MAX6655MEE+ ,Dual Remote/Local Temperature Sensors and Four-Channel Voltage MonitorsApplicationsV 1 16 STBYCCNotebooks WorkstationsDXP1 2 15 SMBCLKThin Clients CommunicationDXN1 14 OV ..
MAX6656MEE ,PLASTIC ENCAPSULATED DEVICESTable of Contents I. ........Device Description V. ........Quality Assurance Information II ..
MAX6656MEE+ ,Dual Remote/Local Temperature Sensors and Four-Channel Voltage MonitorsFeaturesThe MAX6655/MAX6656 are precise voltage and tem-♦ Three Temperature Channelsperature monito ..
MAX6657 ,±1°C, SMBus-Compatible Remote/Local Temperature Sensors with Overtemperature AlarmsApplications ating temperature range.+Denotes a lead(Pb)-free/RoHS-compliant package.Desktop Comput ..
MAX6657 ,±1°C, SMBus-Compatible Remote/Local Temperature Sensors with Overtemperature AlarmsFeaturesThe MAX6657/MAX6658/MAX6659 are precise, two- ♦ Dual Channel Measures Remote and Localchann ..
MAX6657MSA ,1C / SMBus-Compatible Remote/Local Temperature Sensors with Overtemperature AlarmsFeaturesThe MAX6657/MAX6658/MAX6659 are precise, two- Dual Channel: Measures Remote and Localchann ..
MB3763 ,Bidirectional Motor DriverFEATURES• Motor Drive Current: 150 mA maximum (DC)300 mA maximum (tON = 1 s, Duty = 50 %)• Wide Pow ..
MB3763P ,Bidirectional Motor DriverBLOCK DIAGRAMA-VCC B-VCCInputLogicA-OUT B-OUTA-IN GND B-IN2MB3763nnnn ABSOLUTE MAXIMUM RATINGS(Ta = ..
MB3763PF ,Bidirectional Motor DriverFEATURES• Motor Drive Current: 150 mA maximum (DC)300 mA maximum (tON = 1 s, Duty = 50 %)• Wide Pow ..
MB3769A ,SWITCHING REGULATOR CONTROLLERFUJITSU SEMICONDUCTORDS04-27202-3EDATA SHEETASSPSWITCHING REGULATORCONTROLLERMB3769AThe Fujitsu MB3 ..
MB3769A. ,SWITCHING REGULATOR CONTROLLERFUJITSU SEMICONDUCTORDS04-27202-3EDATA SHEETASSPSWITCHING REGULATORCONTROLLERMB3769AThe Fujitsu MB3 ..
MB3771 ,Power Supply MonitorFUJITSU SEMICONDUCTORDS04-27400-8EDATA SHEETASSP For power supply
MAX6655MEE+-MAX6656MEE+
Dual Remote/Local Temperature Sensors and Four-Channel Voltage Monitors
General DescriptionThe MAX6655/MAX6656 are precise voltage and tem-
perature monitors. The digital thermometer reports the
temperature of two remote sensors and its own die tem-
perature. The remote sensors are diode-connected
transistors—typically a low-cost, easily mounted
2N3906 PNP type—that replace conventional thermis-
tors or thermocouples. Remote accuracy is ±1°C for
multiple transistor manufacturers with no calibration
necessary. The remote channels can also measure the
die temperature of other ICs, such as microprocessors,
that contain a substrate-connected PNP with its collec-
tor grounded and its base and emitter available for tem-
perature-sensing purposes. The temperature is
digitized with 11-bit resolution.
The MAX6655/MAX6656 also measure their own supply
voltage and three external voltages with 8-bit resolution.
Each voltage input’s sensitivity is set to give approxi-
mately 3/4-scale output code when the input voltage is
at its nominal value. The MAX6655 operates at +5V
supply and its second voltage monitor is 3.3V. The
MAX6656 operates on a +3.3V supply and its second
voltage monitor is 5V.
The 2-wire serial interface accepts standard SMBus™
Write Byte, Read Byte, Send Byte, and Receive Byte
commands to program the alarm thresholds and to read
data. The MAX6655/MAX6656 also provide SMBus alert
response and timeout functions. The MAX6655/MAX6656
measure automatically and autonomously, with the con-
version rate programmable. The adjustable rate allows
the user to control the supply current.
