MAX6650_12 [MAXIM]
Fan-Speed Regulators and Monitors with SMBus/I2C-Compatible Interface; 风扇转速调节器和监控器，带有SMBus / I²C兼容接口
| 型号： | MAX6650_12 |
| 厂家： | 
MAXIM INTEGRATED PRODUCTS |
| 描述： | Fan-Speed Regulators and Monitors with SMBus/I2C-Compatible Interface |
| 文件： | 总25页 (文件大小：284K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
EVALUATION KIT AVAILABLE
MAX6650/MAX6651
Fan-Speed Regulators and Monitors
2
with SMBus/I C-Compatible Interface
General Description
____________________________Features
♦ Closed/Open-Loop Fan-Speed Control for
The MAX6650/MAX6651 fan controllers use an
SMBus/I2C-compatible interface to regulate and moni-
tor the speed of 5VDC/12VDC brushless fans with built-
in tachometers. They automatically force the fan’s
tachometer frequency (fan speed) to match a prepro-
grammed value in the Fan-Speed Register by using an
external MOSFET or bipolar transistor to linearly regu-
late the voltage across the fan. The MAX6650 regulates
the speed of a single fan by monitoring its tachometer
output. The MAX6651 also regulates the speed of a sin-
gle fan, but it contains additional tachometer inputs to
monitor up to four fans and control them as a single unit
when they are used in parallel.
5V/12V Fans
♦ 2-Wire SMBus/I C-Compatible Interface
2
♦ Monitors Tachometer Output
Single Tachometer (MAX6650)
Up to Four Tachometers (MAX6651)
♦ Programmable Alert Output
♦ GPIOs
♦ Hardware Full-On Override
♦ Synchronize Multiple Fans
♦ Four Selectable Slave Addresses
♦ 3V to 5.5V Supply Voltage
The MAX6650/MAX6651 provide general-purpose
input/output (GPIO) pins that serve as digital inputs,
digital outputs, or various hardware interfaces. Capable
of sinking 10mA, these open-drain inputs/outputs can
drive an LED. To add additional hardware control, con-
figure GPIO1 to fully turn on the fan in case of software
failure. To generate an interrupt when a fault condition
is detected, configure GPIO0 to behave as an active-
low alert output. Synchronize multiple devices by set-
ting GPIO2 (MAX6651 only) as an internal clock output
or an external clock input.
♦ Small Packages
10-Pin µMAX (MAX6650)
16-Pin QSOP (MAX6651)
Ordering Information
PART
TEMP RANGE
-40°C to +85°C
-40°C to +85°C
-40°C to +85°C
-40°C to +85°C
PIN-PACKAGE
10 µMAX
MAX6650EUB
MAX6650EUB+
MAX6651EEE
MAX6651EEE+
10 µMAX
16 QSOP
The MAX6650 is available in a space-saving 10-pin
µMAX® package, and the MAX6651 is available in a
small 16-pin QSOP package.
16 QSOP
+Denotes a lead(Pb)-free/RoHS-compliant package.
________________________Applications
Pin Configurations appear at end of data sheet.
RAID
Desktop Computers
µMAX is a registered trademark of Maxim Integrated Products, Inc.
Servers
Networking
Workstations
Telecommunications
Typical Operating Circuit
V
FAN
5V OR 12V
V
V
CC
CC
3V TO 5.5V
10kΩ
MAX6650
TACH0
FB
SCL
SDA
FAN
2
SMBus/I C
INTERFACE
LED
C
10μF
ALERT
COMP
GPIO0
GPIO1
OUT
ADD
GND
FULL ON
For pricing, delivery, and ordering information, please contact Maxim Direct at
1-888-629-4642, or visit Maxim Integrated’s website at www.maximintegrated.com.
19-1784; Rev 5; 12/12
MAX6650/MAX6651
Fan-Speed Regulators and Monitors
2
with SMBus/I C-Compatible Interface
ABSOLUTE MAXIMUM RATINGS
CC
FB, TACH_ ..........................................................-0.3V to +13.2V
All Other Pins..............................................-0.3V to (V
Output Voltages..........................................-0.3V to (V
Maximum Current
V
to GND..............................................................-0.3V to +6V
Operating Temperature Range ...........................-40°C to +85°C
Junction Temperature .....................................................+150°C
Storage Temperature Range.............................-65°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
Soldering Temperature (reflow)
+ 0.3V)
+ 0.3V)
CC
CC
Into V , GND, V
Into All Other Pins ..........................................................50mA
...................................................100mA
Lead(Pb)-free..............................................................+260°C
Containing lead(Pb)....................................................+240°C
CC
OUT
Continuous Power Dissipation (T = +70°C)
A
µMAX (derate 5.6mW/°C above +70°C) .....................444mW
QSOP (derate 8.3mW/°C above +70°C).....................667mW
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.
ELECTRICAL CHARACTERISTICS
(V
= 3.0V to 5.5V, T = -40°C to +85°C, unless otherwise noted. Typical values are at T = +25°C and V
= 5V.)
CC
A
A
CC
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
POWER SUPPLY (V
Supply Voltage
)
CC
V
3.0
5.5
10
V
CC
Supply Current
I
Full-on mode, I
= 0
mA
CC
OUT
OUTPUT (OUT)
Output Voltage Range
Output Sink Current
Output Source Current
V
I
=
100µA
0.3
10
50
V
CC
- 0.3
V
OUT
OUT
I
V
= 0.5V
mA
mA
SINK
OUT
OUT
I
V
= V
- 1.8V
SOURCE
CC
TACHOMETER INPUTS (TACH_)
5V fan, 0 < V < 4.5V
V
V
+ 0.5
V
V
+1.5
+3
FB
FB
FB
Tachometer Threshold
V
R
V
TACH_
12V fan, 0 < V < 9V
+ 1.0
FB
FB
FB
Tachometer Input Impedance
FEEDBACK (FB)
0 < V
< 9V
70
100
150
kΩ
TACH_
TACH
DAC Differential Nonlinearity
Useful DAC Resolution
Guaranteed monotonicity on FB (Note 1)
Measured at FB (Note 1)
0 < VFB < 9V
5
8
LSB
bits
kΩ
Feedback Input Impedance
R
FB
70
100
150
GENERAL-PURPOSE INPUTS/OUTPUTS (GPIO_) (Note 2)
Input Low Voltage
V
0.8
V
V
IL(GPIO_)
V
V
≤ 3.6V
2
3
CC
Input High Voltage
V
IH(GPIO_)
> 3.6V
CC
Input Hysteresis
Pullup Resistor
V
200
100
mV
kΩ
HYS
R
GPIO_
GPIO_
Output Sink Current
I
V
= 0.4V
10
mA
GPIO_
2
Maxim Integrated
MAX6650/MAX6651
Fan-Speed Regulators and Monitors
2
with SMBus/I C-Compatible Interface
ELECTRICAL CHARACTERISTICS (continued)
(V
= 3.0V to 5.5V, T = -40°C to +85°C, unless otherwise noted. Typical values are at T = +25°C and V
= 5V.)
CC
A
A
CC
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
ADDRESS SELECT (ADD)
ADD Input Low Voltage
ADD Input High Voltage
ADD Input Leakage
V
Selects slave address 90h (Table 1)
Selects slave address 96h (Table 1)
0.1
V
V
IL(ADD)
V
V
- 0.05
IH(ADD)
CC
I
Selects slave address 36h (Table 1) (Note 3)
-1
0
µA
LADD
ADD External Pulldown Resistor
to GND
R
Selects slave address 3Eh (Table 1)
9.5
-80
10.5
-40
kΩ
ADD
ADD
ADD Pulldown Current
I
V
V
= 0.5V (Note 4)
= 0.6V
µA
ADD
2
SMBus/I C INTERFACE (SDA, SCL)
Data Output Sink Current
Input Leakage Current
I
6
mA
µA
V
SDA
SDA
0 < V < V
1
IN
CC
Input Low Voltage
V
0.8
IL
V
V
≤ 3.6V
> 3.6V
2
3
CC
Input High Voltage
V
IH
V
CC
Input Hysteresis
V
HYS
200
mV
TIMING CHARACTERISTICS
(V
= 3.0V to 5.5V, T = -40°C to +85°C, unless otherwise noted. Typical values are at T = +25°C and V
= 5V.)
