19-1852; Rev 1; 3/01
SOT23 Local Temperature Comparators
with SMBus Serial Interface
The MAX1755/MAX1756 are offered in the 6-pin SOT23
package and are specified over the -40°C to +125°C
temperature range. For other temperature sensors with
SMBus interfaces, refer to the MAX1617, MAX1618,
MAX1668, and MAX1805 data sheets.
Features
♦ Programmable Temperature Threshold
♦ Built-In Hysteresis
♦ SMBus 2-Wire Serial Interface
♦ OVERT Thermostat Output (MAX1755)
♦ ALERT Interrupt Output (MAX1756)
♦ Threshold Accuracy
±3°C (+25°C to +100°C)
±5°C (0°C to +125°C)
♦ 200µA (max) Supply Current
♦ +2.375V to +5.5V Supply Range
♦ Tiny 6-Pin SOT23 Package
Ordering Information
PART*
TEMP RANGE
PINPACKAGE
TOP
MARK
MAX1755AAUT-T
-40°C to +125°C
6 SOT23-6
AANM
MAX1755BAUT-T
-40°C to +125°C
6 SOT23-6
AANL
MAX1756AAUT-T
-40°C to +125°C
6 SOT23-6
AANO
Local Temperature Monitoring
MAX1756BAUT-T
-40°C to +125°C
6 SOT23-6
AANL
Desktop Computers
*A and B suffixes on these devices select different combinations of slave addresses. See Table 3 to determine the
appropriate suffix.
________________________Applications
Notebook Computers
Servers
Workstations
Memory Modules
Multichip Modules
Industrial Control Systems
Note: Requires a special solder temperature profile described
in the Absolute Maximum Ratings section.
Typical Operating Circuit
0.1µF
+2.375V TO +5.5V
Automotive
VCC
10kΩ EACH
MAX1755
MAX1756
SMBCLK
CLOCK
SMBDATA
DATA
OVERT
INTERRUPT TO µC
(ALERT)
Pin Configurations appear at end of data sheet.
ADD
GND
SMBus is a trademark of Intel Corp.
( ) ARE FOR MAX1756 ONLY.
________________________________________________________________ Maxim Integrated Products
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.
1
MAX1755/MAX1756
General Description
The MAX1755/MAX1756 are temperature monitors with
±3°C accuracy, each with a built-in comparator and
SMBus™-controlled variable trip threshold. This DACprogrammable threshold allows specific thermal zones
and system responses to overtemperature conditions to
be adjusted under software control. The MAX1755/
MAX1756 feature serial programmability, low cost, wide
supply-voltage range, and a tiny package.
The MAX1755 has a self-clearing thermostat output
(OVERT), and the MAX1756 has a latched interrupt output (ALERT). A variety of slave addresses allow for up
to six devices per system (Table 3).
The MAX1755 has 4°C of built-in hysteresis. The
MAX1756 detects both low-to-high and high-to-low temperature transitions within an 8°C temperature window.
This arrangement supports “windowing” algorithms
where the comparison threshold is reprogrammed “onthe-fly.”
MAX1755/MAX1756
SOT23 Local Temperature Comparators
with SMBus Serial Interface
ABSOLUTE MAXIMUM RATINGS (Note 1)
VCC to GND ..............................................................-0.3V to +6V
ADD to GND ...............................................-0.3V to (VCC + 0.3V)
SMBCLK, SMBDATA, ALERT, OVERT to GND ........-0.3V to +6V
SMBDATA, ALERT, OVERT Current....................-1mA to +50mA
Continuous Power Dissipation (TA = +70°C)
6-Pin SOT23 (derate 9.1mW/°C above +70°C)............727mW
Operating Temperature Range (extended) .......-40°C to +125°C
Junction Temperature ......................................................+150°C
Storage Temperature Range .............................-65°C to +150°C
Note 1: This device is constructed using a unique set of packaging techniques that impose a limit on the thermal profile to which the
device can be exposed during board-level solder attach and rework. The limit permits the use of only the solder profiles recommended in the industry-standard specification, JEDEC 020A, paragraph 7.6, Table 3 for IR/VPR and Convection reflow.
Preheating is required. Hand or wave soldering is not allowed.