In addition to the SMBus ALERToutput, the MAX6655/
MAX6656 feature an OVERToutput, which is used as a
temperature reset that remains active only while the
temperature is above the maximum temperature limit.
The OVERToutput is optimal for fan control or for sys-
tem shutdown.
FeaturesThree Temperature Channels
Two Remote PN Junctions
One Local SensorFour Voltage Channels
+12V, +5V, +3.3V, +2.5V
Three External Monitors
One Internal Supply Monitor11-Bit, 0.125°C ResolutionHigh Accuracy: ±1°C Over +60°C to +100°C
Temperature RangeProgrammable Under/Over-Threshold AlarmsProgrammable Power-Saving Mode No Calibration RequiredSMBus/I2C-Compatible InterfaceOVERT
Output for Fan Control and System
Shutdown
MAX6655/MAX6656
Dual Remote/Local Temperature Sensors and
Four-Channel Voltage MonitorsVCCSTBY
SMBCLK
OVERT
SMBDATA
ALERT
VIN2
VIN1
VIN3
TOP VIEW
MAX6655
MAX6656
QSOPDXP1
DXN1
DXP2
ADD0
ADD1
DXN2
GND
Pin Configuration
Ordering Information19-2117; Rev 1; 5/06
PARTTEMP RANGEPIN-
PACKAGE
PKG
CODE
MAX6655MEE-55°C to +125°C16 QSOPE16-5
MAX6656MEE-55°C to +125°C16 QSOPE16-5
*P,SMBus is a trademark of Intel Corp.
Typical Application Circuit appears at end of data sheet.Notebooks
Thin Clients
Servers
Workstations
Communication
Equipment
Desktop PC
Applications
MAX6655/MAX6656
Dual Remote/Local Temperature Sensors and
Four-Channel Voltage Monitors
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS(VCC= +3.0V to +3.6V for MAX6656, VCC= +4.5V to +5.5V for MAX6655, TA= -55°C to +125°C, unless otherwise noted. Typical values
are at VCC= +3.3V for MAX6656, VCC= +5.0V for MAX6655, TA= +25°C.)
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.
VCCto GND..............................................................-0.3V to +6V
DXN_ to GND........................................................-0.3V to +0.8V
SMBCLK, SMBDATA, ALERT, STBY,
OVERTto GND.....................................................-0.3V to +6V
VIN1to GND............................................................-0.3V to +16V
VIN2to GND..............................................................-0.3V to +6V
VIN3to GND..............................................................-0.3V to +6V
All Other Pins to GND.................................-0.3V to (VCC+ 0.3V)
SMBDATA, ALERT, OVERTCurrent....................-1mA to +50mA
DXN_ Current......................................................................±1mA
ESD Protection (all pins, Human Body Model)..................2000V
Continuous Power Dissipation (TA= +70°C)
16-Pin QSOP (derate 8.30mW/°C above +70°C)........667mW
Operating Temperature Range.........................-55°C to +125°C
Junction Temperature......................................................+150°C
Storage Temperature Range.............................-65°C to +150°C
Lead Temperature (soldering, 10s).................................+300°C
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITSSupply RangeVCC3.05.5V
+60°C ≤ TA ≤ +100°C±1.5Accuracy (Local Sensor)0°C ≤ TA ≤ +125°C±3°C
+60°C ≤ TRJ ≤ +100°C±1Accuracy (Remote Sensor)0°C ≤ TRJ ≤ +120°C±3°C
0.125°CTemperature Measurement
Resolution11Bits
ADC Input ImpedanceZINVIN1, VIN2, VIN3 input resistance100kΩ
ADC Total ErrorVIN1, VIN2, VIN3 between 30% and 120% of
nominal±1±1.5%
VIN ADC Resolution8Bits
Undervoltage Lockout ThresholdUVLOVCC input, disables A/D conversion,
falling edge2.502.702.90V
Undervoltage Lockout
Hysteresis90mV
Power-On Reset (POR)
ThresholdVCC, falling edge11.72.5V
POR Threshold Hysteresis90mV
Standby CurrentSMBus static, STBY = GND310µA
DXP and DXN Leakage CurrentIn standby mode2µA
Average Operating CurrentContinuous temperature mode5501000µA
Conversion Time for Single
Temperature MeasurementtCONFrom stop bit to conversion completed95125155ms
Monitoring Cycle TimetMONITotal of 3 temperature plus 4 voltage
measurements625ms
MAX6655/MAX6656
Dual Remote/Local Temperature Sensors and
Four-Channel Voltage Monitors
ELECTRICAL CHARACTERISTICS (continued)(VCC= +3.0V to +3.6V for MAX6656, VCC= +4.5V to +5.5V for MAX6655, TA= -55°C to +125°C, unless otherwise noted. Typical values
are at VCC= +3.3V for MAX6656, VCC= +5.0V for MAX6655, TA= +25°C.)