CC
A
A
CC
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
500
254
MAX
UNITS
TACHOMETERS
Glitch Rejection
GPIO2 (Note 2)
Clock Frequency
Minimum pulse duration
µs
f
f
kHz
%
CLK
Clock Frequency Uncertainty
V
CC
= 5V
-10
+10
400
CLK
2
SMBus/I C INTERFACE (Figures 3, 4)
SCL Clock Frequency
f
0
kHz
µs
SCL
Bus Free Time Between Stop
and Start Condition
t
1.3
BUF
Hold-Time Start Condition
Low Period of the SCL Clock
High Period of the SCL Clock
Data Hold Time
t
0.6
1.3
0.6
0
µs
µs
µs
µs
ns
HD:STA
t
LOW
t
HIGH
t
(Note 5)
900
HD:DAT
Data Setup Time
t
100
SU:DAT
Rise-Time SDA/SCL Signal
(Receiving)
t
(Note 6)
(Note 6)
20 + 0.1C (pF)
300
300
250
ns
ns
ns
R
B
Fall-Time SDA/SCL Signal
(Receiving)
t
t
20 + 0.1C (pF)
B
F
Fall-Time SDA Signal
(Transmitting)
I
< 6mA (Note 6)
20 + 0.1C (pF)
B
F
SINK
Maxim Integrated
3
MAX6650/MAX6651
Fan-Speed Regulators and Monitors
2
with SMBus/I C-Compatible Interface
TIMING CHARACTERISTICS (continued)
(V
CC
= 3.0V to 5.5V, T = -40°C to +85°C, unless otherwise noted. Typical values are at T = +25°C and V
= 5V.)
A
A
CC
PARAMETER
SYMBOL
CONDITIONS
MIN
0.6
0
TYP
50
MAX
UNITS
µs
Setup Time for Stop Condition
Pulse Width of Spike Suppressed
t
SU:STO
t
ns
SPIKE
Note 1: For proper measurement of V , connect OUT and FB as shown in the Typical Operating Circuit.
FB
Note 2: GPIO2, GPIO3, and GPIO4 only in the MAX6651.
Note 3: Guaranteed by design and not 100% production tested.
Note 4: For R
component test purposes only.
ADD
Note 5: Note that the transition must internally provide at least a hold time to bridge the undefined region (300ns max) of SCL’s
falling edge.
Note 6: C is the total capacitance of one bus line in pF. Tested with C = 400pF. Rise and fall times are measured between 0.3 x
B
B
V
CC
and 0.7 x V
.
CC
Typical Operating Characteristics
(T = +25°C, unless otherwise noted.)
A
INTERNAL OSCILLATOR FREQUENCY
INTERNAL OSCILLATOR FREQUENCY
vs. TEMPERATURE
FEEDBACK VOLTAGE
vs. TEMPERATURE
vs. SUPPLY VOLTAGE
265
300
280
260
240
220
200
2.5
2.4
2.3
2.2
2.1
2.0
1.9
1.8
260
255
250
245
240
V
= 5.5V
CC
V
= 5.5V,
FAN
CC
V
= 5.5V, V = 12.0V
FAN
V
= 3.0V
CC
V
= 12.0V, V = 5.5V
FAN
CC
FAN
V
= 3.0V
-50
0
50
100
-50
0
50
100
3.0
3.5
4.0
4.5
5.0
5.5
SUPPLY VOLTAGE (V)
TEMPERATURE (°C)
TEMPERATURE (°C)
FEEDBACK VOLTAGE vs. SUPPLY
VOLTAGE (DAC SET TO 35)
SUPPLY CURRENT
vs. SUPPLY VOLTAGE
SUPPLY CURRENT vs. TEMPERATURE
3.8
3.6
3.4
3.2
3.0
2.8
2.6
2.4
2.2
2.0
4.0
3.5
3.0
2.5
2.0
1.5
2.20
2.15
2.10
2.05
2.00
1.95
1.90
1.85
1.80
V
= 5.5V
CC
V
= 5.5V
FAN
V
= 12.0V
FAN
V
CC
= 3V
3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0
SUPPLY VOLTAGE (V)
-40 -20
0
20
40
60
80 100
3.0
3.5
4.0
4.5
5.0
5.5
TEMPERATURE (°C)
SUPPLY VOLTAGE (V)
4
Maxim Integrated
MAX6650/MAX6651
Fan-Speed Regulators and Monitors
2
with SMBus/I C-Compatible Interface
Pin Description
PIN
MAX6650
NAME
FUNCTION
MAX6651
1
1
TACH0
Tachometer Input. Used to close the loop around the tachometer.
Tachometer Inputs. Used to monitor tachometers only.
TACH2, TACH3,
TACH1
—
2, 3, 16
2
3
4
5
GND
SDA
Ground
2-Wire Serial-Data Input/Output (open drain)
2-Wire Serial Clock Input
4
6
SCL
5
8
ADD
Slave Address Select Input (Table 1)
General-Purpose Input/Output (open drain)
—
7, 12
GPIO4, GPIO3
General-Purpose Input/Output (open drain). Configurable to act either as an out-
put or as an input (FULL ON or general purpose).
6
7
9
GPIO1
GPIO0
General-Purpose Input/Output (open drain). Configurable to act as a general
input/output line or an active-low ALERT output.
10
11
General-Purpose Input/Output (open drain). Configurable to act as a general
input/output line, an internal clock output, or an external clock input.
—
GPIO2
OUT
8
9
13
14
Output. Drives the external MOSFET or bipolar transistor.
+3.0V to +5.5V Power Supply
V
CC
Feedback Input. Closes the loop around the external MOSFET or bipolar tran-
sistor.
10
15
FB
The second control loop consists of the internal digital
Detailed Description
logic that controls the fan’s speed. The MAX6650/
MAX6651 control fan speed by forcing the tachometer
frequency to equal a reference frequency set by the
Fan-Speed Register, the prescaler, and the internal
oscillator (see the Fan-Speed Register section). When
the tachometer frequency is too high, the value of the
DAC’s input register is increased by the regulator.
Once the DAC voltage increases, the analog control
loop forces the feedback voltage to rise, which reduces
the voltage across the fan. Since fan speed is propor-
tional to the voltage across the fan, the fan slows down.
2
The MAX6650/MAX6651 use an SMBus/I C-Compatible
interface to regulate and monitor the speed of
5VDC/12VDC brush-less fans with built-in open-collec-
tor/drain tachometers. Regulating fan speed propor-
tionally with temperature saves power, increases fan
life, and reduces acoustic noise. Since fan speed is
proportional to the voltage across the fan, the
MAX6650/MAX6651 control the speed by regulating the
voltage on the low side of the fan with an external MOS-
FET or bipolar transistor.
The MAX6650/MAX6651 each contain two internal con-
trol loops. The first loop controls the voltage across the
fan. The internal digital-to-analog converter (DAC) sets
the reference voltage for an internal amplifier (Figure 1),
which then drives the gate of an external N-channel
MOSFET (or the base of a bipolar transistor) to regulate
the voltage on the low side of the fan. As the reference
voltage provided by the DAC changes, the feedback
amplifier automatically adjusts the feedback voltage,
which changes the voltage across the fan.
2
2-Wire SMBus/I C-Compatible
Digital Interface
From a software perspective, the MAX6650/MAX6651
appear as a set of byte-wide registers that contain
speed control, tachometer count, alarm conditions, or
configuration bits. These devices use a standard
2
SMBus/I C-compatible 2-wire serial interface to access
the internal registers.
Maxim Integrated
5
MAX6650/MAX6651
Fan-Speed Regulators and Monitors
2
with SMBus/I C-Compatible Interface
V
= 5V OR 12V
FAN
V
CC
V
CC
2
SMBus/I C
3V TO 5.5V
MAX6650
MAX6651
INTERFACE
90kΩ
FAN SPEED
TACH0
FB
FAN
SCL
SDA
CONFIGURE
ALARM ENABLE
ALARM STATUS
TACH COUNT
COUNT TIME
GPIO DEF
2
SMBus/I C
10kΩ
INTERFACE
TACHOMETER
COUNT
V
OFFSET
90kΩ
OUT
CONTROL
LOGIC
10kΩ
GPIO STATUS
DAC
GPIO0
GPIO1
ALERT
FULL ON
8-BIT
DAC
ADD
GND
ADDRESS
DECODE
V
REF
10kΩ
GPIO
BLOCKS
(FIGURE 5)
Figure 1. Block Diagram
The MAX6650/MAX6651 employ three standard SMBus
protocols: write byte, read byte, and receive byte
(Figure 2). The shorter protocol (receive) allows quicker
transfers, provided that the correct data register was
previously selected by a write or read byte instruction.
Use caution with the shorter protocol in multimaster
systems, since a second master could overwrite the
command byte without informing the first master.
Table 1. Slave Address Decoding (ADD)
ADDRESS
ADD
HEX
BINARY
1001 000
1001 011
0011 011
0011 111
GND
90
V
CC
96
Slave Addresses
The device address can be set to one of four different
values. Accomplish this by pin-strapping ADD so that
more than one MAX6650/MAX6651 can reside on the
same bus without address conflicts (Table 1).
