MAX6678AEP92 [MAXIM]
2-Channel Temperature Monitor with Dual Automatic PWM Fan-Speed Controller and Five GPIOs; 2通道温度监测器，提供双路PWM自动风扇速度控制器和五个GPIO
| 型号： | MAX6678AEP92 |
| 厂家： | 
MAXIM INTEGRATED PRODUCTS |
| 描述： | 2-Channel Temperature Monitor with Dual Automatic PWM Fan-Speed Controller and Five GPIOs |
| 文件： | 总19页 (文件大小：690K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
19-3306; Rev 0; 5/04
2-Channel Temperature Monitor with Dual Automatic
PWM Fan-Speed Controller and Five GPIOs
General Description
Features
The MAX6678 monitors its own temperature and the
temperatures of two external diode-connected transis-
tors, which typically reside on the die of a CPU or other
integrated circuit. The device reports temperature values
in digital form using a 2-wire serial interface. The
MAX6678 provides a programmable alarm output to gen-
erate interrupts, throttle signals, or overtemperature shut-
down signals.
♦ Two Thermal-Diode Inputs
♦ Local Temperature Sensor
♦ Five GPIO Input/Outputs
♦ Two PWM Outputs for Fan Drive (Open Drain; May
Be Pulled Up to +5V)
♦ Programmable Fan-Control Characteristics
♦ Automatic Fan Spin-Up Ensures Fan Start
The 2-wire serial interface accepts standard System
Management Bus (SMBus)™ write byte, read byte, send
byte, and receive byte commands to read the tempera-
ture data and program the alarm thresholds. The tem-
perature 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 maxi-
mum cooling when power dissipation increases.
♦ Controlled Rate of Change Ensures Unobtrusive
Fan-Speed Adjustments
♦ 1°C Remote Temperature Accuracy (+60°C to
+145°C)
♦ Temperature Monitoring Begins at POR for Fail-
Five GPIO pins provide additional flexibility. The GPIO
power-up states are set by connecting the GPIO preset
Safe System Protection
♦ OT Output for Throttling or Shutdown
inputs to ground or V
.
CC
♦ Four Versions Available, Each with a Different
The MAX6678 is available in a 20-pin QSOP package
and a 5mm x 5mm thin QFN package. It operates from
3.0V to 5.5V and consumes just 500µA of supply current.
Address
♦ 5mm x 5mm TQFN Package
Ordering Information
Applications
Desktop Computers
Notebook Computers
Workstations
Servers
Networking Equipment
PIN-
SMBus
PART
TEMP RANGE
PACKAGE ADDRESS
MAX6678AEP90 -40°C to +125°C 20 QSOP
MAX6678AEP92 -40°C to +125°C 20 QSOP
MAX6678AEP94 -40°C to +125°C 20 QSOP
MAX6678AEP96 -40°C to +125°C 20 QSOP
1001000
1001001
1001010
1001011
SMBus is a trademark of Intel Corp.
Pin Configurations
20 Thin
MAX6678ATP90 -40°C to +125°C
QFN-EP*
1001000
1001001
1001010
1001011
20 Thin
MAX6678ATP92 -40°C to +125°C
QFN-EP*
TOP VIEW
20 19 18 17 16
20 Thin
MAX6678ATP94 -40°C to +125°C
QFN-EP*
SMBDATA
SMBCLK
GPIO4
1
2
3
4
5
15 OT
14 GPIO1
13 GPIO2
12 GPIO3
20 Thin
MAX6678ATP96 -40°C to +125°C
QFN-EP*
MAX6678
PRESET4
DXP1
*EP = Exposed paddle.
*CONNECT EXPOSED
PADDLE TO GND
11
PRESET0
6
7
8
9
10
5mm x 5mm THIN QFN
Typical Operating Circuit appears at end of data sheet.
Pin Configurations continued at end of data sheet.
________________________________________________________________ Maxim Integrated Products
1
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.
2-Channel Temperature Monitor with Dual Automatic
PWM Fan-Speed Controller and Five GPIOs
ABSOLUTE MAXIMUM RATINGS
CC
V
to GND..............................................................-0.3V to +6V
Continuous Power Dissipation (T = +70°C)
A
OT, SMBDATA, SMBCLK, PWMOUT_,
GPIO_ to GND......................................................-0.3V to +6V
DXP_ to GND ..........................................-0.3V to + (V + 0.3V)
DXN to GND ..........................................................-0.3V to +0.8V
20-Pin QSOP (derate 9.1mW/°C above +70°C).......... 727mW
20-Pin TQFN (derate 34.5mW/°C above +70°C) .......2759mW
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
CC
PRESET_ to GND ....................................-0.3V to + (V
+ 0.3V)
CC
SMBDATA, OT, PWMOUT_ Current....................-1mA to +50mA
DXN Current ....................................................................... 1mA
ESD Protection (all pins, Human Body Model) ..................2000V
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 +125°C, unless otherwise noted. Typical values are at V
= +3.3V, T = +25°C.)
CC A
CC
A
PARAMETER
SYMBOL
V
CONDITIONS
MIN
TYP
MAX
+5.5
1
UNITS
V
Operating Supply Voltage Range
Operating Current
+3.0
CC
I
Interface inactive, ADC active
0.5
mA
S
+25°C ≤ T ≤ +125°C,
R
1
3
4
T
A
= 60°C
External Temperature Error,
0°C ≤ T ≤ +145°C,
R
V
V
= 3.3V
°C
CC
CC
V
= 3.3V
+25°C ≤ T ≤ +100°C
CC
A
0°C ≤ T ≤ +145°C,
R
0°C ≤ T ≤ +125°C
A
+25°C ≤ T ≤ +100°C
2.5
4
R
Internal Temperature Error
Temperature Resolution
= +3.3V
°C
0°C ≤ T ≤ +125°C
A
1
8
°C
Bits
ms
%
Conversion Time
200
-20
80
8
250
300
+20
120
12
PWM Frequency Tolerance
(Note 1)
High level
Low level
100
10
Remote-Diode Sourcing Current
µA
V
DXN Source Voltage
0.7
DIGITAL INPUTS AND OUTPUTS
Output Low Voltage (Sink Current)
(OT, GPIO_, SMBDATA, PWMOUT_)
V
I
= 6mA
0.4
1
V
µA
V
OL
OUT
Output High Leakage Current
(OT, GPIO_, SMBDATA, PWMOUT_)
I
OH
V
V
V
V
= 3V to 3.6V
= 3.6V to 5.5V
= 3V to 3.6V
= 3.6V to 5.5V
0.8
0.8
CC
CC
CC
CC
Logic-Low Input Voltage (SMBDATA,
SMBCLK, PRESET_, GPIO_)
V
IL
2.1
2.1
Logic-High Input Voltage (SMBDATA,
SMBCLK, PRESET_, GPIO_)
V
V
IH
Input Leakage Current
Input Capacitance
SMBus TIMING
1
µA
pF
C
5
IN
Serial Clock Frequency
f
100
kHz
SCLK
2
_______________________________________________________________________________________
2-Channel Temperature Monitor with Dual Automatic
PWM Fan-Speed Controller and Five GPIOs
ELECTRICAL CHARACTERISTICS (continued)
(V
= +3.0V to +5.5V, T = -40°C to +125°C, unless otherwise noted. Typical values are at V
= +3.3V, T = +25°C.)