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITSHigh level80100140Remote Junction Current
(DXP, DXN)Low level81014µA
SMBus INTERFACE (SMBCLK, SMBDATA, STBY)Logic Input Low VoltageVILVCC = +3.0V to +5.5V0.8V
VCC = +3.0V2.1Logic Input High VoltageVIHVCC = +5.5V2.6V
Input Leakage CurrentILEAKVIN = GND or VCC±1µA
Output Low Sink CurrentIOLVOL = +0.6V6mA
Input CapacitanceCIN5pF
SMBus TimeoutSMBCLK or SMBDATA time low for reset303560ms
ALERT, OVERT
Output Low Sink CurrentVOL = +0.6V6mA
Output High Leakage CurrentVOH = +5.5V1µA
SMBus TIMINGSerial Clock FrequencyfSCL400kHz
Bus Free Time Between STOP
and START ConditiontBUF4.7µs
START Condition Setup Time4.7µs
Repeat START Condition Setup
TimetSU:STA90% to 90%50ns
START Condition Hold TimetHD:STA10% of SMBDATA to 90% of SMBCLK4µs
STOP Condition Setup TimetSU:STO90% of SMBCLK to 10% of SMBDATA4µs
Clock Low PeriodtLOW10% to 10%4.7µs
Clock High PeriodtHIGH90% to 90%4µs
Data Setup TimetSU:DAT90% of SMBDATA to 10% of SMBCLK250ns
Data Hold TimetHD:DAT(Note 1)0µs
Receive SMBCLK/SMBDATA
Rise TimetR1µs
Receive SMBCLK/SMBDATA
Fall TimetF300ns
Pulse Width of Spike
SuppressedtSP050ns
MAX6655/MAX6656
Dual Remote/Local Temperature Sensors and
Four-Channel Voltage Monitors
Typical Operating Characteristics(TA = +25°C, unless otherwise noted.)
REMOTE TEMPERATURE ERROR
vs. PC BOARD RESISTANCE
MAX6655/MAX6656 toc01
LEAKAGE RESISTANCE (MΩ)
REMOTE TEMPERATURE ERROR (
PATH = DXP TO GND
PATH = DXP TO VCC (5V)
REMOTE TEMPERATURE ERROR
vs. REMOTE-DIODE TEMPERATURE
MAX6655/MAX6656 toc02
TEMPERATURE (°C)
REMOTE TEMPERATURE ERROR (
°C)
RANDOM SAMPLE
2N3906
TEMPERATURE ERROR
vs. POWER-SUPPLY NOISE FREQUENCY
MAX6655/MAX6656 toc03
FREQUENCY (MHz)
TEMPERATURE ERROR (
VIN = SQUARE WAVE
APPLIED TO VCC WITH
NO VCC BYPASS CAPACITOR
VIN = 250mVp-p
REMOTE DIODE1020304050
REMOTE TEMPERATURE ERROR
vs. COMMON-MODE NOISE FREQUENCYMAX6655/MAX6656 toc04
FREQUENCY (MHz)
REMOTE TEMPERATURE ERROR (
°C)
VIN = 200mVp-p
VIN = 100mVp-p
VIN = SQUARE WAVE
AC-COUPLED TO DXN50100150200
REMOTE TEMPERATURE ERROR
vs. DXP-DXN CAPACITANCEMAX6655/MAX6656 toc05
DXP-DXN CAPACITANCE (nF)
REMOTE TEMPERATURE ERROR (VCC = +5V101001000
STANDBY SUPPLY CURRENT
vs. CLOCK FREQUENCYMAX6655/MAX6656 toc06
SMBCLK FREQUENCY (kHz)
SUPPLY CURRENT (
SMBCLK IS DRIVEN
RAIL-TO-RAIL
STANDBY SUPPLY CURRENT
vs. SUPPLY VOLTAGE
MAX6655/MAX6656 toc07
SUPPLY VOLTAGE (V)
SUPPLY CURRENT (
ADD0, ADD1 = GND
ADD0, ADD1 = HIGH-Z
RESPONSE TO THERMAL SHOCK
MAX6655/MAX6656 toc08
TIME (s)
TEMPERATURE (
REMOTE DIODE IMMERSED
IN +115°C FLUORINERT BATH
VOLTAGE ACCURACY
vs. TEMPERATURE
MAX6655/MAX6656 toc09
TEMPERATURE (°C)
OUTPUT VOLTAGE (V)
VIN1
VCC
VIN2
VIN3
INPUT VOLTAGES ARE NOMINAL
Detailed DescriptionThe MAX6655/MAX6656 are voltage and temperature
monitors that communicate through an SMBus-compat-
ible interface with a microprocessor or microcontroller
in thermal management applications.
Essentially an 11-bit serial ADC with a sophisticated front
end, the MAX6655/MAX6656 contain a switched-current
source, a multiplexer, an ADC, an SMBus interface, and
the associated control logic. Temperature data from the
ADC is loaded into a data register, where it is automati-