No connection (high-Z)
36
10kΩ resistor to GND
3E
6
Maxim Integrated
MAX6650/MAX6651
Fan-Speed Regulators and Monitors
2
with SMBus/I C-Compatible Interface
S
ADDRESS
ACK
COMMAND
ACK
DATA
ACK
P
WR
7 bits
0
8 bits
8 bits
Slave Address
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).
Figure 2a. SMBus Protocol: Write Byte Format
S
ADDRESS
7 bits
ACK
COMMAND
ACK
WR
0
8 bits
Slave Address
Command byte: Selects
which register you are
reading from.
S
ADDRESS
RD
ACK
DATA
P
A
7 bits
1
8 bits
Slave Address.
Repeated due to
change in data-flow
direction
Data byte: Reads from
the register set by the
command byte.
Figure 2b. SMBus Protocol: Read Byte Format
S
ADDRESS
7 bits
RD
ACK
DATA
P
A
1
8 bits
Slave Address
Data byte: Reads data
from the register com-
manded by the last
read-byte or write-byte
transmission; also
used for SMBus alert
response return address.
Figure 2c. SMBus Protocol: Receive Byte Format
S = Start condition
P = Stop condition
Shaded = Slave transmission
ACK = Acknowledged = 0
WR = Write = 0
RD = Read =1
A = Not acknowledged = 1
Maxim Integrated
7
MAX6650/MAX6651
Fan-Speed Regulators and Monitors
2
with SMBus/I C-Compatible Interface
A
B
C
D
E
F
G
H
I
J
K
L
M
t
t
HIGH
LOW
SMBCLK
SMBDATA
t
t
t
t
HD:DAT
HD:STA
SU:STA
SU:DAT
t
t
SU:STO
BUF
A = START CONDITION
F = ACKNOWLEDGE BIT CLOCKED INTO MASTER
G = MSB OF DATA CLOCKED INTO SLAVE
H = LSB OF DATA CLOCKED INTO SLAVE
I = SLAVE PULLS SMBDATA LINE LOW
J = ACKNOWLEDGE CLOCKED INTO MASTER
K = ACKNOWLEDGE CLOCK PULSE
L = STOP CONDITION, DATA EXECUTED BY SLAVE
M = NEW 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
Figure 3. SMBus Write Timing Diagram
A
B
C
D
E
F
G
H
I
J
K
t
t
HIGH
LOW
SMBCLK
SMBDATA
t
t
t
t
t
BUF
SU:STA HD:STA
SU:DAT
SU:STO
A = START CONDITION
E = SLAVE PULLS SMBDATA LINE LOW
I = ACKNOWLEDGE CLOCK PULSE
J = STOP CONDITION
K = NEW START CONDITION
B = MSB OF ADDRESS CLOCKED INTO SLAVE
C = LSB OF ADDRESS CLOCKED INTO SLAVE
D = R/W BIT CLOCKED INTO SLAVE
F = ACKNOWLEDGE BIT CLOCKED INTO MASTER
G = MSB OF DATA CLOCKED INTO MASTER
H = LSB OF DATA CLOCKED INTO MASTER
Figure 4. SMBus Read Timing Diagram
tachometer pulses, so the required Fan-Speed Register
value (K ) may be calculated as:
Command-Byte Functions
The 8-bit Command-Byte Register (Table 2) is the mas-
ter index that points to the various other registers within
MAX6650/MAX6651. The register’s power-on reset
(POR) state is 0000 0000, so that a receive-byte trans-
mission (a protocol that lacks the command byte)
occurring immediately after POR returns the current
speed setting.
TACH
t
= 1 / (2 x Fan Speed)
TACH
K
= [t
x K
x (f
/ 128)] - 1
CLK
TACH
TACH
SCALE
where the fan speed is in rotations per second (RPS),
is the period of the tachometer signal, f is the
t
TACH
CLK
internal oscillator frequency (254kHz 10%), and
is the prescaler value (see Configuration-Byte
K
SCALE
Fan-Speed Register
In closed-loop mode, the MAX6650/MAX6651 use the
Fan-Speed Register to set the period of the tachometer
signal that controls the fan speed. The Fan-Speed
Register is ignored in all other modes of operation. The
MAX6650/MAX6651 regulate the fan speed by forcing
Register). Since the fan speed is inversely proportional
to the tachometer period, the Fan-Speed Register value
(K
) does not linearly control the fan speed (Table
TACH
3). Select the prescaler value so the fan’s full speed is
achieved with a register value of approximately 64
(0100 0000) to optimize speed range and resolution.
The MAX6651 may be controlled by an external oscilla-
the tachometer period (t
) equal to the scaled reg-
TACH
ister value. One revolution of the fan generates two
8
Maxim Integrated
MAX6650/MAX6651
Fan-Speed Regulators and Monitors
2
with SMBus/I C-Compatible Interface
Table 2. Command-Byte Assignments
POR (DEFAULT)
STATE
REGISTER
COMMAND
READ
FUNCTION
Fan speed
WRITE
SPEED
CONFIG
GPIO DEF
DAC
0000 0000
0000 0010
0000 0100
0000 0110
0000 1000
0000 1010
0000 1100
0000 1110
0001 0000
0001 0010
0001 0100
0001 0110
x
x
x
x
x
x
x
x
x
x
x
x
x
x
00h
0Ah
FFh
00h
00h
00h
00h
00h
00h
00h
1Fh
02h
Configuration
x
GPIO definition
DAC
x
ALARM ENABLE
ALARM
x
Alarm enable
—
—
—
—
—
—
x
Alarm status
TACH0
Tachometer 0 count
Tachometer 1 count
Tachometer 2 count
Tachometer 3 count
GPIO status
TACH1
TACH2
TACH3
GPIO STAT
COUNT
Tachometer count time
Table 3. Fan Speed
t
FAN SPEED (RPS)
FAN SPEED (RPM)
TACH
K
TACH
K
(ms)
K
K
SCALE
SCALE
SCALE
1
4
16
*
1
4
16
*
1
4
16
0000 0000
0000 0001
0000 0010
—
1.0
1.0
1.5
—
*
500
500
330
—
*
30,000
30,000
20,000
—
*
*
*
*
*
*
*
*
*
*
*
*
*
*
—
3.9
4.0
4.2
—
8.2
—
31
—
*
—
—
*
—
—
0001 1110
0001 1111
0010 0000
—
16
16
17
—
32
128
124
120
—
1900
1900
1800
—
7700
7400
7200
—
*
1.0
1.0
—
2.1
—
7.8
31
500
480
—
240
—
64
30,000
29,000
—
30
—
0100 0000
—
33
—
15.3
—
61.1
—
910
3700
—
15,000
—
—
1111 1000
125
4
15.9
240
960
3830
*The minimum allowed tachometer period is 1ms.
tor that overrides the internal oscillator (see General-
Purpose Input/Output). When using an external oscillator
fan’s full speed is achieved with a register value of
approximately 64 (0100 0000) to optimize speed range
and resolution (see the Fan Speed Register section). The
fourth bit selects the fan operating voltage.
(f
), calculate the Fan-Speed Register value with f
CLK
OSC
equal to f
. Codes above F8h (1111 1000) are
OSC
allowed, but will not significantly decrease the frequency.
The fifth and sixth bits configure the operating mode.
The MAX6650/MAX6651 have four modes of operation:
full-on, full-off (shutdown), closed-loop, and open-loop.
In closed-loop operation, the external microcontroller
(µC) sets the desired speed by writing an 8-bit word to
the Fan-Speed Register (see the Fan-Speed Register
section). The MAX6650/MAX6651 monitor the fan’s
tachometer output and automatically adjust the voltage
Configuration-Byte Register
The Configuration-Byte Register (Table 4) adjusts the
prescaler, changes the tachometer threshold voltage,
and sets the mode of operation. The three least-signifi-
cant bits configure the prescaler division used to scale
the tachometer period. Select the prescaler value so the
Maxim Integrated
9
MAX6650/MAX6651
Fan-Speed Regulators and Monitors
2
with SMBus/I C-Compatible Interface
Table 4. Configuration Byte Register
POR (DEFAULT)
BIT
NAME
FUNCTION
STATE
7 (MSB) to 6
—
0
Always 0
Operating Mode:
00 = Software full-on (default)
01 = Software off (shutdown)
10 = Closed-loop operation
11 = Open-loop operation
5 to 4
MODE
00
1
Fan/Tachometer Voltage:
0 = 5V
3
5/12V
1 = 12V (default)
Prescaler Division:
000 = Divide by 1
001 = Divide by 2
010 = Divide by 4 (default)
011 = Divide by 8
2 to 0 (LSB)
SCALE
010
100 = Divide by 16
General-Purpose Input/Output
V
V
CC
CC
The GPIO pins connect to the drain of the internal N-
channel MOSFET and pullup resistor (Figure 5). When
the N-channel MOSFET is off (Table 5), the pullup resis-
tor provides a logic-level high output. However, with the
MOSFET off, the GPIO may serve as an input pin and
its state is read from the GPIO Status Register (Table
6). The MAX6650/MAX6651 power up with the MOSFET
off, so input signals may be safely connected to the
GPIO pins. When using the GPIO pin as a general-pur-
pose output, change the output by writing to the GPIO
Definition Register.