CC A
CC
A
PARAMETER
SYMBOL
CONDITIONS
MIN
4
TYP
MAX
UNITS
µs
Clock Low Period
Clock High Period
t
10% to 10%
90% to 90%
LOW
t
4.7
µs
HIGH
Bus Free Time Between Stop and
Start Conditions
t
4.7
µs
BUF
SMBus Start Condition Setup Time
Start Condition Hold Time
Stop Condition Setup Time
Data Setup Time
t
90% of SMBCLK to 90% of SMBDATA
10% of SMBDATA to 10% of SMBCLK
90% of SMBCLK to 10% of SMBDATA
10% of SMBDATA to 10% of SMBCLK
10% of SMBCLK to 10% of SMBDATA
4.7
4
µs
µs
µs
ns
ns
ns
ns
ms
ms
SU:STA
t
HD:STO
t
t
4
SU:STO
SU:DAT
HD:DAT
250
300
Data Hold Time
t
SMBus Fall Time
t
300
1000
55
F
SMBus Rise Time
t
R
SMBus Timeout
t
29
37
TIMEOUT
Startup Time After POR
t
500
POR
Note 1: Deviation from programmed value in Table 6.
Typical Operating Characteristics
(T = +25°C, unless otherwise noted.)
A
OPERATING SUPPLY CURRENT
vs. SUPPLY VOLTAGE
REMOTE TEMPERATURE ERROR
vs. REMOTE-DIODE TEMPERATURE
600
560
520
480
440
400
2
FAIRCHILD 2N3906
1
0
-1
-2
-3
-4
75
100
3.0
3.5
4.0
4.5
5.0
5.5
0
25
50
125
150
°
SUPPLY VOLTAGE (V)
TEMPERATURE ( C)
_______________________________________________________________________________________
3
2-Channel Temperature Monitor with Dual Automatic
PWM Fan-Speed Controller and Five GPIOs
Typical Operating Characteristics (continued)
(T = +25°C, unless otherwise noted.)
A
LOCAL TEMPERATURE ERROR
REMOTE TEMPERATURE ERROR
vs. POWER-SUPPLY NOISE FREQUENCY
LOCAL TEMPERATURE ERROR
vs. POWER-SUPPLY NOISE FREQUENCY
vs. DIE TEMPERATURE
3
2.0
1.5
1.0
0.5
V
= 250mV SQUARE WAVE APPLIED
CC
V = 250mV SQUARE WAVE APPLIED
IN P-P
IN
P-P
TO V WITH NO BYPASS CAPACITOR
TO V WITH NO BYPASS CAPACITOR
CC
2
1
1.0
0.5
0
0
-0.5
-1.0
-1.5
-2.0
-2.5
0
-1
-2
-3
-0.5
-1.0
-1.5
0
25
50
75
100
125
0.01
0.1
1
10
100
1000
0.01
0.1
1
10
100
1000
°
TEMPERATURE ( C)
FREQUENCY (kHz)
FREQUENCY (kHz)
REMOTE TEMPERATURE ERROR
vs. COMMON-MODE NOISE FREQUENCY
REMOTE TEMPERATURE ERROR
vs. DIFFERENTIAL NOISE FREQUENCY
TEMPERATURE ERROR
vs. DXP-DXN CAPACITANCE
2
1
2.0
1.8
1.0
0.9
V
V
= AC-COUPLED TO DXP
V
V
= AC-COUPLED TO DXP AND DXN
IN
IN
IN
IN
T
= +25°C
A
= 100mV SQUARE WAVE
= 100mV SQUARE WAVE
P-P
P-P
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
-1
-2
-3
-4
-5
-6
0.1
1
10
100
0.01
0.1
1
10
100
1000
0.01
0.1
1
10
100
1000
DXP-DXN CAPACITANCE (nF)
FREQUENCY (kHz)
FREQUENCY (kHz)
PWMOUT FREQUENCY
vs. DIE TEMPERATURE
PWMOUT FREQUENCY
vs. SUPPLY VOLTAGE
GPIO OUTPUT VOLTAGE
vs. GPIO SINK CURRENT
500
400
300
200
100
0
35
34
33
32
31
30
35
34
33
32
31
30
0
5
10 15 20 25 30 35 40
GPIO SINK CURRENT (mA)
-40
-15
10
35
60
°
85
110
3.0
3.5
4.0
4.5
5.0
5.5
TEMPERATURE ( C)
SUPPLY VOLTAGE (V)
4
_______________________________________________________________________________________
2-Channel Temperature Monitor with Dual Automatic
PWM Fan-Speed Controller and Five GPIOs
Pin Description
PIN
NAME
SMBDATA
SMBCLK
DESCRIPTION
THIN QFN
QSOP
SMBus Serial-Data Input/Output, Open Drain. Can be pulled up to 5.5V,
1
3
regardless of V . Open circuit when V = 0.
CC
CC
SMBus Serial-Clock Input. Can be pulled up to 5.5V, regardless of V . Open
CC
2
4
circuit when V
= 0.
CC
3, 12, 13,
14, 16
5, 14, 15,
16, 18
Active-Low, Open-Drain GPIO Pins. Can be pulled up to 5.5V, regardless of
. Open circuit when V = 0.
GPIO0–GPIO4
V
CC
CC
4, 9, 10,
11, 20
2, 6, 11,
12, 13
PRESET0–PRESET4 GPIO Preset Inputs. Connect to GND or V
to set POR value of GPIO0–GPIO4.
CC
Combined Current Source and A/D Positive Input for Remote Diode. Connect to
anode of remote-diode-connected temperature-sensing transistor. Do not leave
floating; connect to DXN if no remote diode is used. Place a 2200pF capacitor
between DXP_ and DXN for noise filtering.