cally compared with data previously stored in over/under-
temperature alarm threshold registers. Temperature data
can be read at any time with 11 bits of resolution.
The MAX6655/MAX6656 can monitor external supply volt-
ages of typically 12V, 2.5V, and 3.3V for the MAX6655
and 5.0V for the MAX6656, as well as their own supply
voltage. All voltage inputs are converted to an 8-bit code
using an ADC. Each input voltage is scaled down by an
on-chip resistive-divider so that its output, at the nominal
input voltage, is approximately 3/4 of the ADC’s full-scale
range, or a decimal count of 198.
ADCThe averaging ADC integrates over a 40ms period (typ)
with excellent noise rejection. The ADC converts a tem-
perature measurement in 125ms (typ) and a voltage
measurement in 62.5ms (typ). For temperature mea-
surements, the multiplexer automatically steers bias
currents through the remote diode, then the forward
voltage is measured and the temperature is computed.
The DXN input is biased at one diode drop above
ground by an internal diode to set up the ADC inputs for
a differential measurement. The worst-case DXP-DXN
differential input voltage range is +0.25V to +0.95V.
Excess resistance in series with the remote diode caus-
es about +1/2°C error/Ω. A 200µV offset voltage at
DXP-DXN causes about -1°C error.
MAX6655/MAX6656
Dual Remote/Local Temperature Sensors and
Four-Channel Voltage Monitors
Pin Description
PINNAMEFUNCTION1VCCSupply Voltage. +5V for MAX6655; +3.3V for MAX6656. Bypass VCC to GND with a 0.1µF capacitor.DXP1External Diode 1 Positive Connection. DXP1 is the combined current source and ADC positive input
for remote-diode 1. If a remote-sensing junction is not used, connect DXP1 to DXN1.DXN1External Diode 1 Negative Connection. DXN1 is the combined current sink and ADC negative input
for remote-diode 1. DXN1 is normally biased to a diode voltage above ground.ADD0SMBus Slave Address Select Input. ADD0 and ADD1 are sampled upon power-up. Table 5 is the
truth table.ADD1SMBus Slave Address Select Input. ADD0 and ADD1 are sampled upon power-up.DXP2External Diode 2 Positive Connection. DXP2 is the combined current source and ADC positive input
for remote-diode 2. If a remote-sensing junction is not used, connect DXP2 to DXN2.DXN2External Diode 2 Negative Connection. DXN2 is the combined current sink and ADC negative input
for remote-diode 2. DXN2 is normally biased to a diode voltage above ground.GNDGround
9VIN3External Voltage Monitor 3. VIN3 is typically used to monitor +2.5V supplies.VIN1External Voltage Monitor 1. VIN1 is typically used to monitor +12V supplies.VIN2External Voltage Monitor 2. VIN2 is typically used to monitor voltage supplies of +3.3V for MAX6655
and +5.0V for MAX6656.ALERTSMBus Alert (Interrupt) Output, Open-DrainSMBDATASMBus Serial-Data Input/Output, Open-DrainOVERTOvertemperature Alarm Output, Open-Drain. OVERT is an unlatched alarm output that responds to
the programmed maximum temperature limit for all temperature channels.SMBCLKSMBus Serial-Clock InputSTBYHardware Standby Input. Drive STBY low for low-power standby mode. Drive STBY high for normal
operating mode. Temperature and comparison threshold data are retained in standby mode.