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
(VCC = +3.3V, TA = 0°C to +125°C, unless otherwise noted. Typical values are at TA = +25°C.) (Note 2)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
TEMPERATURE COMPARATOR AND POWER SUPPLY
DAC Resolution (Note 3)
Monotonicity guaranteed
o
Temperature Comparison Error
o
2
TA = +25 C to +100 C
-3
+3
TA = 0oC to +125oC
-5
+5
Temperature Hysteresis
With respect to temperature comparison
value (MAX1755)
Temperature Window
Lower threshold with respect to temperature
comparison value (MAX1756)
-4.5
-4
o
C
o
C
o
C
-3.5
-9
-8
-7
Sampling Frequency
0.50
1
2.00
kHz
Supply Voltage Range
2.375
5.5
V
Undervoltage Lockout Threshold
VCC input, rising edge
2.05
Undervoltage Lockout Hysteresis
2.2
2.35
30
Power-On Reset Threshold
VCC, falling edge
Operating Supply Current
Standby Supply Current
1.7
2.3
SMBus static, logic inputs forced to VCC or GND
85
200
µA
SMBus static, logic inputs forced to VCC or GND
0.8
4
µA
ADD Input High Voltage
0.2
V
mV
0.8 x VCC
V
V
ADD Input Low Voltage
0.2 x VCC
V
SMBus INTERFACE
2
Logic Input High Voltage
SMBCLK, SMBDATA; VCC = 2.7V to 5.5V
Logic Input Low Voltage
SMBCLK, SMBDATA; VCC = 2.7V to 5.5V
2.1
V
0.8
V
SMBus Output Low Sink Current
SMBDATA forced to 0.4V; VCC = 2.7V to 5.5V
6
mA
ALERT, OVERT Output Low Sink Current
Pin forced to 0.4V
6
mA
ALERT, OVERT Output High Leakage Current
Pin forced to 5.5V
1
-1
+1
µA
Logic Input Current (ADD, SMBCLK, SMBDATA)
Inputs forced to VCC or GND
SMBus Input Capacitance
SMBCLK, SMBDATA
SMBus Clock Frequency
(Note 4)
DC
SMBCLK Clock Low Time
tLOW, 10% to 10% points, VCC = 2.7V to 5.5V
4.7
µs
SMBCLK Clock High Time
tHIGH, 90% to 90% points, VCC = 2.7V to 5.5V
4
µs
SMBus Rise Time
SMBCLK, SMBDATA; 10% to 90% points,
VCC = 2.7V to 5.5V
5
_______________________________________________________________________________________
µA
pF
100
1
kHz
µs
SOT23 Local Temperature Comparators
with SMBus Serial Interface
(VCC = +3.3V, TA = 0°C to +125°C, unless otherwise noted. Typical values are at TA = +25°C.) (Note 2)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
300
ns
SMBus Fall Time
SMBCLK, SMBDATA; 90% to 10% points,
VCC = 2.7V to 5.5V
SMBus Start Condition Setup Time
VCC = 2.7V to 5.5V
4.7
µs
SMBus Repeated Start Condition Setup Time
tSU:STA, 90% to 90% points, VCC = 2.7V to 5.5V
250
ns
SMBus Start Condition Hold Time
tHD:STA, 10% of SMBDATA to 90% of
SMBCLK, VCC = 2.7V to 5.5V
4
µs
SMBus Stop Condition Setup Time
tSU:STO, 90% of SMBCLK to 10% of
SMBDATA; VCC = 2.7V to 5.5V
4
µs
SMBus Data Valid to SMBCLK Rising-Edge
Time
tSU:DAT, 10% or 90% of SMBDATA to 10% of
SMBCLK; VCC = 2.7V to 5.5V
250
ns
SMBus Data Hold Time (Master Transmitter)
tHD:DAT, VCC = 2.7V to 5.5V
0
µs
SMBus Data Hold Time (Slave Transmitter)
tHD:DAT, VCC = 2.7V to 5.5V
300
ns
ELECTRICAL CHARACTERISTICS
(VCC = +3.3V, TA = -40°C to +125°C, unless otherwise noted.) (Note 2)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
TEMPERATURE COMPARATOR AND POWER SUPPLY
DAC Resolution (Note 3)
Monotonicity guaranteed
Temperature Comparison Error
Temperature Hysteresis
With respect to temperature comparison
value (MAX1755)
Temperature Window
Lower threshold with respect to temperature
comparison value (MAX1756)
Supply Voltage Range
Operating Supply Current
2
o
C
-5
+5
o
C
-4.5
-3.5
o
C
-9
-7
2.375
5.5
V
200
µA
SMBus static, logic inputs forced to VCC or GND
SMBus INTERFACE
SMBus Output Low Sink Current
SMBDATA forced to 0.4V; VCC = 2.7V to 5.5V
6
ALERT, OVERT Output Low Sink Current
Pin forced to 0.4V
6
ALERT, OVERT Output High Leakage Current
Pin forced to 5.5V
mA
mA
1
µA
Note 2: Limits are 100% production tested at TA = +25°C. Limits over operating temperature are guaranteed by design.
