIOs. Can be pulled up to 5.5V regardless of V

Fan Driver Output 1. The pullup resistor can be connected to a supply voltage as high as
12V, regardless of the supply voltage. See the PWM Output section for configuration.

Fan Tachometer Input. Accepts logic-level signal from fan’s tachometer output. Can be
connected to a supply voltage as high as 12V, regardless of the supply voltage.

External Thermistor Input 1. Connect a thermistor in series with a fixed resistor between
REF and ground.

Reference Voltage Output. Provides 1V during measurements. High impedance when not
measuring.

External Thermistor Input 2. Connect a thermistor in series with a fixed resistor between
REF and ground.

Fan-Failure Output. Asserts low when either fan fails. Can be pulled up as high as 5.5V
regardless of V

Overtemperature Output. Active low, open drain. Typically used for system shutdown or
clock throttling. Can be pulled up as high as 5.5V regardless of V
when V

CC

Power Supply. 3.3V nominal. Bypass with a 0.1µF capacitor to GND.

SMBus Serial-Data Input/Output. Pull up with a 10kΩ resistor. Can be pulled up as high
as 5.5V regardless of V

SMBus Serial-Clock Input. Pull up with a 10kΩ resistor. Can be pulled up as high as 5.5V
regardless of V

Fan Tachometer Input. Accepts logic-level signal from fan’s tachometer output. Can be
connected to a supply voltage as high as 12V, regardless of the supply voltage.

Fan Driver Output 2. The pullup resistor can be connected to a supply voltage as high as
12V, regardless of the supply voltage. See the PWM Output section for configuration.

. High impedance when VCC = 0V.

CC

= 0V.

. High impedance when VCC = 0V.

CC

. High impedance when VCC = 0V.

CC

.

CC

. High impedance

CC

MAX6615/MAX6616

Detailed Description

The MAX6615/MAX6616 accurately monitor two tem­perature channels, either the internal die temperature
and the temperature of an external thermistor, or the
temperatures of two external thermistors. They report
temperature values in digital form using a 2-wire
SMBus/I2C*-compatible serial interface. The MAX6615/
MAX6616 operate from a supply voltage range of 3.0V
to 5.5V and consume 500µA (typ) of supply current.

The temperature data controls the duty cycles of two
PWM output signals that are used to adjust the speed
of a cooling fan. They also feature an overtemperature
alarm output to generate interrupts, throttle signals, or
shutdown signals.

The MAX6616 also includes six GPIO input/outputs to
provide additional flexibility. The GPIO0 power-up state
is set by connecting the GPIO PRESET input to ground
or VCC.

SMBus Digital Interface

From a software perspective, the MAX6615/MAX6616
appear as a set of byte-wide registers. Their devices use
a standard SMBus 2-wire/I2C-compatible serial interface
to access the internal registers. The MAX6615/MAX6616

have nine different slave addresses available; therefore, a
maximum of nine MAX6615/MAX6616 devices can share
the same bus.

The MAX6615/MAX6616 employ four standard SMBus
protocols: write byte, read byte, send byte, and receive
byte (Figures 1, 2, and 3). The shorter receive byte proto­col allows quicker transfers, provided that the correct
data register was previously selected by a read byte
instruction. Use caution with the shorter protocols in mul­timaster systems, since a second master could overwrite
the command byte without informing the first master.

Temperature data can be read from registers 00h and
01h. The temperature data format for these registers is
8 bits, with the LSB representing 1°C (Table 1) and the

MSB representing 128°C. The MSB is transmitted first.
All values below 0°C clip to 00h.

Table 3 details the register address and function, whether
they can be read or written to, and the power-on reset

Dual-Channel Temperature Monitors and
Fan-Speed Controllers with Thermistor Inputs

6 _______________________________________________________________________________________

WRITE BYTE FORMAT

READ BYTE FORMAT

SEND BYTE FORMAT

RECEIVE BYTE FORMAT

SLAVE ADDRESS: EQUIVA­LENT TO CHIP-SELECT LINE
OF A 3-WIRE INTERFACE

COMMAND BYTE: SELECTS
WHICH REGISTER YOU ARE
WRITING TO

DATA BYTE: DATA GOES INTO THE REG­ISTER SET BY THE COMMAND BYTE (TO
SET THRESHOLDS, CONFIGURATION
MASKS, AND SAMPLING RATE)

SLAVE ADDRESS:
EQUIVALENT TO CHIP­SELECT LINE

COMMAND BYTE:
SELECTS WHICH
REGISTER YOU ARE
READING FROM

SLAVE ADDRESS: REPEAT­ED DUE TO CHANGE IN
DATA- FLOW DIRECTION

DATA BYTE: READS
FROM THE REGISTER
SET BY THE COMMAND
BYTE

COMMAND BYTE: SENDS COM­MAND WITH NO DATA, USUALLY
USED FOR ONE-SHOT COMMAND

DATA BYTE: READS DATA FROM
THE REGISTER COMMANDED BY
THE LAST READ BYTE OR WRITE
BYTE TRANSMISSION; ALSO
USED FOR SMBUS ALERT
RESPONSE RETURN ADDRESS

S = START CONDITION SHADED = SLAVE TRANSMISSION
P = STOP CONDITION /// = NOT ACKNOWLEDGED

Figure 1. SMBus Protocols

S ADDRESS RD ACK DATA /// P

— 7 BITS — — 8 BITS — —

WRS ACK COMMAND ACK P

— — — 8 BITS — —

ADDRESS

7 BITS

P

1

ACK

—

DATA

8 BITS

ACK

—

COMMAND

8 BITS

ACK

—

WR

—

ADDRESS

7 BITS

S

—

S ADDRESS WR ACK COMMAND ACK S ADDRESS

7 BITS——8 BITS——7 BITS—

RD—ACK—DATA

8 BITS

///

—

P

—

*Purchase of I2C components from Maxim Integrated Products,

Inc., or one of its sublicensed Associated Companies, conveys
a license under the Philips I

2

C Patent Rights to use these com-

ponents in an I

2

C system, provided that the system conforms

to the I

2

C Standard Specification as defined by Philips.