MAX6655/MAX6656
ADC Conversion SequenceEach time a conversion begins, all channels are con-
verted, and the results of the measurements are avail-
able after the end of conversion. A BUSY status bit in
the Status Byte shows that the device is actually per-
forming a new conversion; however, even if the ADC is
busy, the results of the previous conversion are always
available. The conversion sequence for the MAX6655
(MAX6656) is External Diode 1, External Diode 2,
Internal Diode, VIN3, VIN2 (VCC), VIN1, VCC (VIN2).
The ADC always converts at maximum speed, but the
time between a sequence of conversions is adjustable.
The Conversion Rate Control Byte (Table 1) shows the
possible delays between conversions. Disabling voltage
or temperature measurements with the Configuration
Byte makes the ADC complete the conversion
sequence faster.
Low-Power Standby Mode Standby mode disables the ADC and reduces the sup-
ply current drain to 3µA (typ). Enter standby mode by
forcing STBYlow or through the RUN/STOP bit in the
Configuration Byte register. Hardware and software
standby modes behave identically; all data is retained
in memory, and the SMBus interface is alive and listen-
ing for reads and writes. Standby mode is not a shut-
down mode. Activity on the SMBus draws extra supply
current (see Typical Operating Characteristics).
Enter hardware standby mode by forcing STBYlow. In
a notebook computer, this line may be connected to
the system SUSTAT# suspend-state signal. The STBY
low state overrides any software conversion command.
If a hardware or software standby command is
received while a conversion is in progress, the conver-
sion cycle is truncated, and the data from that conver-
sion is not latched into the Temperature Reading
register. The previous data is not changed and remains
available.
Supply current during the 125ms conversion is typically
550µA. Between conversions, the instantaneous supply
current is about 25µA, due to the current consumed by
the conversion-rate timer. With very low supply voltages
(under the POR threshold), the supply current is higher
due to the address input bias currents.
Dual Remote/Local Temperature Sensors and
Four-Channel Voltage Monitors
Write Byte Format
Read Byte Format
Send Byte FormatReceive Byte FormatSlave 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)
Slave Address: equiva-
lent 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
Data Byte: writes data to the
register commanded by the
last read byte or write byte
transmission
Data Byte: reads data from
the register commanded
by the last read byte or
write byte transmission;
also used for SMBus alert
response return addressS = Start condition
P = Stop condition
Shaded = Slave transmission
A = Not acknowledged
ACK7 bits
ADDRESSACK8 bits
DATAACKP8 bits
COMMANDWR
ACK7 bits
ADDRESSACKSACK8 bits
DATA7 bits
ADDRESSRD8 bits
SCOMMANDAWR
ACK7 bits
ADDRESS8 bits
COMMANDACKPSWRACK7 bits
ADDRESSRD8 bits
DATAPSAFigure 1. SMBus/I2C Protocols
SMBus Digital InterfaceFrom a software perspective, the MAX6655/MAX6656
appear as a set of byte-wide registers that contain tem-
perature data, voltage data, alarm threshold values,
and control bits. Use a standard SMBus 2-wire serial
interface to read temperature data and write control
bits and alarm threshold data.