Note 3: Guaranteed, but not 100% tested.
Note 4: The SMBus logic block is a static design that works with clock frequencies down to DC. While slow operation is possible,
it violates the 10kHz minimum clock frequency and SMBus specifications and may monopolize the bus.
_______________________________________________________________________________________
3
MAX1755/MAX1756
ELECTRICAL CHARACTERISTICS (continued)
Typical Operating Characteristics
(VCC = +3.3V, TA = +25°C, unless otherwise noted.)
TRIP THRESHOLD ACCURACY PERCENTAGE
OF PARTS SAMPLED vs. ACCURACY
5
0.25
0
-0.25
-0.50
0
-0.50
0
0.50
60
-25
0
25
50
75
100 125 150
VCC = +2.375V
-50
-25
0
TEMPERATURE (°C)
MAX1755/6 toc04
1.7
VCC = +5.5V
75
200
TA = +125°C
160
VOL (mV)
1.3
1.1
50
VOL vs. IOL
1.9
1.5
25
TEMPERATURE (°C)
SUPPLY CURRENT
vs. TEMPERATURE (STANDBY)
SUPPLY CURRENT (µA)
80
1.00
-50
VCC = +3.3V
90
70
1.00
VCC = +5.5V
100
-0.75
ACCURACY (°C)
VCC = +3.3V
120
TA = +25°C
80
TA = -40°C
0.9
40
0.7
VCC = +2.375V
0.5
-50
TEMPERATURE (°C)
110
MAX1755/6 toc05
-1.00
-25
0
25
50
75
0
100 125 150
0
2
4
6
8
10
12
14
TEMPERATURE (°C)
IOL (mA)
S0T23 THERMAL STEP RESPONSE
STARTUP AND POWER-DOWN
16
MAX1755/6 toc07
MAX1755/6 toc06
65
60
55
50
45
40
35
30
25
VCC = 3.3V
TA = +110°C
A
20
15
10
5
0
B
SOT23 IMMERSED IN +60°C
FLUORINERT BATH
-2
-1
0
1
2
TIME (s)
4
120
SUPPLY CURRENT (µA)
ACCURACY (°C)
0.50
MAX1755/6 toc03
15
130
MAX1755/6 toc02
1.00
0.75
0
SUPPLY CURRENT
vs. TEMPERATURE
ACCURACY vs. TEMPERATURE SETTING
MAX1755/6 toc01
20
PERCENTAGE OF PARTS SAMPLED (%)
MAX1755/MAX1756
SOT23 Local Temperature Comparators
with SMBus Serial Interface
3
4
5
A: VCC, 2V/div
B: OVERT, 2V/div
4ms/div
_______________________________________________________________________________________
100 125 150
SOT23 Local Temperature Comparators
with SMBus Serial Interface
PIN
NAME
1
ADD
2
SMBDATA
3
SMBCLK
4
FUNCTION
SMBus Address-Select Pin. See Table 3.
SMBus Serial-Data Input/Output, Open Drain
SMBus Serial-Clock Input
OVERT
Open-Drain Thermostat Output (MAX1755 Only)
ALERT
Open-Drain SMBus Alert (Interrupt) Output (MAX1756 Only)
5
VCC
Supply Voltage Input, +2.375V to +5.5V. Bypass VCC to GND with a 0.1µF capacitor. If supply is
noisy, insert 100Ω resistor in series with this connection.