(POR) state. See Tables 3–7 for all other register functions

and the Register Descriptions section.

Temperature Measurements

The averaging ADC integrates over a 120ms period
(each channel, typically), with excellent noise rejection.
For internal temperature measurements, the ADC and
associated circuitry measure the forward voltage of the
internal sensing diode at low- and high-current levels
and compute the temperature based on this voltage.
For thermistor measurements, the reference voltage
and the thermistor voltage are measured and offset is
applied to yield a value that correlates well to thermistor
temperature within a wide temperature range. Both
channels are automatically converted once the conver­sion process has started. If one of the two channels is
not used, the circuit still performs both measurements,
and the data from the unused channel may be ignored.
If either of the measured temperature values is below

0°, the value in the corresponding temperature register
is clipped to zero when a negative offset is pro­grammed into the thermistor offset register (17h).

Local (internal) temperature data is expressed directly
in degrees Celsius. Two registers contain the tempera­ture data for the local channel. The high-byte register
has an MSB of 128°C and an LSB of 1°C. The low- byte
register contains 3 bits, with an MSB of 0.5°C and an
LSB of 0.125°C. The data format is shown in Table 1.

Thermistors allow measurements of external tempera­tures. Connect a thermistor in series with a resistor,
R

EXT

. The thermistor should be connected between the

TH_ input and ground, and R

EXT

should be connected
between the reference output, REF, and the TH_ input,
as shown in the Typical Application Circuit.

The voltage across R

EXT

is measured by the ADC,

resulting in a value that is directly related to tempera-

MAX6615/MAX6616

Dual-Channel Temperature Monitors and

Fan-Speed Controllers with Thermistor Inputs

_______________________________________________________________________________________ 7

Figure 2. SMBus Write Timing Diagram

Figure 3. SMBus Read Timing Diagram

AB CDEFG

t

t

HIGH

LOW

SMBCLK

SMBDATA

t

SU:STAtHD:STA

A = START CONDITION
B = MSB OF ADDRESS CLOCKED INTO SLAVE
C = LSB OF ADDRESS CLOCKED INTO SLAVE
D = R/W BIT CLOCKED INTO SLAVE

AB CDEFG HIJ

t

LOWtHIGH

SMBCLK

SMBDATA

t

t

HD:STA

SU:STA

A = START CONDITION
B = MSB OF ADDRESS CLOCKED INTO SLAVE
C = LSB OF ADDRESS CLOCKED INTO SLAVE
D = R/W BIT CLOCKED INTO SLAVE
E = SLAVE PULLS SMBDATA LINE LOW

t

SU:DAT

E = SLAVE PULLS SMBDATA LINE LOW
F = ACKNOWLEDGE BIT CLOCKED INTO MASTER
G = MSB OF DATA CLOCKED INTO SLAVE
H = LSB OF DATA CLOCKED INTO SLAVE

t

SU:DAT

F = ACKNOWLEDGE BIT CLOCKED INTO MASTER
G = MSB OF DATA CLOCKED INTO MASTER
H = LSB OF DATA CLOCKED INTO MASTER
I = MASTER PULLS DATA LINE LOW

t

HD:DAT

HIJ

I = MASTER PULLS DATA LINE LOW
J = ACKNOWLEDGE CLOCKED INTO SLAVE
K = ACKNOWLEDGE CLOCK PULSE
L = STOP CONDITION
M = NEW START CONDITION

K

J = ACKNOWLEDGE CLOCKED INTO SLAVE
K = ACKNOWLEDGE CLOCK PULSE
L = STOP CONDITION
M = NEW START CONDITION

t

SU:STO

t

LMK

SU:STOtBUF

M

L

t

BUF

MAX6615/MAX6616

ture. The thermistor data in the temperature register(s)
gives the voltage across R

EXT

as a fraction of the refer­ence voltage. The LSB of the high byte has a nominal
weight of 7.68mV.

OOTT

Output

The OT output asserts when a thermal fault occurs, and
can therefore be used as a warning flag to initiate sys­tem shutdown, or to throttle clock frequency. When
temperature exceeds the OT temperature threshold
and OT is not masked, the OT status register indicates
a fault and OT output becomes asserted. If OT for the
respective channel is masked off, the OT status register
continues to be set, but the OT output does not
become asserted.

The fault flag and the output can be cleared by reading
the OT status register. The OT output can also be
cleared by masking the affected channel. If the OT sta­tus bit is cleared, OT reasserts on the next conversion if
the temperature still exceeds the OT temperature
threshold.

PWM Output

The PWM_ signals are normally used in one of three
ways to control the fan’s speed:

1) PWM_ drives the gate of a MOSFET or the base of a

bipolar transistor in series with the fan’s power sup­ply. The Typical Application Circuit shows the PWM_
driving an n-channel MOSFET. In this case, the PWM
invert bit (D4 in register 02h) is set to 1. Figure 4

shows PWM_ driving a p-channel MOSFET and the
PWM invert bit must be set to zero.

2) PWM_ is converted (using an external circuit) into a

DC voltage that is proportional to duty cycle. This
duty-cycle-controlled voltage becomes the power
supply for the fan. This approach is less efficient
than (1), but can result in quieter fan operation.
Figure 5 shows an example of a circuit that converts
the PWM signal to a DC voltage. Because this circuit

produces a full-scale output voltage when PWM =
0V, bit D4 in register 02h should be set to zero.