3.0V TO 5.5V
MAX6650
MAX6651
C
BYPASS
100kΩ
GPIO
STATUS
REGISTER
GPIO_
GPIO
DEFINITION
REGISTER
GPIO0 may be configured as an ALERT output that will
go low whenever a fault-condition is detected (see the
Alarm-Enable and Status Registers section). GPIO1
may be configured as a FULL ON input to allow hard-
ware control to fully turn on the fan in case of software
or µC failure. GPIO2 (MAX6651 only) may be config-
ured as an internal clock output or as an external clock
input to allow synchronization of multiple devices.
GND
Figure 5. General-Purpose Input/Output Structure
across the fan until the desired speed is reached. Open-
loop operation allows the µC to regulate fan speed direct-
ly. The µC reads the fan speed from the Tach-
ometer-Count Register. Based on the tachometer
count, the µC decides if the fan speed requires adjust-
ment, and changes the voltage across the fan by writ-
ing an 8-bit word to the DAC Register. Full-on mode
applies the maximum voltage across the fan, forcing it
to spin at full speed. Configuring GPIO1 (see the
General-Purpose Input/Output section) as an active-low
input provides additional hardware control that fully
turns on the fan and overrides all software commands.
Alarm-Enable and Status Registers
The alarms are enabled only when the appropriate bits of
the Alarm-Enable Register are set (Table 7). The maxi-
mum and minimum output level alarms function only
when the device is configured to operate in the closed-
loop mode (see the Configuration-Byte Register section).
The Alarm Status Register allows the system to deter-
mine which alarm caused the alert output (Table 8).
The set-alarm and alert outputs clear after reading the
10
Maxim Integrated
MAX6650/MAX6651
Fan-Speed Regulators and Monitors
2
with SMBus/I C-Compatible Interface
Table 5. GPIO Definition Register
POR
(DEFAULT)
STATE
MAX6650
PIN
MAX6651
PIN
BIT
STATE
FUNCTION
GPIO4 outputs a logic-low level.
0
N/A
(must be 1)
7
6
1
1
GPIO4
GPIO3
1
GPIO4 outputs a logic-high level or serves as an input.
GPIO3 outputs a logic-low level.
0
N/A
(must be 1)
1
GPIO3 outputs a logic-high level or serves as an input.
GPIO2 serves as an external clock input.
GPIO2 serves as an internal clock output.
GPIO2 outputs a logic-low level.
00
01
10
11
00
01
10
11
00
01
10
11
N/A
(must be 11)
5:4
3:2
1:0
11
11
11
GPIO2
GPIO1
GPIO0
GPIO2 outputs a logic-high level or serves as an input.
GPIO1 outputs a logic-high level or serves as an input.
GPIO1 serves as a FULL ON input.
GPIO1
GPIO0
GPIO1 outputs a logic-low level.
GPIO1 outputs a logic-high level or serves as an input.
GPIO0 outputs a logic-high level or serves as an input.
GPIO0 serves as an ALERT output.
GPIO0 outputs a logic-low level.
GPIO0 outputs a logic-high level or serves as an input.
The MAX6651 contains three additional tachometer
inputs, which may be used to monitor additional fans. For
accurate control of multiple fans, use identical fans.
Table 6. GPIO Status Register
POR
(DEFAULT
STATE)
BIT
NAME
The Tachometer Count-Time Register sets the integration
time over which the MAX6650/MAX6651 count tachome-
ter pulses. The devices can count up to 255 (FFh) pulses
during the selected count time. If more than 255 pulses
occur, the IC sets the overflow alarm and the Tachometer
Count Register reports the maximum value of 255. Set
the time register so the count register will not overflow
under worst-case conditions (maximum fan speed) while
maximizing resolution. Calculate the maximum measur-
able fan speed and minimum resolution with the following
equations:
7 (MSB) to 5
Always 0
GPIO4 (MAX6651 only)
GPIO3 (MAX6651 only)
GPIO2 (MAX6651 only)
GPIO1
0
1
1
1
1
1
4
3
2
1
0 (LSB)
GPIO0
Alarm Status Register if the condition that caused the
alarm is removed.
Max Fan Speed (in RPS) = 255 / (2 x t
)
COUNT
Min Resolution (in RPS) = 1 / (2 x t
)
COUNT
Tachometer
The Tachometer Count Registers record the number of
pulses on the corresponding tachometer input during the
period defined by the Tachometer Count-Time Register.
where t
is the tachometer count time; 1kHz is the
COUNT
maximum allowable tachometer input frequency for the
MAX6650/MAX6651.
Maxim Integrated
11
MAX6650/MAX6651
Fan-Speed Regulators and Monitors
2
with SMBus/I C-Compatible Interface
Table 7. Alarm-Enable Register Bit Masks
POR
BIT
NAME
(DEFAULT)
STATE
FUNCTION
7 (MSB) to 5
—
0
0
0
0
0
0
Always 0
4
GPIO2 (MAX6651 only)
GPIO2 Alarm Enable/Disable (MAX6651 only)
GPIO1 Alarm Enable/Disable
3
GPIO1
TACH
MIN
2
1
Tachometer Overflow Alarm Enable/Disable
Minimum Output Level Alarm Enable/Disable
Maximum Output Level Alarm Enable/Disable
0 (LSB)
1 = Enabled
MAX
Table 8. Alarm Status Register Bit Assignments
POR
BIT
NAME
(DEFAULT)
STATE
FUNCTION
7 (MSB) to 5
—
0
0
0
0
0
0
Always 0
4
GPIO2 (MAX6651 only)
GPIO2 Alarm. Set when GPIO2 is low (MAX6651 only).
GPIO1 Alarm. Set when GPIO1 is low.
Tachometer Overflow Alarm
3
GPIO1
TACH
MIN
2
1
Minimum Output Level Alarm
0 (LSB)
MAX
Maximum Output Level Alarm
1 = Alarm condition
Upon power-up, the Tachometer Count Registers reset
to 00h and the Tachometer Count-Time Register sets a
1s integration time.
Table 9. Tachometer Count-Time Register
(Assumes two pulses per revolution)
REGISTER
VALUE
COUNT
TIME
(s)
MAXIMUM
FAN SPEED
(RPS)
MINIMUM
RESOLUTION
(Hz/COUNT)
Digital-to-Analog Converter
When using the open-loop mode of operation, the DAC
Register sets the voltage on the low side of the fan. An
internal operational amplifier compares the feedback
(K
COUNT
)
0000 0000
0000 0001
0000 0010
0000 0011
0.25
0.5
512
256
128
64
2
1
voltage (V ) with the reference voltage set by the 8-bit
FB
DAC, and adjusts the output voltage (V
) until the
1.0
0.5
0.25
OUT
two input voltages are equal. The voltage at the FB pin
2.0
may be determined by the following equation:
V
= (10 x V
x K ) / 256
DAC
FB
REF
The first 6 bits of the Tachometer Count-Time Register
are always zero, and the last 2 bits set the count time
(Table 9). The count time may be determined from the
following equation:
and the voltage across the fan is:
90k
10k
K
DAC
⎛
⎞ ⎛
⎞
V
–
+1
⎟ ⎜
V
REF
⎜
⎝
⎟
FAN
t
= 0.25s x 2K
COUNT
⎠ ⎝
⎠
256
COUNT
where K
is the numerical value of the two 2LSBs.