6
GND
Ground
Detailed Description
SMBus Digital Interface
The MAX1755/MAX1756 are temperature comparators
designed to work in conjunction with an external µC or
other digital intelligence through an SMBus interface.
The µC is typically a power-management or keyboard
controller, which generates SMBus serial commands
from a GPIO port or through a dedicated SMBus interface block.
The SMBus block utilizes the read-byte/write-byte protocol. It only supports 8-bit reads and writes, has no
command byte, and does not need to support the Alert
Response feature. From a software perspective, the
MAX1755/MAX1756 appear as write registers containing the DAC comparison value and as read registers
containing the status byte. A standard SMBus 2-wire
serial interface is used to program DAC values and
read the status byte. Figure 2 shows the SMBus data
protocols and Figure 3 details the SMBus timing.
Figure 1 illustrates the circuit blocks in the MAX1755/
MAX1756. The output of an analog temperature-to-voltage converter signal is compared to the temperaturestable output of a serially programmed dual-output
digital-to-analog converter (DAC). Great care is taken in
the design to ensure that the thermal signal is proportional to absolute temperature or PTAT. A precision
comparator with low-input offset voltage is used to
compare the PTAT voltage to the DAC output.
The DAC provides two levels: one is set to the rising trip
threshold and the other is set to the falling threshold
(hysteresis) level by a digital subtractor. This arrangement makes the hysteresis value very accurate with
respect to the rising trip point. Each DAC level is alternately compared to the PTAT voltage through a sampling system that reuses the same comparator, thereby
reducing errors due to comparator offset. The sampling
rate is typically 1kHz.
A simplified SMBus interface enables the system to
program the temperature thresholds and read the part’s
status register.
Programming the TMAX Register
The TMAX register is programmed using a standard
SMBus Send Byte operation. The temperature data format is 6 bits plus sign in two’s complement form, with
each data bit representing 2°C (Table 1). The MSB is
transmitted first, LSB last. The MSB (B7) of the TMAX
register is an embedded control bit, which can either
invert the polarity of the OVERT output (MAX1755) or
mask low-going (THYST) interrupts (MAX1756). The B6
bit of the TMAX register is the sign bit for the remaining
6 bits that make up the binary-coded temperature
threshold.
Software Standby Mode
If the DAC code is set to negative full scale, the device
goes into software standby mode. In standby mode,
supply current is reduced to 0.8µA. At very low supply
voltages (UVLO threshold), the supply current is higher
due to address decoding. Typical supply current can
be as high as 250µA, depending on the ADD setting,
while the typical ADD input current can be as high as
40µA. Exiting standby mode causes the slave address
to be checked again.
_______________________________________________________________________________________
5
MAX1755/MAX1756
Pin Description
MAX1755/MAX1756
SOT23 Local Temperature Comparators
with SMBus Serial Interface
TMAX
REGISTER
SMBCLK
SMB
MUX
MAX1755
MAX1756
8-BIT DAC
SMBDATA
TMAX - THYST
REGISTER
ADD
DECODER
ADD
CONTROL
LOGIC
OVERT (ALERT)
OUTPUT
LOGIC
COMP
PTAT
GENERATOR
2
STATUS
REGISTER
OVERT
UNDERT
( ) FOR MAX1756
Figure 1. MAX1755/MAX1756 Block Diagram
Send Byte Format
S
ADDRESS
READ/WRITE
ACK
DATA
ACK
1 bit
1 bit
8 bits
1 bit
READ/WRITE
ACK
DATA
///
1 bit
1 bit
8 bits
1 bit
7 bits
Receive Byte Format
S
ADDRESS
7 bits
P
P
S = START CONDITION
P = STOP CONDITION
SHADED = SLAVE TRANSMISSION
/// = NOT ACKNOWLEDGED (NACK)
Figure 2. SMBus Protocols
A
tLOW