3) PWM_ directly drives the logic-level PWM speed­control input on a fan that has this type of input. This
approach requires fewer external components and
combines the efficiency of (1) with the low noise of
(2). An example of PWM_ driving a fan with a speed­control input is shown in Figure 6. Bit D4 in register
02h should be set to 1 when this configuration is
used.

Whenever the fan has to start turning from a motionless
state, PWM_ is forced high for 2s. After this spin-up
period, the PWM_ duty cycle settles to the predeter­mined value. Whenever spin-up is disabled (bit 2 in the
configuration byte = 1) and the fan is off, the duty cycle
changes immediately from zero to the nominal value,
ignoring the duty-cycle rate-of-change setting.

The frequency-select register controls the frequency of
the PWM signal. When the PWM signal modulates the
power supply of the fan, a low PWM frequency (usually
33Hz) should be used to ensure the circuitry of the

Dual-Channel Temperature Monitors and
Fan-Speed Controllers with Thermistor Inputs

8 ____________________________________________________

Table 1. Temperature Data Format (High Byte and Low Byte)

Figure 4. Driving a p-Channel MOSFET for Top-Side PWM Fan
Drive

TEMPERATURE (°C)

140.0 1000 1100 8Ch 0000 0000 00h

127.0 0111 1111 7Fh 0000 0000 00h

25.375 0001 1001 19h 0110 0000 60h

25.0 0001 1001 19h 0000 0000 00h

0.5 0000 0000 00h 1000 0000 80h

0.0 0000 0000 00h 0000 0000 00h

<0 0000 0000 00h 0000 0000 00h

BINARY VALUE HEX VALUE BINARY VALUE HEX VALUE

HIGH BYTE LOW BYTE

V

CC

PWM

5V

10kΩ

P

brushless DC motor has enough time to operate. When
driving a fan with a PWM-to-DC circuit as shown in

Figure 5, the highest available frequency (35kHz) should
be used to minimize the size of the filter capacitors.
When using a fan with a PWM control input, the frequen­cy normally should be high as well, although some fans
have PWM inputs that accept low-frequency drive.

The duty cycle of the PWM can be controlled in two ways:

1) Manual PWM control: setting the duty cycle of the fan
directly through the fan target duty-cycle registers
(0Bh and 0Ch).

2) Automatic PWM control: setting the duty cycle based
on temperature.

Manual PWM Duty-Cycle Control

Clearing the bits that select the temperature channels for
fan control (D5 and D4 for PWM1 and D3 and D2 for
PWM2) in the fan-configuration register (11h) enables
manual fan control. In this mode, the duty cycle written to
the fan target duty-cycle register directly controls the

corresponding fan. The value is clipped to a maximum of

240. Any value entered above that is changed to 240
automatically. In this control mode, the value in the maxi­mum duty-cycle register is ignored and does not affect
the duty cycle used to control the fan.

Automatic PWM Duty-Cycle Control

In the automatic control mode, the duty cycle is con­trolled by the local or remote temperature according to
the settings in the control registers. Below the fan-start
temperature, the duty cycle is either 0% or is equal to
the fan-start duty cycle, depending on the value of bit
D3 in the configuration byte register. Above the fan­start temperature, the duty cycle increases by one
duty-cycle step each time the temperature increases by
one temperature step. The target duty cycle is calculat­ed based on the following formula; for temperature >
FanStartTemperature:

where:

DC = DutyCycle

FSDC = FanStartDutyCycle

T = Temperature

FST = FanStartTemperature

DCSS = DutyCycleStepSize

TS = TempStep

Duty cycle is recalculated after each temperature con­version if temperature is increasing. If the temperature
begins to decrease, the duty cycle is not recalculated
until the temperature drops by 5°C from the last peak
temperature. The duty cycle remains the same until the
temperature drops 5°C from the last peak temperature or
the temperature rises above the last peak temperature.
For example, if the temperature goes up to +85°C and
starts decreasing, duty cycle is not recalculated until the
temperature reaches +80°C or the temperature rises
above +85°C. If the temperature decreases further, the
duty cycle is not updated until it reaches +75°C.

For temperature < FanStartTemperature and D2 of
configuration register = 0:

DutyCycle = 0

For temperature < FanStartTemperature and D2 of
configuration register = 1:

DutyCycle = FanStartDutyCycle

Once the temperature crosses the fan-start temperature
threshold, the temperature has to drop below the fan­start temperature threshold minus the hysteresis before

MAX6615/MAX6616

Dual-Channel Temperature Monitors and

Fan-Speed Controllers with Thermistor Inputs

_______________________________________________________________________________________ 9

Figure 5. Driving a Fan with a PWM-to-DC Circuit

Figure 6. Controlling a PWM Input Fan with the MAX6615/
MAX6616s’ PWM Output (Typically, the 35kHz PWM
Frequency Is Used)

+12V

500kΩ

+3.3V

V

CC

PWM

18kΩ

PWM

10kΩ 120kΩ

1µF

27kΩ

+3.3V

4.7kΩ

0.01µF

V

OUT

TO FAN

1µF

5V

0.1µF

DC FSDC T FST

=+ × ( ) -

DCSS

TS

MAX6615/MAX6616

the duty cycle returns to either 0% or the fan-start duty
cycle. The value of the hysteresis is set by D7 of the
fan-configuration register.

The duty cycle is limited to the value in the fan maximum
duty-cycle register. If the duty-cycle value is larger than
the maximum fan duty cycle, it is set to the maximum
fan-duty cycle as in the fan maximum duty-cycle register.
The temperature step is bit D6 of the fan-configuration
register (0Dh).

Notice if temperature crosses FanStartTemperature
going up with an initial DutyCycle of zero, a spin-up of
2s applies before the duty-cycle calculation controls
the value of the fan’s duty cycle.

FanStartTemperature for a particular channel follows the
channel, not the fan. If DutyCycle is an odd number, it is
automatically rounded down to the closest even number.