COUNT
where K
is the numerical value of the DAC Register
DAC
The 0.25 factor has a 10% uncertainty.
and V
= 1.5V. The minimum feedback voltage is
REF
limited by the voltage drop across the external MOS-
FET (R
x I
), and the maximum voltage is limited
ON
FAN
by the fan’s supply voltage (V
). For linear opera-
FAN
12
Maxim Integrated
MAX6650/MAX6651
Fan-Speed Regulators and Monitors
2
with SMBus/I C-Compatible Interface
V
FAN
5V OR 12V
V
V
CC
CC
3V TO 5.5V
10kΩ
MAX6650
TACH0
FB
SCL
SDA
FAN
2
SMBus/I C
INTERFACE
C
10μF
ALERT
COMP
GPIO0
GPIO1
OUT
FULL ON
ADD
GND
Figure 6. Fan Control with a Bipolar Transistor
tion, use DAC values between 08h and B0h (see
Typical Operating Characteristics). When using the
closed-loop mode of operation, the contents of the
DAC Register are ignored. When writing to the DAC,
wait at least 500µs before attempting to read back.
breakdown voltage, current rating, and drain-to-source
on-resistance (R
). Gate-to-source conduction
DS(ON)
threshold must be compatible with available V . The
CC
maximum gate-to-source voltage and the drain-to-
source breakdown voltage rating should both be at
least a few volts higher than the fan supply voltage
Power-on Reset (POR)
The MAX6650/MAX6651 have volatile memory. To pre-
vent ambiguous power-supply conditions from corrupt-
ing the data in the memory and causing erratic
(V
). Choose a MOSFET with a maximum continuous
FAN
drain current rating higher than the maximum fan cur-
rent. R should be as low as practical to maxi-
DS(ON)
mize the feedback voltage range. Maximum power
dissipation in the power transistor can be approximat-
behavior, a POR voltage detector monitors V
and
CC
clears the memory if V
falls below 1.6V. When power
rises above 1.6V, the logic
CC
ed by P = (V
I
) / 4. Bipolar power transis-
FAN X FAN(MAX)
is first applied and V
CC
tors are practical for driving small and midsize fans
(Figure 6). Very-high-current fans may require output
transistor base current greater than the MAX6650’s
50mA drive capability. Bipolar Darlington transistors
will work but have poor saturation characteristics and
could lose up to 2V to 3V of drive voltage.
blocks begin operating (though reads and writes at
levels below 3V are not recommended).
V
CC
Power-up defaults include the following:
• All alarms are disabled.
• Prescale divider is set to 4.
Resistor Selection
• Fan speed is set in full-on mode.
See Table 2 for the default states of all registers.
The tachometer input voltages (V
) and feedback
TACH_
voltage (V ) cannot exceed 13.2V (see Absolute
FB
Maximum Ratings). When using a fan powered by a
Applications Information
MOSFET and Bipolar Transistor
Selection
13.2V or greater supply (V
), protect these inputs
FAN
from overvoltage conditions with series resistors. The
resistance required to protect these pins may be calcu-
lated from the following equation:
The MAX6650/MAX6651 drive an external N-channel
MOSFET that requires five important parameters for
proper selection: gate-to-source conduction threshold,
maximum gate-to-source voltage, drain-to-source
R
= [(V
- 13.2V) x R ] / 13.2V
FAN(MAX) IN
PROTECT
where V
is the worst-case maximum supply
FAN(MAX)
voltage used to power the fan and R is the input
IN
Maxim Integrated
13
MAX6650/MAX6651
Fan-Speed Regulators and Monitors
2
with SMBus/I C-Compatible Interface
impedance of the tachometer input (150kΩ max) or the
feedback input (150kΩ max).
Fan Selection
For closed-loop operation and fan monitoring, the
MAX6650/MAX6651 require fans with tachometer outputs.
A tachometer output is typically specified as an option on
many fan models from a variety of manufacturers. Verify
the nature of the tachometer output (open collector, totem-
pole) and the resultant levels, and configure the connec-
tion to the MAX6650/MAX6651 accordingly. Note how
many pulses per revolution are generated by the
tachometer output (this varies from model to model and
among manufacturers, though two pulses per revolution is
the most common).
Compensation Capacitor
A compensation capacitor is needed from the fan’s low
side to ground to stabilize the analog control loop.
Typically, this capacitor should be 10µF, but depend-
ing on the type of fan being used, a value between 1µF
and 100µF may be required. The proper value has
been selected when no ringing is present on the volt-
age at the fan’s low side.
Table 10 lists the representative fan manufacturers and
the models they make available with tachometer outputs.
Table 10. Fan Manufacturers
Low-Speed Operation
Brushless DC fans increase reliability by replacing
mechanical commutation with electronic commutation. By
lowering the voltage across the fan to reduce its speed,
the MAX6650/MAX6651 are also lowering the supply volt-
age for the electronic commutation and tachometer elec-
tronics. If the voltage supplied to the fan is lowered too
far, the internal electronics may no longer function prop-
erly. Some of the following symptoms are possible:
MANUFACTURER
FAN MODEL OPTION
All DC brushless models can be
ordered with optional tachometer
output.
Comair Rotron
Tachometer output optional on some
models.
EBM-Papst
NMB
All DC brushless models can be
ordered with optional tachometer
output.
• The fan may stop spinning.
• The tachometer output may stop generating a signal.
Panaflo and flat unidirectional
miniature fans can be ordered with
tachometer output.
• The tachometer output may generate more than two
pulses per revolution.
Panasonic
Sunon
The problems that occur, and the supply voltages at
which they occur, depend on which fan is used. As a
Tachometer output optional on some
models.
V
FAN
5V OR 12V
10kΩ
FAN
0
V
CC
V
CC
TACH0
TACH1
3V TO 5.5V
10kΩ
MAX6651
FAN
1
SCL
SDA
2
10kΩ
SMBus/I C
INTERFACE
FAN
2
TACH2
FB
ALERT
GPIO0
GPIO1
OUT
C
COMP
FULL ON
ADD
GND
Figure 7. Using the MAX6651 to Control Parallel Fans
14
Maxim Integrated
MAX6650/MAX6651
Fan-Speed Regulators and Monitors
2
with SMBus/I C-Compatible Interface
VCC
3V OR 5.5V
TACH0
TACH1
TACH2
FAN TACH 1
FAN TACH 2
TO FAN VOLTAGE
5V OR 12V
FAN TACH 3
MAX6651
FAN TACH 4
FAN TACH 5
COM
NO0
NO1
TACH3
MAX4051
FAN TACH 6
FAN TACH 7
NO2
ADDA
ADDB
ADDC
INH
GPIO2
NO3
NO4
NO5
NO6
FAN TACH 8
FAN TACH 9
FAN TACH 10
FAN TACH 11
GPIO3
GPIO4
NO7
V-
GND
Figure 8. Monitoring Multiple Fans
very rough rule of thumb, 12V fans can be expected to
experience problems somewhere around 1/4 to 1/2
their rated speed.
Closed-Loop Mode
The MAX6650 allows the system to read the DAC value
used to regulate the fan speed. For a given speed, a
significant change in the required DAC value may indi-
cate future fan problems.
Predicting Future Fan Failure
In systems that require maximum reliability, such as
servers and network equipment, it can be advantageous
to predict fan failure before it actually happens, to alert
the system operator before the fan fails, minimizing down
time. The MAX6650 allows the user to monitor the fan’s
condition through the following modes.
Monitoring More than 4 Fans
Use the MAX6651 to monitor up to four fans at a time
(Figure 7). For systems requiring more than four fans,
Figure 8 shows an application using an analog multi-
plexer (mux) to monitor 11 fans. GPIO2, GPIO3, and
GPIO4 are connected to the mux’s address pins. By
writing the appropriate value to the GPIO pins, the
desired tachometer gets selected and counted by the
TACH3 input. Because the TACH inputs are double-
buffered, and only sampled every other time slot, it is
important to wait at least 4 times the tachometer count
time before reading the register after changing the mux
address. In the extreme case, a total of 25 fans can be
monitored using three multiplexers connected to
TACH1, TACH2, and TACH3. Do not connect TACH0 to
a mux if the MAX6651 is under closed-loop mode.
Full-On Mode
By occasionally (over a period of days or weeks) turning
the fan on full and measuring the resultant speed, a
failing fan can be detected by a trend of decreasing
speeds at a given power-supply voltage. Power-up is a
convenient time to measure the maximum fan speed.
Open-Loop Mode
The fan’s condition can also be monitored using open-
loop mode. By characterizing the fan while it is new,
fan failure can be determined by writing a predeter-
mined value to the DAC and measuring the resultant
fan speed. A decrease over time of the resultant speed
may be an indication of future fan failure.
N + 1 Fan Application
As shown in Figure 9, if any MAX6650 cannot maintain
speed regulation, all other fans will automatically be
turned on full. This can be useful in high-reliability sys-
tems where any single fan failure should not cause
Maxim Integrated
15
MAX6650/MAX6651
Fan-Speed Regulators and Monitors
2
with SMBus/I C-Compatible Interface
ALERT
TO INT PIN
ON NC
GPIO0
MAX6650
FAN
1
FULL ON
ALERT
GPIO1
GPIO0
MAX6650
FAN
2
FULL ON
ALERT
GPIO1
GPIO0
FAN
3
MAX6650
FULL ON
ALERT
GPIO1
GPIO0
FAN
4
MAX6650
GPIO1
FULL ON
Figure 9. N + 1 Application
downtime. The system should be designed so that the
number of fans used is one more than are actually
needed. This way, there is sufficient cooling even if a
fan fails. With all fans operating correctly, it is unneces-
sary to run the fans at their maximum speed. Reducing
fan speed can reduce noise and increase the life of the
fans. However, once a fan fails, it is important that the
remaining fans spin at their maximum speed.
ured as FULL ON input. The MAX6501 TOVER pin goes
low whenever its temperature goes above a preset value.