5, 7
7, 9
DXP1, DXP2
Combined Remote-Diode Cathode Input. Connect cathode of the remote-diode-
connected transistor to DXN.
6
8
8
DXN
GND
10
Ground. Connect to a clean ground reference.
Active-Low, Open-Drain Over-Temperature Output. Typically used for system
shutdown or clock throttling. Can be pulled up to 5.5V regardless of V . Open
CC
15
17
OT
circuit when V
= 0.
CC
Open-Drain Output to Power Transistor Driving Fan. Connect to the gate of a
MOSFET or base of a transistor. PWMOUT_ requires a pullup resistor. The
pullup resistor can be connected to a supply voltage as high as 5.5V,
regardless of the MAX6678’s supply voltage.
PWMOUT1,
PWMOUT2
17, 19
18
1, 19
20
V
Power-Supply Input. 3.3V nominal. Bypass V
to GND with 0.1µF capacitor.
CC
CC
Block Diagram
Detailed Description
The MAX6678 temperature sensor and fan controller
accurately measures the temperature of either two
remote pn junctions or one remote pn junction and its
own die. The device reports temperature values in digi-
tal form using a 2-wire serial interface. The remote pn
junction is typically the emitter-base junction of a com-
mon-collector pnp on a CPU, FPGA, or ASIC. The
MAX6678 operates from supply voltages of 3.0V to
5.5V and consumes 500µA (typ) of supply current. The
temperature data controls a PWM output signal to
adjust the speed of a cooling fan. The device also fea-
tures an overtemperature alarm output to generate
interrupts, throttle signals, or shutdown signals.
V
CC
DXP1
DXN
TEMPERATURE
PROCESSING
BLOCK
PWM
PWMOUT1
GENERATOR
PWMOUT2
BLOCK
DXP2
OT
GPIO0
LOGIC
SMBus
INTERFACE
AND
GPIO4
PRESET0
SMBDATA
SMBCLK
REGISTERS
Five GPIO input/outputs provide additional flexibility.
The GPIO power-up states are set by connecting the
PRESET4
MAX6678
GPIO preset inputs to ground or V
.
CC
GND
_______________________________________________________________________________________
5
2-Channel Temperature Monitor with Dual Automatic
PWM Fan-Speed Controller and Five GPIOs
Write Byte Format
S
ADDRESS
WR
ACK
COMMAND
ACK
DATA
ACK
P
—
7 bits
—
—
8 bits
—
8 bits
—
1
Slave address: equiva-
lent 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)
Read Byte Format
S
ADDRESS
WR
ACK
COMMAND
ACK
S
ADDRESS
RD
ACK
DATA
///
P
—
7 bits
—
—
8 bits
—
—
7 bits
—
—
8 bits
—
—
Slave address: equivalent
to chip-select 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
Send Byte Format
Receive Byte Format
S
ADDRESS
RD
ACK DATA
8 bits
///
P
S
ADDRESS WR ACK COMMAND ACK
7 bits 8 bits
P
—
7 bits
—
—
—
—
—
—
—
—
—
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
Command byte: sends com-
mand with no data, usually
used for one-shot command
S = Start condition
P = Stop condition
Shaded = Slave transmission
/// = Not acknowledged
Figure 1. SMBus Protocols
MSB representing +128°C. The MSB is transmitted first.
All values below 0°C clip to 00h.
SMBus Digital Interface
From a software perspective, the MAX6678 appears as a
set of byte-wide registers. This device uses a standard
SMBus 2-wire/I2C™-compatible serial interface to access
the internal registers. The MAX6678 has four different
slave addresses available; therefore, a maximum of four
MAX6678 devices can share the same bus.
Table 2 details the register address and function, whether
they can be read or written to, and the power-on reset
(POR) state. See Tables 2–6 for all other register functions
and the Register Descriptions section.
Temperature Reading
The MAX6678 contains two external temperature mea-
surement inputs to measure the die temperature of CPUs
or other ICs having on-chip temperature-sensing diodes,
or discrete diode-connected transistors as shown in the
Typical Operating Circuits. For best accuracy, the dis-
crete diode-connected transistor should be a small-signal
device with its collector and base connected together.
The on-chip ADC converts the sensed temperature and
outputs the temperature data in the format shown in Table
1. Temperature channel 2 can be used to measure either
a remote thermal diode or the internal temperature of the
MAX6678. Bit D1 of register 02h (Table 2) selects local or
remote sensing for temperature channel 2 (1 = local). The
temperature measurement resolution is 1°C for both local
and remote temperatures. The temperature accuracy is
within 1°C for remote temperature measurements from
+60°C to +100°C.
The MAX6678 employs 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 instruc-
tion. 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
2
I C is a trademark of Philips Corp.
2
Purchase of I C components from Maxim Integrated Products,
Inc., or one of its sublicensed Associated Companies, conveys
2
a license under the Philips I C Patent Rights to use these com-
2
ponents in an I C system, provided that the system conforms
2
to the I C Standard Specification as defined by Philips.
6
_______________________________________________________________________________________
2-Channel Temperature Monitor with Dual Automatic
PWM Fan-Speed Controller and Five GPIOs
A
B
C
D
E
F
G
H
I
J
K
L
M
t
t
HIGH
LOW
SMBCLK
SMBDATA
t
t
SU:STO
BUF
t
t
t
SU:DAT
SU:STA HD:STA
A = START CONDITION
E = SLAVE PULLS SMBDATA LINE LOW
I = MASTER PULLS DATA LINE LOW
J = ACKNOWLEDGE CLOCKED INTO SLAVE
K = ACKNOWLEDGE CLOCK PULSE
L = STOP 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 SLAVE
H = LSB OF DATA CLOCKED INTO SLAVE
M = NEW START CONDITION
Figure 2. SMBus Write Timing Diagram
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 MASTER
H = LSB OF DATA CLOCKED INTO MASTER
I = MASTER PULLS DATA LINE LOW
J = ACKNOWLEDGE CLOCKED INTO SLAVE
K = ACKNOWLEDGE CLOCK PULSE
L = STOP CONDITION
B = MSB OF ADDRESS CLOCKED INTO SLAVE
C = LSB OF ADDRESS CLOCKED INTO SLAVE
D = R/W BIT CLOCKED INTO SLAVE
M = NEW START CONDITION
E = SLAVE PULLS SMBDATA LINE LOW
Figure 3. SMBus Read Timing Diagram
The DXN input is biased at 0.60V above ground by an
internal diode to set up the analog-to-digital inputs for a
differential measurement. The worst case DXP-DXN dif-
ferential input voltage range is from 0.25V to 0.95V.