Duty-Cycle Rate-of-Change Control

To reduce the audibility of changes in fan speed, the
rate of change of the duty cycle is limited by the values
set in the duty-cycle rate-of-change register. Whenever
the target duty cycle is different from the instantaneous
duty cycle, the duty cycle increases or decreases at
the rate determined by the duty-cycle rate-of-change
byte until it reaches the target duty cycle. By setting the
rate of change to the appropriate value, the thermal
requirements of the system can be balanced against
good acoustic performance. Slower rates of change
are less noticeable to the user, while faster rates of
change can help minimize temperature variations.
Remember that the fan controller is part of a complex
control system. Because several of the parameters are
generally not known, some experimentation may be
necessary to arrive at the best settings.

Fan-Fail

When the fan tachometer count is larger than the fan
tachometer limit, the fan is considered failing. The
MAX6615/MAX6616 PWM_ drives the fan with 100%
duty cycle for about 2s immediately after detecting a
fan-fail. At the end of that period, another measurement
is initiated. If the fan fails both measurements, the
FAN_FAIL bit, as well as the FAN_FAIL output, assert if
the pin is not masked. If the fan fails only the first mea­surement, the fan goes back to normal settings.

If one fan fails, it can be useful to drive the other fan
with 100% duty cycle. This can be enabled with bit D0
of the fan-status register (1Ch).

Slave Addresses

The MAX6615/MAX6616 appear to the SMBus as one
device having a common address for both ADC chan­nels. The devices’ address can be set to one of nine
different values by pinstrapping ADD0 and ADD1 so
that more than one MAX6615/MAX6616 can reside on
the same bus without address conflicts (see Table 2).

The address input states are checked regularly, and
the address data stays latched to reduce quiescent
supply current due to the bias current needed for high­impedance state detection.

Power-On Defaults

At power-on, or when the POR bit in the configuration
byte register is set, the MAX6615/MAX6616 have the
default settings indicated in Table 3. Some of these set­tings are summarized below:

• Temperature conversions are active.

• Channel 1 and channel 2 are set to report the
remote temperature channel measurements.

• Channel 1 OT limit = +110°C.

• Channel 2 OT limit = +80°C.

• Manual fan mode.

• Fan duty cycle = 0.

• PWM invert bit = 0.

Dual-Channel Temperature Monitors and
Fan-Speed Controllers with Thermistor Inputs

10 ______________________________________________________________________________________

Figure 7. Automatic PWM Duty Control

DUTY CYCLE

FAN-START

DUTY CYCLE

REGISTER 02h,

BIT D3 = 1

FAN-START

TEMPERATURE

REGISTER 02h,
BIT D3 = 0

TEMP
STEP

DUTY-CYCLE

TEMPERATURE

STEP SIZE

GPIO Inputs/Outputs and

Preset (MAX6616)

The MAX6616 has six GPIO ports. GPIO0 has a POR
control pin (PRESET). When PRESET is connected to
GND at POR, GPIO0 is configured as an output and is
low. When PRESET is connected to VCCat POR, GPIO0
is configured as an input. Since GPIO0 is a high­impedance node in this state, it can be connected to a
pullup resistor and also serve as an output (high). The
rest of the GPIO ports, GPIO5–GPIO1, are configured
as high-impedance outputs after power-on, so they will
be in the high state if connected to pullup resistors. All
GPIOs are at their preset values within 1ms of power­up. During power-up, GPIO1 and GPIO2 are low while
the remaining GPIOs go into high-impedance state.

Figure 8 shows the states of the GPIO lines during
power-up. After power has been applied to the
MAX6616, the GPIO functions can be changed through
the SMBus interface.

Register Descriptions

The MAX6615/MAX6616 contain 32/34 internal regis­ters. These registers store temperature data, allow con­trol of the PWM outputs, determine if the devices are
measuring from the internal die or the thermistor inputs,
and set the GPIO as inputs or outputs.

Temperature Registers (00h and 01h)

The temperature registers contain the results of temper­ature measurements. The value of the MSB is 128°C and
the value of the LSB is 1°C. Temperature data for ther­mistor channel 1 is in the temperature channel 1 register
(00h). Temperature data for thermistor channel 2 (01h)
or the local sensor (selectable by bit D2 in the configura­tion byte) is in the temperature channel 2 register.

Configuration Byte (02h)

The configuration byte register controls timeout condi­tions and various PWM signals. The POR state of the
configuration byte register is 18h. See Table 4 for con­figuration byte definitions.

Channel 1 and Channel 2

OOTT

Limits (03h and 04h)

Set channel 1 (03h) and channel 2 (04h) temperature
thresholds with these two registers. Once the temperature
is above the threshold, the OT output is asserted low (for
the temperature channels that are not masked). The POR
state of the channel 1 OT limit register is 6Eh, and the
POR state of the channel 2 OT limit register is 50h.

OOTT

Status (05h)

A 1 in D7 or D6 indicates that an OT fault has occurred
in the corresponding temperature channel. Only read­ing its contents clears this register. Reading the con­tents of the register also clears the OT output. If the
fault is still present on the next temperature measure­ment cycle, the bits and the OT output are set again.
The POR state of the OT status register is 00h.

OOTT

Mask (06h)

Set bit D7 to 1 in the OT mask register to prevent the
OT output from asserting on faults in channel 1. Set bit

D6 to 1 to prevent the OT output from asserting on
faults in channel 2. The POR state of the OT mask reg­ister is 00h.