This pulls the FULL ON pin (GPIO1) low, forcing the fan to
spin at its maximum speed. Figure 12 shows the use of
multiple MAX6501s. The MAX6501 has an open-drain
output, allowing multiple devices to be wire ORed to the
FULL ON input. This configuration allows fail-safe moni-
toring of multiple locations around the system.
In Figure 9, all the GPIO0s are configured as ALERT
outputs, and all the GPIO1s are configured as
FULL ON inputs. If any MAX6650 generates an ALERT
(indicating failure), the remaining MAX6650s will auto-
matically turn their fans on full.
Hot-Swap Application
Hot swapping of a fan can be detected using the circuit
in Figure 13 where GPIO2 is configured to generate an
alert whenever it is pulled low. As long as the fan card
is connected, GPIO2 is high. However, when the fan
card is removed, a 2.2kΩ resistor pulls GPIO2 low,
causing an interrupt. This signals to the system that a
hot swap is occurring.
Temperature Monitoring and Fan Control
The circuit shown in Figure 10 provides complete tem-
perature monitoring and fan control. The MAX1617A (a
remote/local temperature serial interface with SMBus)
monitors temperature with a diode-connected transis-
tor. Based on the temperature readings provided by
the MAX1617A, the µC can adjust the fan speed pro-
portionally with temperature. Connecting the ALERT
output of the MAX1617A to the FULL ON input of the
MAX6650/MAX6651 (see the General-Purpose Input/
Output section) allows the fan to turn on fully if the
MAX1617A detects an overtemperature condition.
Step-by-Step Part Selection
and Software Setup
Determining the Fan System Topology
The MAX6650/MAX6651 support three fan system
topologies. These are single fan control, parallel fan
control, and synchronized fan control.
Single Fan Control
The simplest configuration is a single MAX6650 for
each fan. If two or more fans are required per system,
then additional MAX6650 controllers are used (one per
fan). The advantage of this configuration is the ability to
MAX6501 Hardware Fail-Safe
Figure 11 shows an application using a MAX6501 as a
hardware fail-safe. The MAX6650 has its GPIO1 config-
16
Maxim Integrated
MAX6650/MAX6651
Fan-Speed Regulators and Monitors
2
with SMBus/I C-Compatible Interface
GND
ADD1
MAX1617A
ADD0
STBY
TEMPERATURE
SENSOR
DXP
V
CC
DXN
SCL
SDA
INTERRUPT TO μC
ALERT
V
CC
ALERT
GPIO0
GPIO1
FULL ON
V
CC
V
= 5V OR 12V
FAN
MAX6650
MAX6651
μC
TACH0
FB
FAN
SCL
SDA
SCL
SDA
GND
OUT
V
CC
ADD
GND
Figure 10. Temperature Monitoring and Fan Control
Maxim Integrated
17
MAX6650/MAX6651
Fan-Speed Regulators and Monitors
2
with SMBus/I C-Compatible Interface
V
FAN
5V OR 12V
V
V
CC
CC
3V TO 5.5V
10kΩ
MAX6650
TACH0
FB
SCL
FAN
2
SMBus/I C
INTERFACE
SDA
ALERT
GPIO0
C
10μF
COMP
OUT
MAX6501
FULL ON
GPIO1
ADD
GND
TOVER
Figure 11. MAX6501 Hardware Fail-Safe
V
FAN
5V OR 12V
V
V
CC
CC
3V TO 5.5V
10kΩ
MAX6650
TACH0
FB
SCL
FAN
2
SMBus/I C
INTERFACE
SDA
ALERT
GPIO0
MAX6501
TOVER
C
10μF
COMP
OUT
FULL ON
GPIO1
ADD
GND
MAX6501
TOVER
MAX6501
TOVER
Figure 12. MAX6501 Hardware Fail-Safe
independently control each fan. The disadvantage is
cost, size, and complexity.
savings. If all the fans connected in parallel are the
same type, they will tend to run at similar speeds.
However, if one or more of the fans are wearing out,
speed mismatches can occur. The MAX6651 allows the
system to monitor up to four fans, ensuring any signifi-
cant speed mismatches can be detected.
For single fan control, use the MAX6650 (unless addi-
tional GPIOs are needed).
Parallel Fan Control
If multiple fans are required but independent control is
not, then a single MAX6650/MAX6651 connected to
two or more fans in parallel may make sense (Figure 7).
The obvious advantage is simplicity, size, and cost
For parallel fan control while monitoring up to four fan
speeds, select the MAX6651.
18
Maxim Integrated
MAX6650/MAX6651
Fan-Speed Regulators and Monitors
2
with SMBus/I C-Compatible Interface
V
FAN
HOT-SWAP SECTION
5V OR 12V
V
V
CC
CC
3V TO 5.5V
10kΩ
MAX6651
TACH0
FB
SCL
FAN
2
SMBus/I C
INTERFACE
SDA
ALERT
GPIO0
C
10μF
COMP
OUT
FULL ON
GPIO1
ADD
GND
V
ID
GPIO2
2.2kΩ
Figure 13. Hot-Swap Application
For parallel fan control while monitoring only a single
fan, select the MAX6650.
For synchronized fan control, select the MAX6651.
Combination
Synchronized Fan Control (MAX6651 Only)
In systems with multiple fans, an audible beat frequency
can sometimes be detected due to fan speed mismatch.
This happens in systems where fans are connected in
parallel or in systems with a MAX6650 controlling each
fan. In parallel fan systems, speed mismatches occur
because no two fans are identical. Slight mechanical
variations or loading differences can result in enough of
a speed mismatch to cause an audible beat.
In more complex systems, a combination of some or all
of the above control types may be needed.
Choosing a Fan
Once the topology is chosen, the next step is to choose
a fan. See the appropriate section.
Enter a zero in bit 3 of the configuration register for a
5V fan and 1 for a 12V fan.
Configuring this bit also adjusts the tachometer input
threshold voltage. This optimizes operation of the
MAX6650/MAX6651 for the operating voltage of the fan
being used.
Even in systems where there is a MAX6650/MAX6651
for each fan, there can still be speed mismatches. This
is primarily due to the oscillator tolerance. The
MAX6650/MAX6651 oscillator tolerance is specified to
be 10%. In the worst case, this could result in a 20%
(one 10% high, one 10% low) speed mismatch.
Setting the Mode of Operation
The MAX6650/MAX6651 have four modes of operation
as determined by bits 5 and 6 of the configuration reg-
ister: full on, full off, open loop, and closed loop.
The solution is to use a single MAX6651 for each fan,
and configure the parts to use a shared clock. The
shared clock can either be an external system clock or
one of the MAX6651’s internal clocks. If an external
clock is used, its frequency can range from approxi-
mately 50kHz to 500kHz.
Full-On
The full-on mode applies the maximum available volt-
age across the fan, guaranteeing maximum cooling.
Full-on mode can be entered through software or hard-
Maxim Integrated
19
MAX6650/MAX6651
Fan-Speed Regulators and Monitors
2
with SMBus/I C-Compatible Interface
ware control. To enter full-on mode through hardware,
see the Setting Up the GPIOs section. Note that a hard-
ware full-on overrides all other modes.
Below is a possible strategy for controlling the fan
under open-loop mode:
1) On power-up, put the device in open-loop mode with
a DAC value of 00 (full speed).
Configure the MAX6650/MAX6651 to run in software
full-on mode by entering 00 into bits 5 and 4 of the con-
figuration register.
2) Allow the fan speed to settle.
3) Read the TACH register to determine the speed.
Full-Off
The full-off mode removes all the voltage across the
fan, causing the fan to stop. Because the MAX6650/
MAX6651 work by controlling the voltage on the low
side of the fan, either 5V or 12V will be on both leads.
4) Gradually increase the DAC register value (in steps
of 1 or 2) until the desired speed is obtained.
In open-loop mode, any one of the four tachometer regis-
ters (MAX6651) can be used to measure and regulate the
fan’s speed. This is especially useful in parallel fan sys-
tems where up to four fans will be controlled as one unit.
Enter full-off mode by entering 01 into bits 5 and 4 of
the configuration register.
Care must be taken with this mode to prevent instabili-
ty, which can be caused by trying to update the fan
speed too often or in increments that are too large.