The MAX6655/MAX6656 employ four standard SMBus
protocols: Write Byte, Read Byte, Send Byte, and
Receive Byte (Figures 1, 2, and 3). The two shorter pro-
tocols (Receive and Send) allow quicker transfers, pro-
vided that the correct data register was previously
selected by a Write or Read Byte instruction. Use cau-
tion with the shorter protocols in multimaster systems,
since a second master could overwrite the Command
Byte without informing the first master.
MAX6655/MAX6656
Dual Remote/Local Temperature Sensors and
Four-Channel Voltage MonitorsSMBCLKCDEFGHIJK
SMBDATA
tSU:STAtHD:STA
tLOWtHIGH
tSU:DATtHD:DATtSU:STOtBUF
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 M
F = ACKNOWLEDGE BIT CLOCKED INTO MASTER
G = MSB OF DATA CLOCKED INTO SLAVE
H = LSB OF DATA CLOCKED INTO SLAVE
I = MASTER PULLS DATA LINE LOW
J = ACKNOWLEDGE CLOCKED INTO SLAVE
K = ACKNOWLEDGE CLOCK PULSE
L = STOP CONDITION
M = NEW START CONDITION
Figure 2. SMBus/I2C Write Timing Diagram
SMBCLKCDEFGHIJK
SMBDATA
tSU:STAtHD:STA
tLOWtHIGH
tSU:DATtHD:DATtSU:STOtBUFM
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
M = NEW START CONDITION
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
Figure 3. SMBus/I2C Read Timing Diagram
MAX6655/MAX6656The temperature data is stored in internal registers
RRTE, RRT2, and RLTS as 7 bits + sign in two’s com-
plement form with each LSB representing 1°C.
Additionally, the 3MSBs of the Extended Temperature
register contain fractional temperature data with
+0.125°C resolution (Tables 2 and 3). The voltage data
is stored in RV0, RV1, RV2, and RV3 as 8 bits in binary
form (Table 4).
OVERTOutputOVERToutput is an unlatched open-drain output that
behaves as a thermostat for fan control or system shut-
down (Figure 4). This output responds to the current
temperature. If the current temperature is above THIGH,
OVERTactivates and does not go inactive until the tem-
perature drops below THIGH.
Diode Fault AlarmA continuity fault detector at DXP detects whether the
remote diode has an open-circuit condition, short-cir-
cuit to GND, or short-circuit DXP-to-DXN condition. At
the beginning of each conversion, the diode fault is
checked, and the Status Byte is updated. This fault
detector is a simple voltage detector; if DXP rises
above VCC- 1V (typ) or below VDXN+ 50mV (typ), a
fault is detected. Note that the diode fault isn’t checked
until a conversion is initiated, so immediately after POR,
the status byte indicates no fault is present, even if the
diode path is broken.
If the remote channel is shorted (DXP to DXN or DXP to
GND), the ADC reads 1111 1111 so as not to trip either
the THIGHor TLOWalarms at their POR settings.
Similarly, if DXP_ is short circuited to VCC, the ADC
reads -1°C for both remote channels, and the ALERT
outputs are activated.
AlertInterrupts Normally, the ALERTinterrupt output signal is latched
and can be cleared either by responding to the Alert
Response Address or by reading the Status register.
Interrupts are generated in response to THIGHand
TLOW, VHIGHand VLOWcomparisons, and when the
remote diode is faulted. The interrupt does not halt auto-
matic conversions; new temperature data continues to
be available over the SMBus interface after ALERTis
asserted. The interrupt output pin is open-drain so multi-
ple devices can share a common interrupt line.
The interface responds to the SMBus Alert Response
address, an interrupt pointer return-address feature
(see the Alert Response Address section). Before tak-
ing corrective action, always check to ensure that an
interrupt is valid by reading the current temperature.
The alert activates only once per crossing of a given
temperature threshold to prevent any reentrant inter-
rupts. To enable a new interrupt, rewrite the value of the
violated temperature threshold.
Alert Response AddressThe SMBus Alert Response interrupt pointer provides
quick fault identification for simple slave devices that lack
the complex, expensive logic needed to be a bus master.