PWM Start Duty Cycle (07h and 08h)

The PWM start duty-cycle register determines the PWM
duty cycle where the fan starts spinning. Bit D2 in the
configuration byte register (MIN DUTY CYCLE) deter­mines the starting duty cycle. If the MIN DUTY CYCLE
bit is 1, the duty cycle is the value written to the fan­start duty-cycle register at all temperatures below the
fan-start temperature. If the MIN DUTY CYCLE bit is

MAX6615/MAX6616

Dual-Channel Temperature Monitors and

Fan-Speed Controllers with Thermistor Inputs

______________________________________________________________________________________ 11

Table 2. Slave Address Decoding (ADD0
and ADD1)

Note: High-Impedance means that the pin is left unconnected
and floating.

POR (INTERNAL)

V

CC

GPIO0

GPIO1, GPIO2

GPIO3, GPIO4,

GPIO5

HIGH-IMPEDANCE STATE

STATE DETERMINED BY

PRESET

HIGH-IMPEDANCE STATE

1ms

Figure 8. Power-On GPIO States

ADDO ADD1 ADDRESS

GND GND 0011 000

GND High-Impedance 0011 001

GND V

High-Impedance GND 0101 001

High-Impedance High-Impedance 0101 010

High-Impedance V

V

CC

V

CC

V

CC

CC

CC

GND 1001 100

High-Impedance 1001 101

V

CC

0011 010

0101 011

1001 110

MAX6615/MAX6616

Dual-Channel Temperature Monitors and
Fan-Speed Controllers with Thermistor Inputs

12 ______________________________________________________________________________________

Table 3. Register Map

R/W ADD

R 00h

R 01h

R/W 02h

R/W 03h

R/W 04h

R 05h

R/W 06h

R/W 07h

R/W 08h

R/W 09h

R/W 0Ah

POR

STATE

0000
0000

0000
0000

0001
1000

0110
1110

0101
0000

00xx
xxxx

00xx
xxxx

0110
000x
96 =
40%

0110
000x
96 =
40%

1111

000x
240 =
100%

1111

000x
240 =
100%

FUNCTION D7 D6 D5 D4 D3 D2 D1 D0

Temperature

channel 1

Temperature

channel 2

C onfi g ur ati on

b yte

Temperature

channel 1

OT limit

Temperature

channel 2

OT limit

OT status

OT mask

PWM1 start

duty cycle

PWM2 start

duty cycle

PWM1 max

duty cycle

PWM2 max

duty cycle

MSB

(128°C)

MSB

(128°C)

Standby:

0 = run;

1 =

standby

MSB — — — — — — LSB (1°C)

MSB — — — — — — LSB (1°C)

C hannel 1:

1 = faul t

C hannel 1:

1 =

m asked

MSB

(128/240)

MSB

(128/240)

MSB

(128/240)

MSB

(128/240)

— — — — — — LSB (1°C)

— — — — — — LSB (1°C)

Timeout:

POR:

1 = reset

C hannel

2: 1 =

faul t

Channel

2: 1 =

masked

——— — —

——— — —

——— — —

——— — —

0 =

enabled;

1 =

disabled

——————

——————

Fan 1
PWM
invert

Fan 2

PWM

invert

Min duty

cycle: 0 =

0%; 1 =
fan-start

duty

cycle

Temp

Ch2

sources:

1 = local;

0 =

remote2

LSB

(2/240)

LSB

(2/240)

LSB

(2/240)

LSB

(2/240)

disable: 0
= enable;

Spin-up

1 =

disable

—

—

—

—

R/W 0Bh

R/W 0Ch

0000

000x

0000

000x

PWM1 target

duty cycle

PWM2 target

duty cycle

MSB

(128/240)

MSB

(128/240)

——— — —

——— — —

LSB

(2/240)

LSB

(2/240)

—

—

MAX6615/MAX6616

Dual-Channel Temperature Monitors and

Fan-Speed Controllers with Thermistor Inputs

______________________________________________________________________________________ 13

Table 3. Register Map (continued)

R/W ADD

R 0Dh

R 0Eh

R/W 0Fh

R/W 10h

R/W 11h

R/W 12h

R/W 13h

R/W 14h

R/W 15h

R/W 16h

R/W 17h

R 18h

R 19h

R/W 1Ah

R/W 1Bh

POR

STATE

0000

000x

0000

000x

0000

0000

0000

0000

0000

000x

1011

01xx

0101

0101

010x

xxxx

xx00

000*

xx11

111*

( N ote 1)

0000

0000

1111

1111

1111

1111

1111

1111

1111

1111

FUNCTION D7 D6 D5 D4 D3 D2 D1 D0

PWM1
instantan­eous duty

cycle

PWM2
instantan­eous duty

cycle

Channel 1

fan-start

temperature

Channel 2

fan-start

temperature

Fan

configuration

Duty-cycle

rate of

change

Duty-cycle

step size

PWM

frequency

select

GPIO

function

GPIO value — — GPIO5 GPIO4 GPIO3 GPIO2 GPIO1 GPIO0

Thermistor

offset

register

Tach1 value

register

Tach2 value

register

Tach1 limit

register

Tach2 limit

register

MSB

(128/240)

MSB

(128/240)

MSB — — — — — — LSB

MSB — — — — — — LSB

Hysteresis:

0 = 5°C, 1

= 10°C

Fan 1 MSB —

Fan 1 MSB — —

Select A Select B Select C — — — — —

——

Th1 MSB

(sign)

— ———————

— ———————

— ———————

— ———————

——— — —

——— — —

Temp

step : 0 =

1°C,

——

Fan 1:

control 1

= Ch 1

Fan 1

LSB

GPIO5:

0 =

output; 1

= input

Fan 1:

control 1

= Ch 2

Fan 2

MSB

Fan 1

LSB

GPIO4:

0 =

output; 1

= input

Th1 LSB

(2°C)

Fan 2:

control 1

= Ch 1

—

Fan 2

MSB

GPIO3:

0 =

output; 1

= input

Th2 MSB

(sign)

Fan 2:

control 1

= Ch 2

Fan 2

LSB

——

GPIO2:

0 =

output; 1

= input

——

LSB

(2/240)

LSB

(2/240)

——

——

GPIO1:

0 =

output; 1

= input

—

—

Fan 2

LSB

GPIO0:

0 =

output; 1

= input

Th2 LSB

(2°C)

MAX6615/MAX6616

Dual-Channel Temperature Monitors and
Fan-Speed Controllers with Thermistor Inputs

14 ______________________________________________________________________________________

Table 3. Register Map (continued)

*GPIO0 POR values are set by PRESET.