B
tHIGH
C
D
E
F
G
H
I
J
K
L
M
SMBCLK
SMBDATA
tSU:STA
tHD:STA
A = START CONDITION
B = MSB OF ADDRESS CLOCKED INTO SLAVE
C = LSB OF ADDRESS CLOCKED INTO SLAVE
D = R/W BIT CLOCKED INTO SLAVE
E = SLAVE PULLS SMBDATA LINE LOW
tSU:DAT
tHD:DAT
F = ACKNOWLEDGE BIT CLOCKED INTO MASTER
G = MSB OF DATA CLOCKED INTO SLAVE (OP/SUS BIT)
H = LSB OF DATA CLOCKED INTO SLAVE
I = SLAVE PULLS SMBDATA LINE LOW
tSU:STO tBUF
J = ACKNOWLEDGE CLOCKED INTO MASTER
K = ACKNOWLEDGE CLOCK PULSE
L = STOP CONDITION, DATA EXECUTED BY SLAVE
M = NEW START CONDITION
Figure 3. SMBus Write Timing Diagram
6
_______________________________________________________________________________________
SOT23 Local Temperature Comparators
with SMBus Serial Interface
MAX1755/MAX1756
Table 1. TMAX Register Data Format (Two’s Complement)
DAC REGISTER VALUE
MASK/POLARITY
BIT POR STATE
NOMINAL TEMPERATURE
THRESHOLD (°C)
MASK OR
POLARITY
SIGN
MSB
1/0
X
0
111111
+126
1/0
X
0
111110
+124
LSB
1/0
X
0
110010
+100 (POR State)
1/0
X
0
001100
+24
1/0
X
0
000001
+2
1/0
X
0
000000
0
1/0
X
1
111111
-2
1/0
X
1
111110
-4
1/0
X
1
101100
-40
1/0
X
1
100101
1/0
Between (X 1 100101) and (X 1 000001)
-54
-54
1/0
X
1
000001
-54
1/0
X
1
000000
Standby Mode
POLARITY bit: 0 = OVERT is active low; 1 = OVERT is active high (MAX1755).
MASK bit: A logic 1 disables THYST (negative-going) interrupts (MAX1756).
OVERT Thermostat Output
(MAX1755)
The OVERT output is a self-clearing interrupt output
that is activated when the current temperature equals
or exceeds T MAX . OVERT normally goes low when
active, but this polarity can be changed through the
POLARITY bit in the TMAX register. The latch is cleared
when the current temperature reading is equal to or
less than the current TMAX value minus 4°C, which provides for 4°C of hysteresis ( Figure 4).
ALERT Interrupt (MAX1756)
The ALERT interrupt output signal is latched and can
only be cleared by reading the status register or writing
a new TMAX value. Interrupts are generated when the
device temperature goes above the current TMAX DAC
setting or below the current DAC setting minus 8°C
(THYST). The POR state masks undertemperature interrupts (see Figure 4).
Status Register
The bit functions for the status register are summarized
in Table 2. The OVER bit (MSB) indicates when the
TMAX threshold is crossed. The UNDER bit (B6) indicates when the THYST threshold is crossed. For the
MAX1755, the UNDER bit is not masked during POR; if
the die temperature is less than 96°C, this bit asserts.
For the MAX1756, the UNDER bit is masked to zero
during POR. OVER and UNDER are cleared by any
read or write operations. In the MAX1755, the OUTPUT
bit (B5) mirrors the state of the OVERT signal. The OUTPUT bit is not used in the MAX1756 and returns zero.
The DEFAULT bit (B1) indicates that the DAC is at the
default value. This condition implies that the POR
threshold was crossed. The DEFAULT bit is cleared
using a Send Byte operation. The ERROR bit (LSB)
indicates that the UVLO threshold was crossed, and
therefore the conversion is inaccurate. Use a Send Byte
operation and make sure that VCC is in the valid operating range to clear this bit. The remaining bits in the status register (B4, B3, B2) are reserved for future use.
Slave Addresses
The MAX1755/MAX1756 appear to the SMBus as one
address for both reads and writes. The device address
can be set to one of three different values by pin-strapping ADD, so up to six ICs can reside on the same bus
without address conflicts (Table 3). A test for the current state of the address pins is done at POR and when
exiting standby mode. The results of this test are
latched as the current address. This scheme allows for
the use of three-way pin-strapping to set the address
(VCC, GND, floating).
