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19-1852; Rev 1; 3/01
SOT23 Local Temperature Comparators with SMBus Serial Interface
General Description
The MAX1755/MAX1756 are temperature monitors with 3C accuracy, each with a built-in comparator and SMBusTM-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 4C of built-in hysteresis. The MAX1756 detects both low-to-high and high-to-low temperature transitions within an 8C temperature window. This arrangement supports "windowing" algorithms where the comparison threshold is reprogrammed "onthe-fly." The MAX1755/MAX1756 are offered in the 6-pin SOT23 package and are specified over the -40C to +125C temperature range. For other temperature sensors with SMBus interfaces, refer to the MAX1617, MAX1618, MAX1668, and MAX1805 data sheets. o Built-In Hysteresis o SMBus 2-Wire Serial Interface o OVERT Thermostat Output (MAX1755) o ALERT Interrupt Output (MAX1756) o Threshold Accuracy 3C (+25C to +100C) 5C (0C to +125C) o 200A (max) Supply Current o +2.375V to +5.5V Supply Range o Tiny 6-Pin SOT23 Package
Features
o Programmable Temperature Threshold
MAX1755/MAX1756
Ordering Information
PART* MAX1755AAUT-T MAX1755BAUT-T MAX1756AAUT-T MAX1756BAUT-T TEMP RANGE -40C to +125C -40C to +125C -40C to +125C -40C to +125C PINPACKAGE 6 SOT23-6 6 SOT23-6 6 SOT23-6 6 SOT23-6 TOP MARK AANM AANL AANO AANL
________________________Applications
Local Temperature Monitoring Desktop Computers Notebook Computers Servers Workstations Memory Modules Multichip Modules Industrial Control Systems Automotive
*A and B suffixes on these devices select different combinations of slave addresses. See Table 3 to determine the appropriate suffix. Note: Requires a special solder temperature profile described in the Absolute Maximum Ratings section.
Typical Operating Circuit
0.1F +2.375V TO +5.5V
VCC MAX1755 MAX1756 SMBCLK SMBDATA OVERT (ALERT)
10k EACH
CLOCK DATA INTERRUPT TO C
Pin Configurations appear at end of data sheet. SMBus is a trademark of Intel Corp.
( ) ARE FOR MAX1756 ONLY.
ADD
GND
________________________________________________________________ Maxim Integrated Products
1
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SOT23 Local Temperature Comparators with SMBus Serial Interface MAX1755/MAX1756
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 = +70C) 6-Pin SOT23 (derate 9.1mW/C above +70C)............727mW Operating Temperature Range (extended) .......-40C to +125C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +150C
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 = 0C to +125C, unless otherwise noted. Typical values are at TA = +25C.) (Note 2)
PARAMETER TEMPERATURE COMPARATOR AND POWER SUPPLY DAC Resolution (Note 3) Temperature Comparison Error Temperature Hysteresis Temperature Window Sampling Frequency Supply Voltage Range Undervoltage Lockout Threshold Undervoltage Lockout Hysteresis Power-On Reset Threshold Operating Supply Current Standby Supply Current ADD Input High Voltage ADD Input Low Voltage SMBus INTERFACE Logic Input High Voltage Logic Input Low Voltage SMBus Output Low Sink Current ALERT, OVERT Output Low Sink Current ALERT, OVERT Output High Leakage Current Logic Input Current (ADD, SMBCLK, SMBDATA) SMBus Input Capacitance SMBus Clock Frequency SMBCLK Clock Low Time SMBCLK Clock High Time SMBus Rise Time SMBCLK, SMBDATA; VCC = 2.7V to 5.5V SMBCLK, SMBDATA; VCC = 2.7V to 5.5V SMBDATA forced to 0.4V; VCC = 2.7V to 5.5V Pin forced to 0.4V Pin forced to 5.5V Inputs forced to VCC or GND SMBCLK, SMBDATA (Note 4) tLOW, 10% to 10% points, VCC = 2.7V to 5.5V tHIGH, 90% to 90% points, VCC = 2.7V to 5.5V SMBCLK, SMBDATA; 10% to 90% points, VCC = 2.7V to 5.5V DC 4.7 4 1 -1 5 100 6 6 1 +1 2.1 0.8 V V mA mA A A pF kHz s s s VCC, falling edge SMBus static, logic inputs forced to VCC or GND SMBus static, logic inputs forced to VCC or GND 0.8 x VCC 0.2 x VCC 0.2 VCC input, rising edge Monotonicity guaranteed TA = +25 C to +100 C TA = 0oC to +125oC With respect to temperature comparison value (MAX1755) Lower threshold with respect to temperature comparison value (MAX1756)
o o
CONDITIONS
MIN 2 -3 -5 -4.5 -9 0.50 2.375 2.05
TYP
MAX
UNITS
o
C C
+3 +5 -4 -8 1 2.2 30 1.7 85 0.8 2.3 200 4 -3.5
o
o
C