Table 4. Configuration Byte Definition (02h)

R/W ADD

R/W 1Ch

R 1Eh

R 1Fh

R FDh

R FEh

R FFh

BIT NAME

7 RUN/STANDBY 0 Set to zero for normal operation. Set to 1 to suspend conversions and PWM outputs.

6 POR 0 Set to 1 to perform reset of all device registers.

5 TIMEOUT 0

4 FAN1 PWM INVERT 1

3 FAN2 PWM INVERT 1

2 MIN DUTY CYCLE 0

1

0 SPIN-UP DISABLE 0 Set spin-up disable to 1 to disable spin-up. Set to zero for normal fan spin-up.

POR

STATE

0000
0000

0000
0000

0000
0000

0000
0001

0110
1000

0100
1101

TEMPERATURE

SOURCE SELECT

FUNCTION D7 D6 D5 D4 D3 D2 D1 D0

Read device

Read device

m anufactur er

Fan status

byte

Channel 1

temp LSBs

Channel 2

temp LSBs

revision

ID

Read

ID

POR

STATE

1 =

1 = fan 1

failure

MSB

(1/2°C)

MSB

(1/2°C)

0 0000001

0 1101000

0 1001101

Set TIMEOUT to zero to enable SMBus timeout for prevention of bus lockup. Set to 1 to
disable this function.

Set fan PWM invert to zero to force PWM1 low when the duty cycle is 100%. Set to 1 to
force PWM1 high when the duty cycle is 100%.

Set fan PWM invert to zero to force PWM2 low when the duty cycle is 100%. Set to 1 to
force PWM2 high when the duty cycle is 100%.

Set min duty cycle to zero for a 0% duty cycle when the measured temperature is below the
fan-temperature threshold in automatic mode. When the temperature equals the fan­temperature threshold, the duty cycle is the value in the fan-start duty-cycle register, and it
increases with increasing temperature.
Set min duty cycle to 1 to force the PWM duty cycle to the value in the fan-start duty-cycle
register when the measured temperature is below the fan-temperature threshold. As the
temperature increases above the temperature threshold, the duty cycle increases as
programmed.

Selects either local or remote 2 as the source for temperature channel 2 register data.

0

When D1 = 0, the MAX6615/MAX6616 measure remote 2 and when D1 = 1, the
MAX6615/MAX6616 measure the internal die temperature.

1 = fan 2

failure

—

—

disabled

fan 1

tach

LSB

(1/8°C)

LSB

(1/8°C)

1 =

disabled

fan 2 tach

—————

—————

FUNCTION

1 =

measure

fan 1

when it is

full speed

1 =

measure

fan 2

when it is

full speed

1 = m ask

FAN_FAI L

p i n

1 = fan 1

fail sets

fan 2

100%

zero, the duty cycle is zero below the fan-start tempera­ture and has this value when the fan-start temperature
is reached. A value of 240 represents 100% duty cycle.
Writing any value greater than 240 causes the fan
speed to be set to 100%. The POR state of the fan-start
duty-cycle register is 96h, 40%.

PWMOUT Max Duty Cycle (09h and 0Ah)

The PWM maximum duty-cycle register sets the maxi­mum allowable PWM duty cycle between 2/240 (0.83%
duty cycle) and 240/240 (100% duty cycle). Any values
greater than 240 are recognized as 100% maximum
duty cycle. The POR state of the PWM maximum duty­cycle register is F0h, 100%. In manual-control mode,
this register is ignored.

PWM Target Duty Cycle (0Bh and 0Ch)

In automatic fan-control mode, this register contains the
present value of the target PWM duty cycle, as deter­mined by the measured temperature and the duty­cycle step size. The actual duty cycle requires time
before it equals the target duty cycle if the duty-cycle
rate-of-change register is set to a value other than zero.
In manual fan-control mode, write the desired value of
the PWM duty cycle directly into this register. The POR
state of the fan-target duty-cycle register is 00h.

PWM1 Instantaneous Duty Cycle,

PWM2 Instantaneous Duty Cycle (0Dh, 0Eh)

These registers always contain the duty cycle of the
PWM signals presented at the PWM output.

The POR state of the PWM instantaneous duty-cycle
register is 00h.

Channel 1 and Channel 2 Fan-Start Temperature

(0Fh and 10h)

These registers contain the temperatures at which fan
control begins (in automatic mode). See the Automatic
PWM Duty-Cycle Control section for details on setting
the fan-start thresholds. The POR state of the channel 1
and channel 2 fan-start temperature registers is 00h.

Fan Configuration (11h)

The fan-configuration register controls the hysteresis
level, temperature step size, and whether the remote or
local diode controls the PWM2 signal (see Table 3). Set

bit D7 of the fan-configuration register to zero to set the
hysteresis value to 5°C. Set bit D7 to 1 to set the hys­teresis value to 10°C. Set bit D6 to zero to set the fan­control temperature step size to 1°C. Set bit D6 to 1 to
set the fan-control temperature step size to +2°C. Bits
D5 to D2 select which PWM_ channel 1 or channel 2
controls (see Table 3). If both are selected for a given
PWM_, the highest PWM value is used. If neither is
selected, the fan is controlled by the value written to the

fan-target duty-cycle register. Also in this mode, the value
written to the target duty-cycle register is not limited by
the value in the maximum duty-cycle register. It is, howev­er, clipped to 240 if a value above 240 is written. The
POR state of the fan-configuration register is 00h.