Excess resistance in series with the remote diode causes
about +0.5°C error per ohm. Likewise, a 200µV offset
voltage forced on DXP-DXN causes about 1°C error.
Table 1. Temperature Data Byte Format
ROUNDED TEMP
TEMP (°C)
DIGITAL OUTPUT
(°C)
241
+241
+240
+126
+25
+1
1111 0001
1111 0000
0111 1110
0001 1001
0000 0001
0000 0000
1110 1111
1111 1111
240
126
High-frequency EMI is best filtered at DXP and DXN with
an external 2200pF capacitor. This value can be
increased to about 3300pF (max), including cable capac-
itance. Capacitance higher than 3300pF introduces
errors due to the rise time of the switched current source.
25
0.50
0.00
0
Diode fault (open)
Diode fault (short)
—
—
_______________________________________________________________________________________
7
2-Channel Temperature Monitor with Dual Automatic
PWM Fan-Speed Controller and Five GPIOs
PWM Output
1) The PWMOUT_ signals are normally used in one of
+12V
three ways to control the fan’s speed: PWMOUT_ dri-
ves the gate of a MOSFET or the base of a bipolar
500kΩ
transistor in series with the fan’s power supply. The
Typical Application Circuit shows the PWMOUT_ dri-
ving an n-channel MOSFET. In this case, the PWM
invert bit (D4 in register 02h) is set to 1. Figure 4
+3.3V
0.01µF
shows PWMOUT_ driving a p-channel MOSFET and
V
OUT
the PWM invert bit must be set to zero.
18kΩ
10kΩ
1µF
120kΩ
TO FAN
PWMOUT
2) PWMOUT_ 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 effi-
cient 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 PWMOUT
= 0V, bit D4 in register 02h should be set to zero.
1µF
0.1µF
27kΩ
+3.3V
Figure 5. Driving a Fan with a PWM-to-DC Circuit
V
CC
3) PWMOUT_ directly drives the logic-level PWM
speed-control input on a fan that has this type of
input. This approach requires fewer external compo-
nents and combines the efficiency of 1) with the low
noise of 2). An example of PWMOUT_ driving a fan
with a speed-control input is shown in Figure 6. Bit
D4 in register 02h should be set to 1 when this con-
figuration is used.
5V
4.7kΩ
PWMOUT
Figure 6. Controlling a PWM Input Fan with the MAX6678’s
PWM Output (Typically, the 35kHz PWM Frequency Is Used)
V
CC
Whenever the fan has to start turning from a motionless
state, PWMOUT_ is forced high for 2s. After this spin-up
period, the PWMOUT_ duty cycle settles to the prede-
termined 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.
5V
10kΩ
P
PWMOUT
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
brushless DC motor has enough time to operate. When
driving a fan with a PWM-to-DC circuit as 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.
Figure 4. Driving a P-Channel MOSFET for Top-Side PWM Fan
Drive
8
_______________________________________________________________________________________
2-Channel Temperature Monitor with Dual Automatic
PWM Fan-Speed Controller and Five GPIOs
The duty cycle of the PWM can be controlled in two ways:
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.
1) Manual PWM control by setting the duty cycle of the
fan directly through the fan target duty-cycle regis-
ters (0Bh and 0Ch).
For temperature < FanStartTemperature and D2 of
configuration register = 0:
2) Automatic PWM control by setting the duty cycle
based on temperature.
DutyCycle = 0
Manual PWM Duty-Cycle Control
Clearing the bits that select the temperature channels for
fan control (D5 and D4 for PWMOUT1 and D3 and D2 for
PWMOUT2) 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 con-
trols 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.
For temperature < FanStartTemperature and D2 of
configuration register = 1:
DutyCycle = FanStartDutyCycle
Once the temperature crosses the fan-start tempera-
ture threshold, the temperature has to drop below the
fan-start temperature threshold minus the hysteresis
before 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.
Automatic PWM Duty-Cycle Control
In the automatic control mode, the duty cycle is con-
trolled 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 fan-
start 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:
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. When a fan switches channels, the
start temperature also changes to that of the new channel.
DCSS
DC = FSDC + (T - FST)×
TS
If DutyCycle is an odd number, it is automatically
rounded down to the closest even number.
where:
DC = DutyCycle
FSDC = FanStartDutyCycle
T = Temperature
DUTY CYCLE
FST = FanStartTemperature
DCSS = DutyCycleStepSize
TS = TempStep
REGISTER 02h,
BIT D3 = 1
DUTY-CYCLE
STEP SIZE
FAN-START
DUTY CYCLE
TEMP
STEP
Duty cycle is recalculated after each temperature con-
version 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
REGISTER 02h,
BIT D3 = 0
TEMPERATURE
FAN-START
TEMPERATURE
Figure 7. Automatic PWM Duty Control
_______________________________________________________________________________________
9
2-Channel Temperature Monitor with Dual Automatic
PWM Fan-Speed Controller and Five GPIOs
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 sys-
tem. Because several of the parameters are generally
not known, some experimentation may be necessary to
arrive at the best settings.
GPIO Inputs/Outputs and Presets
The MAX6678 contains five GPIO pins (GPIO0 through
GPIO4). When set as an output, the GPIO pin connects
to the drains of internal n-channel MOSFETs. When the
n-channel MOSFET is off, the pullup resistor (see the
Typical Operating Circuit) provides a logic-level high
output. When a GPIO pin is configured as an input, read
the state of GPIO_ from the GPIO value register (15h).
The MAX6678 powers up with GPIO0, GPIO1, and
GPIO2 high impedance and GPIO3 and GPIO4 pulled
low. After 2ms, the GPIOs go to their assigned preset
values. The preset values are set by connecting the
associated PRESET inputs to either GND or V . With
CC
PRESET“N” connected to GND, GPIO“N” pulls low; with
PRESET“N” connected to V , GPIO“N” pulls high
CC
through the pullup resistor. After power-up, the functions
and states of the GPIOs can be read and controlled
using registers 15h and 16h.
Register Descriptions
The MAX6678 contains 26 internal registers. These reg-
isters store temperature, allow control of the PWM out-
puts, determine if the MAX6678 is measuring from the
internal or remote temperature sensors, and set the
GPIO as inputs or outputs.
Temperature Registers (00h and 01h)
These registers contain the results of temperature mea-
surements. The value of the MSB is +128°C, and the
value of the LSB is +1°C. Temperature data for remote
diode 1 is in the temperature channel 1 register.