_______________________________________________________________________________________
7
MAX1755/MAX1756
SOT23 Local Temperature Comparators
with SMBus Serial Interface
T THRESHOLD = 65°C
THYST = 4°C
61°C
MAX1755
OVERT
T THRESHOLD = 80°C
THYST = 8°C
72°C
MAX1756
ALERT
READ/WRITE
Figure 4. OVERT, ALERT Response
Table 2. Status Register Bit Assignments
BIT
NAME
POR STATE
FUNCTION
7
(MSB)
OVER
0
This bit indicates that the TMAX threshold was crossed. Send a Send Byte or
Receive Byte to clear this bit.
6
UNDER
5
OUTPUT
Not latched
4, 3, 2
RFU
N/A
1
DEFAULT
0
This bit indicates that the DAC is at the default value; POR threshold was
crossed. Send a Send Byte to clear this bit.
0
ERROR
1
This bit indicates that the conversion is inaccurate; UVLO threshold was
crossed. Send a Send Byte and bring VCC back up to a valid operating
range to clear this bit.
Not masked (MAX1755)
0 (MAX1756)
This bit indicates that the THYST threshold was crossed. Send a Send Byte or
Receive Byte to clear this bit. See Table 1.
MAX1755: This bit exactly follows the current state of the OVERT pin
(self-clearing). MAX1756: This bit is reserved for future use (returns 0).
Reserved for future use.
POR and UVLO
The MAX1755/MAX1756 have volatile memory. To prevent power-supply conditions from corrupting the data
in memory and causing erratic behavior, a POR voltage
detector monitors VCC and clears the memory if VCC
falls below 1.5V (typ, see the Electrical Characteristics).
When power is first applied and VCC rises above 1.5V
(typ), the logic blocks begin operating, although the
address decoder is not enabled until VCC > UVLO voltage. The UVLO comparator prevents the DAC and tem-
8
perature comparator from operating until there is sufficient headroom (VCC ≥ 2.2V typ).
Power-Up Defaults:
• TMAX register is set to +100°C.
•
ALERT and OVERT are reset to High-Z state.
•
OVERT polarity is set to active-low (MAX1755 only).
•
ALERT low-going hysteresis interrupts are masked
(MAX1756 only).
•
Status byte is cleared.
_______________________________________________________________________________________
SOT23 Local Temperature Comparators
with SMBus Serial Interface
ADD PIN
CONNECTED TO
ADDRESS
MAX1755A
VCC
1001 000
MAX1755A
GND
1001 001
MAX1755A
Floating
0101 001
MAX1755B
VCC
0011 000
MAX1755B
GND
0011 001
MAX1755B
Floating
0101 000
MAX1756A
VCC
1001 010
MAX1756A
GND
1001 011
MAX1756A
Floating
0101 011
MAX1756B
VCC
0011 010
MAX1756B
GND
0011 011
MAX1756B
Floating
0101 010
DEVICE
Applications Information
Thermal Considerations
The MAX1755/MAX1756 supply current is typically
85µA. When used to drive high-impedance loads, the
devices dissipate negligible power. Therefore, the die
temperature is essentially the same as the package
temperature. The key to accurate temperature monitoring is good thermal contact between the MAX1755/
MAX1756 package and the device being monitored. In
some applications, the 6-pin SOT23 package may be
small enough to fit underneath a socketed µP, allowing
the device to monitor the µP’s temperature directly.
Accurate temperature monitoring depends on the thermal resistance between the device being monitored
and the MAX1755/MAX1756 die. Heat flows in and out
of plastic packages primarily through the leads. Short,
wide copper traces leading to the temperature monitor
ensure that heat transfers quickly and reliably. The rise
in die temperature due to self-heating is given by the
following formula:
∆TJ = PDISSIPATION ✕ θJA
where P DISSIPATION is the power dissipated by the
MAX1755/MAX1756, and θJA is the package’s thermal
resistance.
The typical thermal resistance is 110°C/W. To limit the
effects of self-heating, minimize the output currents. For
example, if the MAX1755 or MAX1756 sink 6mA, the
output voltage is guaranteed to be less than 0.4V.
Therefore, an additional 2.4mW of power is dissipated
within the IC. This corresponds to a 0.26°C shift in the
die temperature.