Instability can result in the fan speeding up and slowing
down repeatedly. Determining the proper update rate,
as shown in the following steps, depends largely on the
fan’s mechanical time constant and the system’s loop
gain (DAC step sizes):
Open Loop
In open-loop mode, the MAX6650/MAX6651 do not
actually regulate the fan speed. Speed regulation
requires an external µC. Although open-loop mode
allows maximum flexibility, it also requires the most
software/processor overhead.
In open-loop mode, the MAX6650/MAX6651 act as an
2
1) Enter open-loop mode by setting bits 5 and 4 of the
control register to 11.
SMB/I C-controlled voltage regulator. The µC adjusts
the voltage across the fan by writing an 8-bit value to
the DAC register. This gives the µC direct control of the
voltage across the fan. Speed regulation is accom-
plished by periodically reading the tachometer regis-
ter(s) and adjusting the DAC register appropriately. The
DAC value controls the voltage across the fan accord-
ing to the following equation:
2) Determine the speed of the fan(s) by reading the
TACH register(s).
3) Increase or decrease the DAC register to decrease
or increase the voltage across the fan, thereby
adjusting its speed.
Closed Loop
V
FAN
= V
- [((R2) / R1) + 1] x V
x K
/
FAN_SUPPLY
256
REF
DAC
2
In closed-loop mode, the SMBus/I C master (usually a
µC) writes a desired fan speed to the MAX6650/
MAX6651, and the device automatically adjusts the
voltage across the fan to maintain this speed. This
operation mode requires less software/processor over-
head than the open-loop mode. Once the desired
speed has been written, the MAX6650/MAX6651 con-
trol the fan’s speed independently, with no intervention
required from the master. If desired, the MAX6650/
MAX6651 can be configured to generate an interrupt if
it is unable to regulate the fan’s speed at the desired
value (see Setting Up Alarms). The MAX6650/MAX6651
can regulate only the speed of the fan connected to the
TACH0 input. Fans connected in parallel to the TACH0
fan will tend to run at similar speeds (assuming similar
fans). When going from full-off to closed-loop-mode, it
is recommended following this sequence:
where V
= the voltage across the fan, V
FAN_SUPPLY
FAN
= the supply voltage for the fan (5V or 12V), R2 = 90kΩ
(typ), R1 = 10kΩ (typ), V = 1.5V (typ), and K
the value in the DAC register.
=
DAC
REF
Note several important things in this equation. First, the
voltage across the fan moves in the opposite direction
of the DAC value. In other words, low DAC values cor-
respond to higher voltages across the fan and therefore
higher speeds. Second, DAC values greater than 180
will result in 0V across a 12V fan. Similarly, DAC values
greater than 76 will produce 0V across a 5V fan. This
limits the useful range of the DAC from 0 to 180 for 12V
fans and 0 to 76 for 5V fans.
Remember that device tolerances can cause the output
voltage value to vary significantly from unit to unit and
over temperature. However, because this voltage is
within a closed speed-control loop, such errors are cor-
rected by the loop.
1) Full-off mode
2) Full-on mode (with sufficient pause to initiate
movement)
3) Closed-loop mode
20
Maxim Integrated
MAX6650/MAX6651
Fan-Speed Regulators and Monitors
2
with SMBus/I C-Compatible Interface
The MAX6650 regulates fan speed in the following
speed register equals approximately 64 (decimal).
Although 64 is a good target value, values between 20
and 100 will work fine.
manner. The output of an internal 254kHz oscillator is
divided by 128, generating a roughly 2kHz signal. This
signal is divided by 1 plus the value in the speed regis-
ter and is used as a reference frequency. For example,
02h in the speed register will result in a 667Hz [2kHz /
(02h+1)] reference frequency, which is then compared
against the frequency at the tachometer input divided
by the prescaler value. The MAX6650/MAX6651
attempt to keep the tachometer frequency divided by
the prescaler equal to the reference frequency by
adjusting the voltage across the fan. If the tachometer
frequency divided by the prescaler value is less than
the reference frequency, the voltage across the fan is
increased. Remember that the tachometer will give two
pulses per revolution of the fan. The following equations
describe the operation.
The prescaler value also affects the response time and
the stability of the speed-control loop. Adjusting the
prescaler value effectively adjusts the loop gain. A larg-
er prescaler value will slow the response time and
increase stability, while a smaller prescaler value will
yield quicker response time. The optimum prescaler
value for response time and stability depends on the
fan’s mechanical time constant. Small, fast-spinning
fans will tend to have small mechanical time constants
and can benefit from smaller prescaler values. A good
rule of thumb is to try the selected prescaler value in
the target system. Set K
to around 75% of full
TACH
scale, and watch for overshoot or oscillation in the fan
speed. Also look for overshoot or oscillation when
When in regulation:
K
TACH
is changed from one value to another (e.g., from
75% of full-scale speed to 90% of full scale). If there is
unacceptable overshoot or if the fan speeds up and
[f
/ (128 x (K
+ 1))] = 2 x FanSpeed / K
CLK
TACH SCALE
where f
= oscillator frequency (either the 254kHz
CLK
slows down with K
, set it to a constant value;
TACH
internal oscillator or the externally applied clock), K
TACH
increase the prescaler value.
= the value in the speed register, FanSpeed = the
speed of the fan in revolutions per second (Hz),
Enter the appropriate prescaler value in bits zero to 2 of
the configuration register.
K
= the prescaler value (1, 2, 4, 8, or 16).
SCALE
Fan speed is a trade-off between cooling requirements,
noise, power, and fan wear. In general, it is desirable
(within limits) to run the fan at the slowest speed that
will accomplish the cooling goals. This will reduce
power consumption, increase fan life, and minimize
noise. When calculating the desired fan speed, remem-
ber that the above equations are written in rotations per
second (RPS), where most fans are specified in rota-
tions per minute (RPM).
Solving for all four variables:
K
K
= [(f
x K
) / (256 x FanSpeed)] - 1
SCALE
TACH
CLK
= [256 x FanSpeed x (K
+ 1)] / f
CLK
SCALE
TACH
FanSpeed = K
x f
/ [256 x (K + 1)]
TACH
SCALE
CLK
f
= 256 x FanSpeed x (K
+ 1) / K
TACH SCALE
CLK
If the internal oscillator is used, setting f
can further reduce the equations:
to 254kHz
+ 1) / 992
CLK
Equation 1: K
Equation 2: K
= FanSpeed x (K
Write the desired fan speed to the speed register.
SCALE
TACH
= (992 x K
/ FanSpeed) - 1
TACH
SCALE
Example:
Assume the following:
Equation 3: FanSpeed = 992 x K
/ (K
+ 1)
SCALE
TACH
Enter closed-loop mode by entering 10 into bits 5 and 4
of the configuration register.
• 12V fan is rated at 2000RPM at 12V.
• Use the internal oscillator (f
= 254kHz).
CLK
Note that in equation 3, the fan speed is inversely pro-
• Desired fan speed = 1500RPM (25RPS).
portional to (K
+ 1). This means the regulated fan
TACH
First, calculate an appropriate prescaler value
speed is a nonlinear function of the value written to the
speed register. Low values written to the speed register
can result in large relative changes in fan speed. For
best results, design the system so that small values
(such as 02h) are not needed. This is easily accom-
plished because an 8-bit speed register is used, and
fan-speed control should rarely need more than 16
speeds. A good compromise is to design the system
(by selecting the appropriate prescaler value) so that
the maximum-rated speed of the fan occurs when the
(K
) using equation 1. Attempt to get K
as
TACH
SCALE
close to 64 as possible for the maximum speed of
2000RPM.
• Set FanSpeed = 33.3RPS (2000RPM/60).
• Set K
= 64.
TACH
• Solving equation 1 gives K
= 2.18.
SCALE
Maxim Integrated
21
MAX6650/MAX6651
Fan-Speed Regulators and Monitors
2
with SMBus/I C-Compatible Interface
We will start with K
could be tried, or to improve response time, a 1 could
be tried).
= 2 (to increase stability, a 4
Digital Out Low
All GPIOs can be configured to output a logic-level low.
The MAX6650/MAX6651 are designed to sink up to
10mA. This high sink current can be especially useful
for driving LEDs.
SCALE
Second, calculate the appropriate value for the Speed
Register (K
) using equation 2.
TACH
On the MAX6651, for GPIO3 and GPIO4, write a zero to
the appropriate location in the GPIO definition register.
• Set FanSpeed = 25RPS (1500PRM/60).
• Solving for equation 2 gives K = 78 for K
TACH
SCALE
For GPIO0, GPIO1, and (MAX6551 only) GPIO2, write a
10 to the appropriate location in the GPIO definition
register.
= 2, K
K = 4.
= 39 for K
= 1, or K = 158 for
TACH
SCALE
TACH
Determining the Tachometer Count Time
Digital Out High
All GPIOs can be configured to generate a logic-level
high. An output high is generated using an open-drain
output stage with an internal pullup resistor of nominally
100kΩ. The MAX6650/MAX6651 power-up default state
is with all GPIOs configured as output highs.