Temperature data for remote diode 2 OR the local sen-
sor (selectable by bit D1 in the configuration byte) is
stored in the temperature channel 2 register.
Power-Up Defaults
At power-up, or when the POR bit in the configuration
byte register is set, the MAX6678 has the default set-
tings indicated in Table 2. 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.
Configuration Byte (02h)
The configuration byte register controls timeout condi-
tions and various PWMOUT signals. The POR state of
the configuration byte register is 00h. See Table 3 for
configuration byte definitions.
Channel 1 and Channel 2 OT Limits (03h and 04h)
Set channel 1 (03h) and channel 2 (04h) temperature
thresholds with these two registers. Once the tempera-
ture 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.
• PWMOUT_ are high.
• When using an NMOS or npn transistor, the fan starts
at full speed on power-up.
OT Output
When temperature exceeds the OT temperature thresh-
old and OT is not masked, the OT status register indi-
cates a fault and OT output becomes active. If OT for
the respective channel is masked off, the OT status
register continues to be set, but the OT output does not
become active.
The fault flag and the output can be cleared only by
reading the OT status register and the temperature reg-
ister of that channel. If the OT status bit is cleared, OT
reasserts on the next conversion if the temperature still
exceeds the OT temperature threshold.
10 ______________________________________________________________________________________
2-Channel Temperature Monitor with Dual Automatic
PWM Fan-Speed Controller and Five GPIOs
Table 2. Register Map
REGISTER
NO.
/ADDRESS
READ/
WRITE
POR
STATE
FUNCTION
D7
D6
D5
D4
D3
D2
D1
D0
Temperature
channel 1
MSB
(+128°C)
LSB
(+1°C)
R
R
00h
01h
0000 0000
0000 0000
—
—
—
—
—
—
—
—
—
—
Temperature
channel 2
MSB
(+128°C)
LSB
(+1°C)
—
—
Min duty
cycle: 0 channel 2
= 0%,
1 = fan - = local, 0 disable
start duty = remote
cycle
Temp
Timeout:
0 =
enabled,
1 =
PWMOUT PWMOUT
Configuration Reserved; Reserved;
source: 1 Spin-up
R/W
02h
0001 1000
1 PWM
invert
2 PWM
invert
byte
set to 0
set to 0
disabled
2
Temperature
channel 1 OT
limit
LSB
(+1°C)
R/W
R/W
R
03h
04h
05h
06h
0110 1110
0101 0000
00xx xxxx
00xx xxxx
MSB
MSB
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
Temperature
channel 2 OT
limit
LSB
(+1°C)
—
—
—
—
—
—
Channel Channel
1: 1 =
fault
2: 1 =
fault
OT status
OT mask
—
—
Channel Channel
1: 1 =
R/W
2: 1 =
masked
masked
0110 000x
(96 = 40%)
PWMOUT1 start
duty cycle
MSB
(128/240)
LSB
(2/240)
R/W
R/W
R/W
R/W
R/W
R/W
07h
08h
09h
0Ah
0Bh
0Ch
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
0110 000x
(96 = 40%)
PWMOUT2 start
duty cycle
MSB
(128/240)
LSB
(2/240)
1111 000x
(240 = 100%)
PWMOUT1 max
duty cycle
MSB
(128/240)
LSB
(2/240)
1111 000x
(240 = 100%)
PWMOUT2 max
duty cycle
MSB
(128/240)
LSB
(2/240)
PWMOUT1
target duty cycle (128/240)
MSB
LSB
(2/240)
0000 000x
0000 000x
PWMOUT2 MSB
target duty cycle (128/240)
LSB
(2/240)
PWMOUT1
MSB
LSB
(2/240)
R
0Dh
0000 000x
instantaneous
(128/240)
duty cycle
—
—
—
—
—
—
***GPIO0 through GPIO4 POR values set by Preset0 through Preset4.
______________________________________________________________________________________ 11
2-Channel Temperature Monitor with Dual Automatic
PWM Fan-Speed Controller and Five GPIOs
Table 2. Register Map (continued)
REGISTER
NO.
/ADDRESS
READ/
WRITE
POR
STATE
FUNCTION
D7
D6
—
—
—
D5
—
—
—
D4
—
—
—
D3
—
—
—
D2
—
—
—
D1
D0
—
PWMOUT2
instantaneous
duty cycle
MSB
(128/240)
LSB
(2/240)
R
0Eh
0Fh
10h
0000 000x
0000 0000
0000 0000
Temperature
channel 1 fan-
start temperature
R/W
R/W
MSB
MSB
—
—
LSB
LSB
Temperature
channel 2 fan-
start temperature
Temp PWMOUT PWMOUT PWMOUT PWMOUT
step: 0 = 1 control: 1 control: 2 control: 2 control:
Hysteresis:
0 = 5°C,
1 = 10°C
Fan
configuration
R/W
11h
0000 000x
—
—
—
1°C,
1 =
1 =
1 =
1 =
1 = 2°C channel1 channel 2 channel 1 channel 2
Duty-cycle rate PWMOUT
of change 1 MSB
PWMOUT PWMOUT
PWMOUT
2 LSB
R/W
R/W
R/W
12h
13h
14h
1011 01xx
0101 0101
010x xxxx
—
—
—
—
—
—
1 LSB
2 MSB
Duty-cycle step PWMOUT
PWMOUT PWMOUT
PWMOUT
2 LSB
—
—
—
size
1 MSB
1 LSB
—
2 MSB
—
PWM frequency
select
Select A Select B Select C
—
GPIO4: 0 GPIO3: 0 GPIO2: 0 GPIO1: 0 GPIO0: 0
= output, = output, = output, = output, = output,
1 = input 1 = input 1 = input 1 = input 1 = input
R/W
15h
xxx0 0000
GPIO function
GPIO value
—
—
—
R/W
R
16h
FDh
xxx***
—
0
—
0
—
0
GPIO4
0
GPIO3
0
GPIO2
0
GPIO1
0
GPIO0
1
Read device
revision
0000 0001
R
R
FEh
FFh
1000 0110
0100 1101
Read device ID
1
0
0
1
0
0
0
0
0
1
1
1
1
0
0
1
Read
manufacturer ID
***GPIO0 through GPIO4 POR values set by Preset0 through Preset4.
OT Status (05h)
Read the OT status register to determine which channel
recorded an overtemperature condition. Bit D7 is high if
the fault reading occurred from channel 1. Bit D6 is
high if the fault reading occurred in channel 2. The OT
status register is cleared only by reading its contents.