Duty-Cycle Rate of Change (12h)

Bits D7, D6, and D5 (channel 1) and D4, D3, and D2
(channel 2) of the duty-cycle rate-of-change register set
the time between increments of the duty cycle. Each
increment is 2/240 of the duty cycle (see Table 5). This
allows the time from 33% to 100% duty cycle to be
adjusted from 5s to 320s. The rate-of-change control is
always active in manual mode. To make instant changes,
set bits D7, D6, and D5 (channel 1) or D4, D3, and D2
(channel 2) = 000. The POR state of the duty-cycle rate­of-change register is B4h (1s between increments).

MAX6615/MAX6616

Dual-Channel Temperature Monitors and

Fan-Speed Controllers with Thermistor Inputs

______________________________________________________________________________________ 15

Table 5. Setting the Time Between Duty­Cycle Increments

Table 6. Setting the Duty-Cycle Step Size

D7:D5, D4:D2

000 0 0

001 0.0625 5

010 0.125 10

011 0.25 20

100 0.5 40

101 1 80

110 2 160

111 4 320

D7:D4, D3:D0

0000 0 0

0001 2/240 80

0010 4/240 40

0011 6/240 27

0100 8/240 20

0101 10/240 16

… … ...

1000 16/240 10

... ... ...

1111 31/240 5

TIME BETWEEN

INCREMENTS (s)

CHANGE IN DUTY

CYCLE PER

TEMPERATURE

STEP

TIME FROM 33%

TO 100% (s)

TEMPERATURE

RANGE FOR FAN

CONTROL

(1°C STEP, 33%

TO 100%)

MAX6615/MAX6616

Duty-Cycle Step Size (13h)

Bits D7–D4 (channel 1) and bits D3–D0 (channel 2) of the
duty-cycle step-size register change the size of the duty­cycle change for each temperature step. The POR state
of the duty-cycle step-size register is 55h (see Table 6).

PWM Frequency Select (14h)

Set bits D7, D6, and D5 (select A, B, and C) in the PWM
frequency-select register to control the PWM frequency
(see Table 7). The POR state of the PWM frequency­select register is 40h, 33Hz. The lower frequencies are
usually used when driving the fan’s power-supply pin as
in the Typical Application Circuit, with 33Hz being the
most common choice. The 35kHz frequency setting is
used for controlling fans that have logic-level PWM input
pins for speed control. The minimum duty-cycle resolution
is decreased from 2/240 to 4/240 at the 35kHz frequen­cy setting. For example, a result that would return a value
of 6/240 is truncated to 4/240.

GPIO Function Register (15h) (MAX6616)

The GPIO function register (15h) sets the GPIO states.
Write a zero to set a GPIO as an output. Write a 1 to set
a GPIO as an input.

GPIO Value Register (16h) (MAX6616)

The GPIO value register (16h) contains the state of
each GPIO input when a GPIO is configured as an
input. When configured as an output, write a 1 or zero
to set the value of the GPIO output.

Thermistor Offset Register (17h)

The thermistor offset register contains the offset for both
of the thermistors in two’s complement. Bits D7, D6, D5,
and D4 set the offset for temperature channel 1. Bits
D3, D2, D1, and D0 set the offset for temperature chan­nel 2. The values in this register allow the thermistor
temperature readings to be shifted to help compensate
for different thermistor characteristics or different values
of R

EXT

and apply to thermistor measurements only.

The MSB is the sign bit and the LSB is 2°C. The POR
state for this register is 00h.

Tachometer Value Registers (18h and 19h)

The tachometer value registers contain the tachometer
count values for each fan. The MAX6615/MAX6616
measure the tachometer signal every 67s. It counts the
number of clock cycles between two tachometer pulses
and stores the value in the corresponding channel reg­ister. The POR state of this register is 00h.

Tachometer Limit Registers (1Ah and 1Bh)

The tachometer limit registers contain the tachometer
limits for each fan. If the value in the tach1 value regis­ter (18h) ever exceeds the value stored in 1Ah, a chan­nel 1 fan failure is detected. If the value in the Tach2
value register (19h) ever exceeds the value stored in
1Bh, a channel 2 fan failure is detected. The POR state
of these registers is 00h.

Fan Configuration/Status Register (1Ch)

The fan configuration/status register contains the status
and tachometer control bits for both fans. Bits D7 and
D6 indicate whether a fan has failed the maximum
tachometer limits in registers 1Ah and 1Bh. Setting bits
D5 and D4 disables the tachometer for each fan. The
speed is not measured when these bits are set. Setting
bits D3 and D2 measure the fan speed only during
spin-up or when it reaches 100% duty cycle. Bit D1 is
the FAN_FAIL output mask. Bit D0 is the FAN_FAIL
cross drive enable. Setting this bit enables fan 2 to go
to full speed when fan 1 fails or vice versa.

Extended Temperature Registers

(1Eh and 1Fh)

The extended temperature registers contain the low-byte
results of temperature measurements. The value of the
MSB is 0.5°C and the value of D5 is 0.125°C. The POR
states of these registers are 00h.

Dual-Channel Temperature Monitors and
Fan-Speed Controllers with Thermistor Inputs

16 ______________________________________________________________________________________

Table 7. PWM Frequency Select

Note: At 35kHz, duty-cycle resolution is decreased from a res­olution of 2/240 to 4/240.