-7 2.00 5.5 2.35 kHz V V mV V A A V V
2
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SOT23 Local Temperature Comparators with SMBus Serial Interface
ELECTRICAL CHARACTERISTICS (continued)
(VCC = +3.3V, TA = 0C to +125C, unless otherwise noted. Typical values are at TA = +25C.) (Note 2)
PARAMETER SMBus Fall Time SMBus Start Condition Setup Time SMBus Repeated Start Condition Setup Time SMBus Start Condition Hold Time SMBus Stop Condition Setup Time SMBus Data Valid to SMBCLK Rising-Edge Time SMBus Data Hold Time (Master Transmitter) SMBus Data Hold Time (Slave Transmitter) CONDITIONS SMBCLK, SMBDATA; 90% to 10% points, VCC = 2.7V to 5.5V VCC = 2.7V to 5.5V tSU:STA, 90% to 90% points, VCC = 2.7V to 5.5V tHD:STA, 10% of SMBDATA to 90% of SMBCLK, VCC = 2.7V to 5.5V tSU:STO, 90% of SMBCLK to 10% of SMBDATA; VCC = 2.7V to 5.5V tSU:DAT, 10% or 90% of SMBDATA to 10% of SMBCLK; VCC = 2.7V to 5.5V tHD:DAT, VCC = 2.7V to 5.5V tHD:DAT, VCC = 2.7V to 5.5V 4.7 250 4 4 250 0 300 MIN TYP MAX 300 UNITS ns s ns s s ns s ns
MAX1755/MAX1756
ELECTRICAL CHARACTERISTICS
(VCC = +3.3V, TA = -40C to +125C, unless otherwise noted.) (Note 2)
PARAMETER TEMPERATURE COMPARATOR AND POWER SUPPLY DAC Resolution (Note 3) Temperature Comparison Error Temperature Hysteresis Temperature Window Supply Voltage Range Operating Supply Current SMBus INTERFACE SMBus Output Low Sink Current ALERT, OVERT Output Low Sink Current ALERT, OVERT Output High Leakage Current SMBDATA forced to 0.4V; VCC = 2.7V to 5.5V Pin forced to 0.4V Pin forced to 5.5V 6 6 1 mA mA A SMBus static, logic inputs forced to VCC or GND With respect to temperature comparison value (MAX1755) Lower threshold with respect to temperature comparison value (MAX1756) Monotonicity guaranteed 2 -5 -4.5 -9 2.375 +5 -3.5
o o o
CONDITIONS
MIN
TYP
MAX
UNITS C C
C
-7 5.5 200 V A
Note 2: Limits are 100% production tested at TA = +25C. 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.
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SOT23 Local Temperature Comparators with SMBus Serial Interface MAX1755/MAX1756
Typical Operating Characteristics
(VCC = +3.3V, TA = +25C, unless otherwise noted.)
TRIP THRESHOLD ACCURACY PERCENTAGE OF PARTS SAMPLED vs. ACCURACY
MAX1755/6 toc01
ACCURACY vs. TEMPERATURE SETTING
MAX1755/6 toc02
SUPPLY CURRENT vs. TEMPERATURE
MAX1755/6 toc03
20 PERCENTAGE OF PARTS SAMPLED (%)
1.00 0.75 0.50 ACCURACY (C) 0.25 0 -0.25 -0.50 -0.75
130 120 SUPPLY CURRENT (A) 110 100 90 80 70 60 VCC = +2.375V -50 -25 0 25 50 75 VCC = +3.3V VCC = +5.5V
15
0
5
0 -1.00 -0.50 0 ACCURACY (C) 0.50 1.00
1.00 -50 -25 0 25 50 75 100 125 150 TEMPERATURE (C)
100 125 150
TEMPERATURE (C)
SUPPLY CURRENT vs. TEMPERATURE (STANDBY)
MAX1755/6 toc04
VOL vs. IOL
200 TA = +125C 160 VOL (mV) 120 TA = +25C 80 TA = -40C 40
MAX1755/6 toc05
1.9 1.7 SUPPLY CURRENT (A) 1.5 1.3 1.1 0.9 0.7 0.5 -50 -25 0 25 50 75 VCC = +2.375V VCC = +3.3V VCC = +5.5V
0 0 2 4 6 8 IOL (mA) 10 12 14 16
100 125 150
TEMPERATURE (C)
S0T23 THERMAL STEP RESPONSE
65 60 55 50 45 40 35 30 25 20 15 10 5 0 -2
MAX1755/6 toc06
STARTUP AND POWER-DOWN
MAX1755/6 toc07
TEMPERATURE (C)
A
VCC = 3.3V TA = +110C
SOT23 IMMERSED IN +60C FLUORINERT BATH -1 0 1 2 3 4 5
B
TIME (s)
A: VCC, 2V/div B: OVERT, 2V/div
4ms/div
4
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SOT23 Local Temperature Comparators with SMBus Serial Interface
Pin Description
PIN 1 2 3 4 NAME ADD SMBDATA SMBCLK OVERT ALERT VCC GND SMBus Address-Select Pin. See Table 3. SMBus Serial-Data Input/Output, Open Drain SMBus Serial-Clock Input Open-Drain Thermostat Output (MAX1755 Only) Open-Drain SMBus Alert (Interrupt) Output (MAX1756 Only) Supply Voltage Input, +2.375V to +5.5V. Bypass VCC to GND with a 0.1F capacitor. If supply is noisy, insert 100 resistor in series with this connection. Ground FUNCTION
MAX1755/MAX1756
5 6
Detailed Description
The MAX1755/MAX1756 are temperature comparators designed to work in conjunction with an external C or other digital intelligence through an SMBus interface. The C is typically a power-management or keyboard controller, which generates SMBus serial commands from a GPIO port or through a dedicated SMBus interface block. 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.