After reading the OT status register, a temperature reg-
ister read must be done. Reading the contents of the
register also makes the OT output high impedance. If
the fault is still present on the next temperature mea-
surement cycle, the corresponding bits and the OT out-
put are set again. The POR state of the OT status regis-
ter is 00h.
OT 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 reg-
ister is 00h.
12 ______________________________________________________________________________________
2-Channel Temperature Monitor with Dual Automatic
PWM Fan-Speed Controller and Five GPIOs
Table 3. Configuration Byte Definition (02h)
BIT
NAME
POR STATE
FUNCTION
7
Reserved; set to 0
Reserved; set to 0
TIMEOUT
—
—
0
—
6
—
Set TIMEOUT to zero to enable SMBus timeout for prevention of bus lockup. Set
to 1 to disable this function.
5
Set FAN PWM INVERT to zero to force PWMOUT1 low when the duty cycle is
100%. Set to 1 to force PWMOUT1 high when the duty cycle is 100%.
4
3
FAN1 PWM INVERT
FAN2 PWM INVERT
0
0
Set FAN PWM INVERT to zero to force PWMOUT2 low when the duty cycle is
100%. Set to 1 to force PWMOUT2 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 fan-temperature 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.
2
MIN DUTY CYCLE
0
Selects either local or remote 2 as the source for temperature channel 2 register
data. When D1 = 0, the MAX6678 measures remote 2 and when D1 = 1, the
MAX6678 measures the internal die temperature.
TEMPERATURE
SOURCE SELECT
1
0
0
0
SPIN-UP DISABLE
Set SPIN-UP DISABLE to 1 to disable spin-up. Set to zero for normal fan spin-up.
PWMOUT Start Duty Cycle (07h and 08h)
PWMOUT Target Duty Cycle (0Bh and 0Ch)
In automatic fan-control mode, this register contains the
present value of the target PWM duty cycle, as deter-
mined by the measured temperature and the duty-
cycle 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.
PWMOUT1 Instantaneous Duty Cycle,
PWMOUT2 Instantaneous Duty Cycle (0Dh, 0Eh)
These registers always contain the duty cycle of the
PWM signals presented at the PWM output.
The PWMOUT 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
fan-start duty-cycle register at all temperatures below
the fan-start temperature. If the MIN DUTY CYCLE bit is
zero, the duty cycle is zero below the fan-start tempera-
ture 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 PWMOUT maximum duty-cycle register sets the
maximum allowable PWMOUT 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
PWMOUT maximum duty-cycle register is F0h, 100%.
In manual control mode, this register is ignored.
The POR state of the PWMOUT 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.
______________________________________________________________________________________ 13
2-Channel Temperature Monitor with Dual Automatic
PWM Fan-Speed Controller and Five GPIOs
Table 4. Setting the Time Between Duty-
Cycle Increments
Table 5. Setting the Duty-Cycle Change
TEMPERATURE
RANGE FOR FAN
CONTROL
(1°C STEP, 33%
TO 100%)
CHANGE IN DUTY
TIME BETWEEN
INCREMENTS (s)
TIME FROM 33%
TO 100% (s)
CYCLE PER
TEMPERATURE
STEP
D7:D5, D4:D2
D7:D4, D3:D0
000
001
010
011
100
101
110
111
0
0.0625
0.125
0.25
0.5
0
5
0000
0001
0010
0011
0100
0101
…
0
2/240
4/240
6/240
8/240
10/240
…
0
10
20
40
80
160
320
80
40
27
20
16
...
1
2
4
1000
...
16
10
...
...
Fan Configuration (11h)
The fan-configuration register controls the hysteresis
level, temperature step size, and whether the remote or
local diode controls the PWMOUT2 signal (see Table
2). 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
fan-control 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 PWMOUT_ channel 1 or
channel 2 controls (see Table 2). If both are selected
for a given PWMOUT_, 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 4). This
allows the time from 33% to 100% duty cycle to be adjust-
ed 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 rate-of-change
register is B4h (1s between increments).
1111
31
5
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 duty-
cycle change for each temperature step. The POR state
of the duty-cycle step size register is 55h (see Table 5).
PWM Frequency Select (14h)
Set bits D7, D6, and D5 (select A, B, and C) in the PWM
frequency-select register to control the PWMOUT frequen-
cy (see Table 6). The POR state of the PWM frequency-
select 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 frequen-
cy setting. For example, a result that would return a value
of 6/240 is truncated to 4/240.
Table 6. PWM Frequency Select
PWM
FREQUENCY (Hz)
SELECT A
SELECT B
SELECT C
20
33
0
0
1
1
X
0
1
0
1
X
0
0
0
0
1
50
100
35k
Note: At 35kHz, duty-cycle resolution is decreased from a res-
olution of 2/240 to 4/240.
14 ______________________________________________________________________________________
2-Channel Temperature Monitor with Dual Automatic
PWM Fan-Speed Controller and Five GPIOs
GPIO Function Register (15h)
The GPIO function register (15h) sets the GPIO_ states.
Write a zero to set a GPIO as an output. Write a one to
set a GPIO as an input.
As an example, assume the MAX6678 is configured
with a CPU that has an ideality factor of 1.002. If the
diode has no series resistance, the measured data is
related to the real temperature as follows:
GPIO Value Register (16h)
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 one or
zero to set the value of the GPIO output.
n
1.008
1.002
NOMINAL
T
= T
= T
= T (1.00599)
M
ACTUAL
M
M
n
1
For a real temperature of +85°C (358.15K), the mea-
sured temperature is +82.87°C (356.02K), which is an
error of -2.13°C.
Applications Information
Remote-Diode Considerations
Temperature accuracy depends upon having a good-
quality, diode-connected, small-signal transistor.
Accuracy has been experimentally verified for all the
devices listed in Table 7. The MAX6678 can also direct-
ly measure the die temperature of CPUs and other ICs
with on-board temperature-sensing diodes.
Effect of Series Resistance
Series resistance in a sense diode contributes addition-
al errors. For nominal diode currents of 10µA and
100µA, change in the measured voltage is:
∆V =R (100µA −10µA) = 90µA ×R
S
M
S
The transistor must be a small-signal type with a rela-
tively high forward voltage. This ensures that the input
voltage is within the A/D input voltage range. The for-
ward voltage must be greater than 0.25V at 10µA at the
highest expected temperature. The forward voltage
must be less than 0.95V at 100µA at the lowest expect-
ed temperature. The base resistance has to be less
than 100Ω. Tight specification of forward-current gain
(+50 to +150, for example) indicates that the manufac-
turer has good process control and that the devices
have consistent characteristics.