ALERT Software Model Example of
Temperature Windowing
The MAX1756 ALERT interrupt output is designed so
effective clock-throttling and/or fan-speed control can
be done by the host firmware, while keeping the commands and registers of the MAX1756 very simple.
At initial power-up, TMAX is set to +100°C. The user can
modify this as needed using Send Byte. The POR state
masks undertemperature interrupts. The initial temperature is assumed to be very low, below the THYST undertemperature threshold. No ALERT signal is issued,
since THYST interrupts are masked.
If the system heats up sufficiently to exceed TMAX, this
generates the first interrupt. The host reads the status
byte, determining that the T MAX temperature was
exceeded. The interrupt, although cleared, is immediately asserted again since the temperature still exceeds
TMAX. The host now immediately writes a new value to
the DAC register, approximately 4°C above the old D/A
value. This hopefully places the (TMAX - THYST) window
centered around the current device temperature. This
new DAC value also writes a zero to the MASK bit,
enabling THYST interrupts. Writing a new value clears the
status byte and ALERT latch. The system then takes
whatever corrective action is needed (clock throttling or
fan control).
If the temperature continues to increase, another corrective action routine is done, similar to above. If the
temperature decreases below THYST, an interrupt is
generated and the host can remove the corrective
action (if desired), set a new TMAX value, and clear status, similar to the routine above.
If desired, the host can now mask undertemperature
interrupts to avoid nuisance interrupts as the system
cools.
Temperature Measurement Using a
Software SAR
A useful application of the MAX1755 is to determine the
ambient temperature with a successive approximation
algorithm to set the TMAX register. Like a successive
approximation register (SAR) analog-to-digital converter, the C code routine provided in Listing 1 tests 1 bit at
a time in the TMAX register form MSB to LSB. Based on
the response of the OVERT signal, the bit is either set
or cleared. This simple set and check algorithm repeats
until all of the bits are set.
_______________________________________________________________________________________
9
MAX1755/MAX1756
Table 3. Slave Address Decoding (ADD Pin)
MAX1755/MAX1756
SOT23 Local Temperature Comparators
with SMBus Serial Interface
Listing 1. Measuring Temperature by Successive Approximation
/* Use successive approximation to measure the ambient temperature.
** Returns the measured temperature in degrees C.
**
** global variables:
** __int8 shadow_command contains the last command we wrote
** __int8 shadow_status contains the last status value we read
**
** External functions:
** SMBusSendByte(__int8 address, __int8 command);
** SMBusReceiveByte(__int8 address, __int8* received_data);
** Delay_msec(int delay_time_msec);
*/
int MeasureTemperature()
{
/* Write a test temperature, then read the status byte.
** Use the state of the OVER bit to determine each successive bit.
*/
signed __int8 Thi = 127;
/*upper limit starts at maximum */
signed __int8 Tlo = -128;
/*lower limit starts at minimum */
while ( (Thi - Tlo) > 0 ) {
signed __int8 Ttest = (Thi + Tlo) / 2;
SMBusSendByte(address, Ttest);
shadow_command = Ttest;
Delay_msec(SAR_delay_time);
/* guess between Thi and Tlo */
/* set new threshold */
SMBusReceiveByte(address, &shadow_status);
/* get status byte */
if (shadow_status & 0x80) {
/* over temperature? */
if (Tlo == Ttest)
break;
/* close enough, exit loop */
Tlo = Ttest;
/* move lower limit up */
} else {
/* not over temperature */
if (Thi == Ttest)
break; /* close enough, exit loop */
Thi = Ttest;
/* move upper limit down */
}
}
return Ttest*2;
/* temperature in degrees C = twice the threshold number */
}
Pin Configurations
TOP VIEW
ADD 1
SMBDATA 2
MAX1755
SMBCLK 3
6
GND
ADD 1
5
VCC
SMBDATA 2
4
OVERT
SOT23
MAX1756
SMBCLK 3
6
GND
5
VCC
4
ALERT
SOT23
Chip Information
TRANSISTOR COUNT: 2963
10
______________________________________________________________________________________
SOT23 Local Temperature Comparators
with SMBus Serial Interface
6LSOT.EPS
PACKAGE OUTLINE, SOT-23, 6L
21-0058
F
1
1
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.
11 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2001 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.
MAX1755/MAX1756
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.)