SMBus Digital Interface
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.
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 2C (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 0.8A. At very low supply voltages (UVLO threshold), the supply current is higher due to address decoding. Typical supply current can be as high as 250A, depending on the ADD setting, while the typical ADD input current can be as high as 40A. Exiting standby mode causes the slave address to be checked again.
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SOT23 Local Temperature Comparators with SMBus Serial Interface MAX1755/MAX1756
SMBCLK SMB SMBDATA TMAX - THYST REGISTER CONTROL LOGIC STATUS REGISTER PTAT GENERATOR COMP OUTPUT LOGIC OVERT (ALERT) TMAX REGISTER
MUX
8-BIT DAC
MAX1755 MAX1756
ADD
ADD DECODER
2 OVERT UNDERT
( ) FOR MAX1756
Figure 1. MAX1755/MAX1756 Block Diagram
Send Byte Format S ADDRESS 7 bits Receive Byte Format S ADDRESS 7 bits
READ/WRITE 1 bit
ACK 1 bit
DATA 8 bits
ACK 1 bit
P
READ/WRITE 1 bit
ACK 1 bit
DATA 8 bits
/// 1 bit
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
tSU:DAT
tHD:DAT
tSU:STO tBUF
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
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
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
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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 1/0 1/0 1/0 1/0 1/0 1/0 1/0 1/0 1/0 1/0 1/0 1/0 1/0 X X MASK OR POLARITY X X X X X X X X X X NOMINAL TEMPERATURE THRESHOLD (C) SIGN 0 0 0 0 0 0 1 1 1 1 1 1 MSB 111111 111110 110010 001100 000001 000000 111111 111110 101100 100101 000001 000000 LSB +126 +124 +100 (POR State) +24 +2 0 -2 -4 -40 -54 -54 -54 Standby Mode
Between (X 1 100101) and (X 1 000001)
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 4C, which provides for 4C 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 8C (THYST). The POR state masks undertemperature interrupts (see Figure 4).
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).
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 96C, this bit asserts.
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7
SOT23 Local Temperature Comparators with SMBus Serial Interface MAX1755/MAX1756
T THRESHOLD = 65C THYST = 4C MAX1755 OVERT 61C
T THRESHOLD = 80C THYST = 8C MAX1756 72C
ALERT
READ/WRITE
Figure 4. OVERT, ALERT Response
Table 2. Status Register Bit Assignments
BIT 7 (MSB) 6 5 4, 3, 2 1 NAME OVER UNDER OUTPUT RFU DEFAULT POR STATE 0 Not masked (MAX1755) 0 (MAX1756) Not latched N/A 0 FUNCTION This bit indicates that the TMAX threshold was crossed. Send a Send Byte or Receive Byte to clear this bit. 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. This bit indicates that the DAC is at the default value; POR threshold was crossed. Send a Send Byte to clear this bit. 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.
0
ERROR
1
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-
perature comparator from operating until there is sufficient headroom (VCC 2.2V typ). Power-Up Defaults: * TMAX register is set to +100C. * * * * 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.
8
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SOT23 Local Temperature Comparators with SMBus Serial Interface
Table 3. Slave Address Decoding (ADD Pin)
DEVICE MAX1755A MAX1755A MAX1755A MAX1755B MAX1755B MAX1755B MAX1756A MAX1756A MAX1756A MAX1756B MAX1756B MAX1756B ADD PIN CONNECTED TO VCC GND Floating VCC GND Floating VCC GND Floating VCC GND Floating ADDRESS 1001 000 1001 001 0101 001 0011 000 0011 001 0101 000 1001 010 1001 011 0101 011 0011 010 0011 011 0101 010
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 +100C. 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 4C 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.
MAX1755/MAX1756
Applications Information
Thermal Considerations
The MAX1755/MAX1756 supply current is typically 85A. 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 P, allowing the device to monitor the P'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 JA where P DISSIPATION is the power dissipated by the MAX1755/MAX1756, and JA is the package's thermal resistance. The typical thermal resistance is 110C/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 0.26C shift in the die temperature.
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.
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9
SOT23 Local Temperature Comparators with SMBus Serial Interface MAX1755/MAX1756
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 6 GND ADD 1 6 GND
SMBDATA 2
MAX1755
5
VCC
SMBDATA 2
MAX1756
5
VCC
SMBCLK 3
4
OVERT
SMBCLK 3
4
ALERT
SOT23
SOT23
Chip Information
TRANSISTOR COUNT: 2963
10 ______________________________________________________________________________________
SOT23 Local Temperature Comparators with SMBus Serial Interface
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.)
6LSOT.EPS
MAX1755/MAX1756
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 (c) 2001 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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