Since 1°C corresponds to 198.6µV, series resistance
contributes a temperature offset of:
µV
90
°C
Ω
= 0.453
µV
Ω
198.6
C
Assume that the diode being measured has a series
resistance of 3Ω. The series resistance contributes an
offset of:
Effect of Ideality Factor
The accuracy of the remote-temperature measurements
depends on the ideality factor (n) of the remote “diode”
(actually a transistor). The MAX6678 is optimized for n =
1.008, which is the typical value for the Intel Pentium® III
and the AMD Athlon™ MP model 6. If a sense transistor
with a different ideality factor is used, the output data is
different. Fortunately, the difference is predictable.
°C
Ω
3Ω × 0.453
=1.36°C
The effects of the ideality factor and series resistance
are additive. If the diode has an ideality factor of 1.002
and series resistance of 3Ω, the total offset can be cal-
culated by adding error due to series resistance with
error due to ideality factor:
1.36°C - 2.13°C = -0.77°C
Assume a remote-diode sensor designed for a nominal
ideality factor n
is used to measure the tem-
NOMINAL
for a diode temperature of +85°C.
perature of a diode with a different ideality factor, n .
1
In this example, the effect of the series resistance and
the ideality factor partially cancel each other.
The measured temperature T can be corrected using:
M
For best accuracy, the discrete transistor should be a
small-signal device with its collector connected to GND
and base connected to DXN. Table 7 lists examples of
discrete transistors that are appropriate for use with the
MAX6678.
n
1
T
= T
M
ACTUAL
n
NOMINAL
where temperature is measured in Kelvin.
As mentioned above, the nominal ideality factor of the
MAX6678 is 1.008.
Pentium is a registered trademark of Intel Corp.
Athlon is a trademark of AMD.
______________________________________________________________________________________ 15
2-Channel Temperature Monitor with Dual Automatic
PWM Fan-Speed Controller and Five GPIOs
ADC Noise Filtering
Table 7. Remote-Sensor Transistor
Manufacturers
The integrating ADC has inherently good noise rejec-
tion, especially of low-frequency signals such as
MANUFACTURER
Central Semiconductor (USA)
Rohm Semiconductor (USA)
Samsung (Korea)
MODEL NO.
CMPT3906
SST3906
60Hz/120Hz power-supply hum. Micropower operation
places constraints on high-frequency noise rejection.
Lay out the PC board carefully with proper external
noise filtering for high-accuracy remote measurements
in electrically noisy environments.
KST3906-TF
SMBT3906
Siemens (Germany)
Filter high-frequency electromagnetic interference
(EMI) at DXP and DXN with an external 2200pF capaci-
tor connected between the two inputs. This capacitor
can be increased to about 3300pF (max), including
cable capacitance. A capacitance higher than 3300pF
introduces errors due to the rise time of the switched-
current source.
3) Route the DXP and DXN traces parallel and close to
each other, away from any high-voltage traces such
as +12VDC. Avoid leakage currents from PC board
contamination. A 20MΩ leakage path from DXP
ground causes approximately +1°C error.
4) Connect guard traces to GND on either side of the
DXP/DXN traces. With guard traces, placing routing
near high-voltage traces is no longer an issue.
Twisted Pairs and Shielded Cables
For remote-sensor distances longer than 8in, or in partic-
ularly noisy environments, a twisted pair is recommend-
ed. Its practical length is 6ft to 12ft (typ) before noise
becomes a problem, as tested in a noisy electronics labo-
ratory. For longer distances, the best solution is a shield-
ed twisted pair like that used for audio microphones. For
example, Belden 8451 works well for distances up to
100ft in a noisy environment. Connect the twisted pair to
DXP and DXN and the shield to ground, and leave the
shield’s remote end unterminated. Excess capacitance at
DXN or DXP limits practical remote-sensor distances (see
the Typical Operating Characteristics).
5) Route as few vias and crossunders as possible to
minimize copper/solder thermocouple effects.
6) When introducing a thermocouple, make sure that
both the DXP and the DXN paths have matching
thermocouples. In general, PC board-induced ther-
mocouples are not a serious problem. A copper sol-
der thermocouple exhibits 3µV/°C, and it takes
approximately 200µV of voltage error at DXP/DXN to
cause a +1°C measurement error, so most parasitic
thermocouple errors are swamped out.
For very long cable runs, the cable’s parasitic capaci-
tance often provides noise filtering, so the recommend-
ed 2200pF capacitor can often be removed or reduced
in value. Cable resistance also affects remote-sensor
accuracy. A 1Ω series resistance introduces about
+1/2°C error.
7) Use wide traces. Narrow traces are more inductive
and tend to pick up radiated noise. The 10-mil widths
and spacings recommended are not absolutely nec-
essary (as they offer only a minor improvement in
leakage and noise), but use them where practical.
8) Placing an electrically clean copper ground plane
between the DXP/DXN traces and traces carrying
high-frequency noise signals helps reduce EMI.
PC Board Layout Checklist
1) Place the MAX6678 as close as practical to the
remote diode. In a noisy environment, such as a
computer motherboard, this distance can be 4in to
8in, or more, as long as the worst noise sources
(such as CRTs, clock generators, memory buses,
and ISA/PCI buses) are avoided.
2) Do not route the DXP/DXN lines next to the deflection
coils of a CRT. Also, do not route the traces across a
fast memory bus, which can easily introduce +30°C
error, even with good filtering. Otherwise, most noise
sources are fairly benign.
16 ______________________________________________________________________________________
2-Channel Temperature Monitor with Dual Automatic
PWM Fan-Speed Controller and Five GPIOs
Typical Application Circuit
V
FAN
(5V OR 12V)
5.0V
3.0V TO 5.5V
V
CC
CPU
V
FAN
(5V OR 12V)
DXP1
DXN
PWMOUT1
5V
REMOTE 1
PWMOUT2
DXP2
3.0V TO 5.5V
3.0V TO 5.5V
MAX6678
TO CLOCK THROTTLE OR
SYSTEM SHUTDOWN
OT
REMOTE 2
GPU
SMBDATA
SMBCLK
TO SMBus
MASTER
3.0V TO 5.5V
3.0V TO 5.5V
GPIO0
3.0V TO 5.5V
3.0V TO 5.5V
GPIO3
GPIO1
GPIO2
GPIO4
GND
PRESET_
5
Pin Configurations (continued)
Chip Information
TRANSISTOR COUNT: 23,618
PROCESS: BiCMOS
TOP VIEW
PWMOUT2
PRESET3
SMBDATA
SMBCLK
GPIO4
1
2
3
4
5
6
7
8
9
20 V
CC
19 PWMOUT1
18 GPIO0
17 OT
MAX6678
16 GPIO1
15 GPIO2
PRESET4
DXP1
14
GPIO3
DXN
13 PRESET0
12 PRESET1
11 PRESET2
DXP2
GND 10
QSOP
______________________________________________________________________________________ 17
2-Channel Temperature Monitor with Dual Automatic
PWM Fan-Speed Controller and Five GPIOs
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.)