2
To monitor the fan speed using the SMBus/I C, the next
step is to determine the tachometer count time. In sys-
tems running in open-loop mode, this is necessary. In
closed-loop or full-speed mode, reading the tachome-
ter can serve as a valuable check to ensure the fan and
the control loop are operating properly.
On the MAX6651, for GPIO3 and GPIO4, write a 1 to
the appropriate location in the GPIO definition register.
The MAX6650/MAX6651 use an 8-bit counter to count
the tachometer pulses. This means the device can
count from 0 to 255 tachometer pulses before overflow-
ing. The MAX6650/MAX6651 can accommodate a large
range of fan speeds by allowing the counting interval to
be programmed. Smaller/faster fans should use smaller
count times. Although larger fans could also use small-
er count times, resolution would suffer. Choose the
slowest count time that will not overflow under worst-
case conditions. Fans are mechanical devices, and
their speeds are subject to large tolerance variations. If
an overflow does occur, the counter will read 255. The
MAX6650/MAX6651 can be configured to generate an
alert if an overflow is encountered (see Setting Up
Alarms). Note that the prescaler value has no effect on
the TACH0 register.
For GPIO0, GPIO1, and (MAX6551 only) GPIO2, write
an 11 to the appropriate location in the GPIO definition
register.
Digital Input
Since a logic-level high output is open drain with an
internal pullup, an external device can actively pull this
pin low. The MAX6650/MAX6651 allow the user to read
the GPIO value through the GPIO status register.
• Configure the GPIO as an output logic level high (see
above).
• Read the state of the GPIO by reading the GPIO sta-
tus register.
Alert Output
GPIO0 can also serve as an ALERT output. The ALERT
output is designed to drive an interrupt on a µC. The
ALERT output goes low whenever an enabled alarm
condition occurs (see Setting Up Alarms).
Enter the appropriate count-time value in the tachometer
count-time register.
Example:
Assume a 12V fan rated at 2000 RPM.
To accommodate large tolerance variations, choose a
count time appropriate for a maximum speed of
3000RPM; 3000RPM is 50RPS and generates a 100Hz
(2 pulses/revolution) tachometer signal. Table 9 indi-
cates a count time of 2s will optimize resolution. With a
2s count time, speeds as fast as 3825RPM can be
monitored without overflow. The minimum resolution will
be 15RPM or 0.75% of the rated speed of 2000RPM.
Configure GPIO0 as an ALERT output by writing a 01 to
bits 1 and 0 of the GPIO definition register.
Full-On Input
GPIO1 can also be configured as a full-on input. When
the full-on pin is pulled low, the MAX6650/MAX6651
apply the full available voltage across the fan. This hap-
pens independently of the software mode of operation.
This is a particularly valuable feature in high-reliability
systems, designed to prevent software malfunctions
from causing system overheating.
Setting Up the GPIOs
To increase versatility, the MAX6650/MAX6651 have
two and five general-purpose digital inputs/outputs,
respectively. These GPIOs can be configured through
Configure GPIO1 as a full-on input by writing a 01 to
bits 3 and 2 of the GPIO definition register.
2
the SMBus/I C.
22
Maxim Integrated
MAX6650/MAX6651
Fan-Speed Regulators and Monitors
2
with SMBus/I C-Compatible Interface
Synchronizing Fans
GPIO2 can be configured to allow multiple MAX6651s
MAX6501
GPIO2
FAN
1
to synchronize the speeds of the fans they are driving
(Figure 14). Synchronization is accomplished by having
one of the MAX6651s (or an external clock) serve as
the clock master by configuring one of the GPIO2s in
the system as a clock output. The remaining GPIO2s in
the system need to be configured as clock inputs:
CLOCK OUT
• Electrically connect all MAX6651 GPIO2s together.
MAX6501
GPIO2
FAN
2
• Configure one of the MAX6651’s GPIO2s to be a
clock output, using the GPIO Definition Register (set
bits 5 and 4 to 01).
CLOCK IN
• Configure the rest of the GPIO2s as clock inputs, using
the GPIO Definition Register (set bits 5 and 4 to 00).
• Configure all MAX6651s in closed-loop mode.
• Configure all prescaler values to be equal.
• Write identical values to all speed registers.
MAX6501
GPIO2
FAN
3
CLOCK IN
Setting Up Alarms
The MAX6650/MAX6651 can be configured to generate
an ALERT output on GPIO0 whenever certain events,
such as control loop out of regulation, tachometer over-
flow, or GPI01/GPI02 being driven low, occur. This is
designed to enhance the “set and forget” functionality
of the fan control system.
Figure 14. Synchronizing Fans
Enable the minimum/maximum output level alarm by
setting bits 0 and 1 of the alarm enable register to 11.
Tachometer Overflow Alarm
If any tachometer counter overflows (reaches a count of
255), this alarm will be set.
Configure GPIO0 to be an ALERT output (see above).
Minimum/Maximum Output Level Alarm
The minimum/maximum output level alarms are
designed to warn the system when the MAX6650/
MAX6651 are unable to maintain speed regulation in
closed-loop mode. The MAX6650/MAX6651 maintain
speed regulation by adjusting the voltage across the
fan. If the desired speed can’t be attained, one of these
alarms will be generated. Possible causes for failure to
attain the desired speed include system programming
problems, incipient fan failure, and a programmed
speed that the fan cannot support.
Enable the overflow output level alarm by setting bit 2
of the alarm enable register bit to 1.
GPIO1/2 Pulled Low
Enabling this alarm causes the ALERT output to go low
whenever GPIO1 or GPIO2 is pulled low. This will occur
independent of the configuration of GPIO1 or GPIO2.
Enable the GPIO1/GPIO2 output level alarms by setting
bits 3 and/or 4 of the alarm enable register bit to 1.
Clearing the ALERT
Once an ALERT is generated, determine which alarm
caused the ALERT pin to go low. Do this by reading the
Alarm Status Register. An ALERT output will stay active
(low) even if the condition that caused the alert is
removed. Reading the Alarm Status Register clears the
ALERT, if the condition that caused the alert is gone. If
the condition has not gone away, the ALERT will stay
active. Disabling the alarm with the Alarm Enable
Register will cause the ALERToutput to go inactive.
The minimum output alarm occurs when the DAC out-
put is 00h. A DAC value of 00h means that the
MAX6650/MAX6651 have applied the largest available
voltage across the fan. This typically means the fan is
unable to spin as fast as the desired speed.
The maximum output alarm occurs when the DAC value
is FFh. A DAC value of FFh means the MAX6650/MAX6651
have tried to reduce the voltage across the fan to 0.
Although this would seem to indicate the fan is spinning
faster than the desired speed, this should rarely hap-
pen. If this alarm occurs, it probably indicates some
type of system error.
Read the Alarm Status Register.
Maxim Integrated
23
MAX6650/MAX6651
Fan-Speed Regulators and Monitors
2
with SMBus/I C-Compatible Interface
Pin Configurations
TOP VIEW
TACH0
TACH2
TACH3
GND
1
2
3
4
5
6
7
8
16 TACH1
15 FB
TACH0
GND
SDA
1
2
3
4
5
10 FB
9
8
7
6
V
CC
MAX6650
14
V
CC
OUT
MAX6651
13 OUT
SCL
GPIO0
GPIO1
SDA
12 GPIO3
11 GPIO2
10 GPIO0
ADD
SCL
μMAX
GPIO4
ADD
9
GPIO1
QSOP
Package Information
For the latest package outline information and land patterns (footprints), 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.
PACKAGE TYPE
10 µMAX
PACKAGE CODE
U10-2
OUTLINE NO.
21-0061
LAND PATTERN NO.
90-0330
16 QSOP
E16-1
21-0055
90-0167
24
Maxim Integrated
MAX6650/MAX6651
Fan-Speed Regulators and Monitors
2
with SMBus/I C-Compatible Interface
Revision History
REVISION REVISION
PAGES
DESCRIPTION
CHANGED
NUMBER
DATE
Added lead-free parts to the Ordering Information
1
4
7/10
Updated Table 5 to include the pins for both the MAX6650 and MAX6651
Updated the ADD parameters in the Electrical Characteristics table; updated the
11
5
12/12
conditions notes for t
Table 1
, t , and t in the Timing Characteristics table; updated
3, 6
HD:DAT
R
F
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 specifications 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.
Maxim Integrated 160 Rio Robles, San Jose, CA 95134 USA 1-408-601-1000
25
© 2012 Maxim Integrated Products, Inc.
Maxim Integrated and the Maxim Integrated logo are trademarks of Maxim Integrated Products, Inc.
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