PWM

FREQUENCY (Hz)

20 000

33 010

50 100

100 1 1 0

35k X X 1

SELECT A SELECT B SELECT C

Applications Information

Thermistor Considerations

NTC thermistors are resistive temperature sensors
whose resistance decreases with increasing tempera­ture. They are available in a wide variety of packages
that are useful in difficult applications such as measure­ment of air or liquid temperature. Some can operate
over temperature ranges beyond that of most ICs. The
relationship between temperature and resistance in an
NTC thermistor is very nonlinear and can be described
by the following approximation:

where T is absolute temperature in Kelvin, R is the ther­mistor’s resistance, and A, B, and C are coefficients that
vary with manufacturer and material characteristics.

The highly nonlinear relationship between temperature
and resistance in an NTC thermistor makes it somewhat
more difficult to use than a digital-output temperature­sensor IC. However, by connecting the thermistor in
series with a properly chosen resistor and using the
MAX6615/MAX6616 to measure the voltage across the
resistor, a reasonably linear transfer function can be
obtained over a limited temperature range. Accuracy
increases over smaller temperature ranges.

Figures 9 and 10 show a good relationship between
temperature and data. This data was taken using a
popular thermistor model, the Betatherm 10K3A1, with
R

EXT

= 1.6kΩ. Using these values produces data with

good conformance to real temperature over a range of
about +30°C to +100°C. Different combinations of ther­mistors and R

EXT

result in different curves.

ADC Noise Filtering

The integrating ADC has inherently good noise rejec­tion, especially at low-frequency signals such as
60Hz/120Hz power-supply hum. Lay out the PC board
carefully with proper external noise filtering for high­accuracy thermistor measurements in electrically noisy
environments.

Filter high-frequency electromagnetic interference
(EMI) at TH_ and REF with an external 100pF capacitor
connected between the two inputs. This capacitor can
be increased to about 2000pF (max), including cable
capacitance. A capacitance higher than 2000pF intro­duces errors due to the rise time of the switched cur­rent source.

Chip Information

PROCESS: BiCMOS

MAX6615/MAX6616

Dual-Channel Temperature Monitors and

Fan-Speed Controllers with Thermistor Inputs

______________________________________________________________________________________ 17

Figure 9. Data Error vs. Temperature Using a Betatherm
10K3A1 Thermistor

0

40

20

80

60

100

120

-50 500 100 150

MEASUREMENT vs. TEMPERATURE

TEMPERATURE (°C)

MEASUREMENT (°C)

Figure 10. Measured Temperature vs. Actual Temperature

MAX6615/MAX6616 ERROR

4

2

0

-2

-4

ERROR (°C)

-6

-8

-10

-12

OPTIMIZED FOR +30°C TO +100°C

0406020

TEMPERATURE (°C)

100 120 140

80

1

A B In R C In R=+ +() [ ()]

T

3

MAX6615/MAX6616

Dual-Channel Temperature Monitors and
Fan-Speed Controllers with Thermistor Inputs

18 ______________________________________________________________________________________

Typical Application Circuits

24

23

22

21

20

19

18

17

1

2

3

4

5

6

7

8

V

CC

SCL

SDA

PWM2

OT

TACH1

PWM1

GPIO5

TACH2

GPIO4

GPIO3

PRESET

GND

REF

ADD1

ADD0

GPIO2

GPIO1

GPIO0

16

15

14

13

9

10

11

12

FAN_FAIL

N.C.

N.C.

TH1

TH2

MAX6616

QSOP

1

2

3

4

5

6

7

8

V

CC

SCL

SDA

PWM2

OT

TACH1

PWM1

TOP VIEW

TACH2

GND

GND

REF

ADD1

ADD0

16

15

14

13

9

10

11

12

FAN_FAIL

TH1

TH2

MAX6615

QSOP

Pin Configurations

V

V

CC

10kΩ

V

FAN

3.0V TO 5.5V

BETATHERM 10K3A1

THERMISTOR

BETATHERM 10K3A1

1.6kΩ

1.6kΩ

100pF

100pF

THERMISTOR

TO SMBus

MASTER

V

CC

TH1

MAX6615

REF

TH2

SDA

SCL

GND(10)GND(5) ADD0 ADD1

FAN_FAIL

PWM1

TACH1

PWM2

TACH2

OT

FAN

(5V OR 12V)

4.7kΩ

V

CC

10kΩ

TO CLOCK THROTTLE OR
SYSTEM SHUTDOWN

V

FAN

4.7kΩ

V

FAN

(5V OR 12V)

MAX6615/MAX6616

Dual-Channel Temperature Monitors and

Fan-Speed Controllers with Thermistor Inputs

______________________________________________________________________________________ 19

Typical Application Circuits (continued)

BETATHERM 10K3A1

THERMISTOR

BETATHERM 10K3A1

THERMISTOR

1.6kΩ

1.6kΩ

TO SMBus

MASTER

100pF

100pF

10kΩ

V

V

CC

10kΩ

V

FAN

3.0V TO 5.5V

V

FAN_FAIL

CC

TH1

PWM1

MAX6616

REF

TH2

SDA

SCL

V

CC

GPIO0

V

CC

TACH1

PWM2

TACH2

OT

GPIO3

FAN

(5V OR 12V)

4.7kΩ

V

CC

10kΩ

TO CLOCK THROTTLE OR
SYSTEM SHUTDOWN

V

CC

10kΩ

V

CC

V

FAN

(5V OR 12V)

V

FAN

4.7kΩ

10kΩ

10kΩ

GPIO1

V

CC

GPIO2

PRESET

GND ADD0 ADD1

GPIO4

GPIO5

10kΩ

V

CC

10kΩ

MAX6615/MAX6616

Dual-Channel Temperature Monitors and
Fan-Speed Controllers with Thermistor Inputs

Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.

20 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600

© 2005 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products, Inc.

Package Information

(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information,
go to www.maxim-ic.com/packages

.)

QSOP.EPS

PACKAGE OUTLINE, QSOP .150", .025" LEAD PITCH

1

21-0055

E

1