PACKAGE OUTLINE, QSOP .150", .025" LEAD PITCH
1
21-0055
E
1
18 ______________________________________________________________________________________
2-Channel Temperature Monitor with Dual Automatic
PWM Fan-Speed Controller and Five GPIOs
Package Information (continued)
(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.)
D2
0.15
C A
D
b
0.10 M
C A B
C
L
D2/2
D/2
k
PIN # 1
I.D.
0.15
C
B
PIN # 1 I.D.
0.35x45∞
E/2
E2/2
C
(NE-1) X
e
L
E2
E
k
L
DETAIL A
e
(ND-1) X
e
DETAIL B
e
L
C
L
C
L
L1
L
L
e
e
0.10
C
A
0.08
C
C
A1 A3
PACKAGE OUTLINE
16, 20, 28, 32, 40L, THIN QFN, 5x5x0.8mm
1
E
21-0140
2
COMMON DIMENSIONS
20L 5x5 28L 5x5
EXPOSED PAD VARIATIONS
DOWN
BONDS
ALLOWED
PKG.
D2
E2
16L 5x5
32L 5x5
40L 5x5
PKG.
CODES
SYMBOL MIN. NOM. MAX. MIN. NOM. MAX. MIN. NOM. MAX. MIN. NOM. MAX. MIN. NOM. MAX.
MIN. NOM. MAX. MIN. NOM. MAX.
T1655-1 3.00 3.10 3.20 3.00 3.10 3.20
NO
A
0.70 0.75 0.80 0.70 0.75 0.80 0.70 0.75 0.80 0.70 0.75 0.80 0.70 0.75 0.80
T1655-2 3.00 3.10 3.20 3.00 3.10 3.20 YES
NO
T2055-3 3.00 3.10 3.20 3.00 3.10 3.20 YES
A1
-
0
0.02 0.05
0.20 REF.
0
0.02 0.05
0.20 REF.
0
0.02 0.05
0.20 REF.
0
0.02 0.05
0.20 REF.
0
0.05
0.20 REF.
T2055-2 3.00 3.10 3.20 3.00 3.10 3.20
A3
b
0.25 0.30 0.35 0.25 0.30 0.35 0.20 0.25 0.30 0.20 0.25 0.30 0.15 0.20 0.25
4.90 5.00 5.10 4.90 5.00 5.10 4.90 5.00 5.10 4.90 5.00 5.10 4.90 5.00 5.10
4.90 5.00 5.10 4.90 5.00 5.10 4.90 5.00 5.10 4.90 5.00 5.10 4.90 5.00 5.10
NO
T2055-4 3.00 3.10 3.20 3.00 3.10 3.20
D
E
T2855-1 3.15 3.25 3.35 3.15 3.25 3.35
T2855-2 2.60 2.70 2.80 2.60 2.70 2.80
NO
NO
e
0.80 BSC.
0.25
0.65 BSC.
0.25
0.50 BSC.
0.25
0.50 BSC.
0.25
0.40 BSC.
T2855-3 3.15 3.25 3.35 3.15 3.25 3.35 YES
k
-
-
-
-
-
-
-
-
0.25 0.35 0.45
YES
NO
T2855-4 2.60 2.70 2.80 2.60 2.70 2.80
T2855-5 2.60 2.70 2.80 2.60 2.70 2.80
T2855-6 3.15 3.25 3.35 3.15 3.25 3.35
L
0.30 0.40 0.50 0.45 0.55 0.65 0.45 0.55 0.65 0.30 0.40 0.50 0.40 0.50 0.60
L1
-
-
-
-
-
-
-
-
-
-
-
-
0.30 0.40 0.50
NO
N
ND
16
4
4
20
5
5
28
7
7
32
8
8
40
10
10
2.80
3.20
T2855-7 2.60 2.70
T3255-2
T3255-3 3.00 3.10
2.60 2.70 2.80 YES
NO
3.00 3.10 3.20 YES
NO
3.00 3.10
3.00 3.10 3.20
NE
3.20
WHHB
WHHC
WHHD-1
WHHD-2
-
JEDEC
T3255-4 3.00 3.10 3.20 3.00 3.10 3.20
T4055-1 3.20 3.30 3.40 3.20 3.30 3.40 YES
NOTES:
1. DIMENSIONING & TOLERANCING CONFORM TO ASME Y14.5M-1994.
2. ALL DIMENSIONS ARE IN MILLIMETERS. ANGLES ARE IN DEGREES.
3. N IS THE TOTAL NUMBER OF TERMINALS.
4. THE TERMINAL #1 IDENTIFIER AND TERMINAL NUMBERING CONVENTION SHALL CONFORM TO JESD 95-1
SPP-012. DETAILS OF TERMINAL #1 IDENTIFIER ARE OPTIONAL, BUT MUST BE LOCATED WITHIN THE
ZONE INDICATED. THE TERMINAL #1 IDENTIFIER MAY BE EITHER A MOLD OR MARKED FEATURE.
5. DIMENSION b APPLIES TO METALLIZED TERMINAL AND IS MEASURED BETWEEN 0.25 mm AND 0.30 mm
FROM TERMINAL TIP.
6. ND AND NE REFER TO THE NUMBER OF TERMINALS ON EACH D AND E SIDE RESPECTIVELY.
7. DEPOPULATION IS POSSIBLE IN A SYMMETRICAL FASHION.
8. COPLANARITY APPLIES TO THE EXPOSED HEAT SINK SLUG AS WELL AS THE TERMINALS.
9. DRAWING CONFORMS TO JEDEC MO220, EXCEPT EXPOSED PAD DIMENSION FOR T2855-1,
T2855-3 AND T2855-6.
PACKAGE OUTLINE
10. WARPAGE SHALL NOT EXCEED 0.10 mm.
16, 20, 28, 32, 40L, THIN QFN, 5x5x0.8mm
2
E
21-0140
2
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.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 19
© 2004 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.
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