Upon receiving an ALERTinterrupt signal, the host mas-
ter can broadcast a Receive Byte transmission to the
Alert Response slave address (0001100). Any slave
device that generated an interrupt then attempts to identi-
fy itself by putting its own address on the bus (Table 5).
The Alert Response can activate several different slave
devices simultaneously, similar to the I2C General Call.
If more than one slave attempts to respond, bus arbitra-
tion rules apply, and the device with the lower address
code wins. The losing device does not generate an
acknowledgment and continues to hold the ALERTline
low until serviced (implies that the host interrupt input is
level sensitive). The alert is cleared after the slave
address is returned to the host.
Command Byte FunctionsThe 8-bit Command Byte register (Table 6) is the mas-
ter index that points to the other registers within the
MAX6655/MAX6656. The register’s POR state is 0000
0000, so a Receive Byte transmission (a protocol that
lacks the Command Byte) that occurs immediately after
POR returns the current internal temperature data.
Dual Remote/Local Temperature Sensors and
Four-Channel Voltage MonitorsVCC
OVERT
MAX6655
MAX6656
SMBDATA
SMBCLK
ADD0
ADD1
GND
DXP2
DXN2
2200pF2N3906
TO SYSTEM
SHUTDOWN
SMBus
SERIAL
INTERFACE
(TO HOST)
+3V TO +5.5V
ALERT
Figure 4. System Shutdown Application
Alarm Threshold RegistersSeventeen registers store ALARMand OVERTthresh-
old data. The MAX6655/MAX6656 contain three regis-
ters for high-temperature (THIGH), three for low-
temperature (TLOW), four for high-voltage (VHIGH), four
for low-voltage (VLOW) thresholds, and three more reg-
isters store OVERTdata. If a measured temperature or
voltage exceeds the corresponding alarm threshold
value, an ALARMinterrupt is asserted. OVERTasserts
when temperature exceeds the corresponding alarm
threshold value. The POR state of the THIGHregister is
full scale (0111 1111 or +127°C). The POR state of the
TLOWregister is 1100 1001 or -55°C.
Configuration Byte FunctionsConfiguration Bytes 1 and 2 (Tables 7 and 8) are used
to mask (disable) interrupts, disable temperature and
voltage measurements, and put the device in software
standby mode. The serial interface can read back the
contents of these registers.
Status Byte FunctionsThe two Status Byte registers (Tables 9 and 10) indi-
cate which (if any) temperature or voltage thresholds
have been exceeded. Status Byte 1 also indicates
whether the ADC is converting and whether there is a
fault in the remote-diode DXP-DXN path. After POR, the
normal state of all the flag bits is zero, except the MSB,
assuming none of the alarm conditions are present. The
MSB toggles between 1 and 0 indicating whether the
ADC is converting or not. A Status Byte is cleared by
any successful read of that Status Byte. Note that the
ALERTinterrupt latch clears when the status flag bit is
read, but immediately asserts after the next conversion
if the fault condition persists.
High and low alarm conditions can exist at the same time
in the Status Byte because the MAX6655/MAX6656 are
correctly reporting environmental changes.
Applications Information
Remote-Diode SelectionRemote temperature accuracy depends on having a
good-quality, diode-connected transistor. See Table 11
for appropriate discrete transistors. The MAX6655/
MAX6656 can directly measure the die temperature of
CPUs and other ICs with on-board temperature-sensing
transistors.
The transistor must be a small-signal type with a rela-
tively high forward voltage. This ensures that the input
voltage is within the ADC 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 controls and that the devices
have consistent VBEcharacteristics. Do not use power
transistors.
Self-HeatingThermal mass can significantly affect the time required
for a temperature sensor to respond to a sudden
change in temperature. The thermal time constant of
the 16-pin QSOP package is about 140s in still air.
When measuring local temperature, it senses the tem-
perature of the PC board to which it is soldered. The
leads provide a good thermal path between the PC
board traces and the MAX6655/MAX6656 die. Thermal
conductivity between the MAX6655/MAX6656 die and
the ambient air is poor by comparison. Because the
thermal mass of the PC board is far greater than that of
the MAX6655/MAX6656, the device follows temperature
changes on the PC board with little or no perceivable
delay.
When measuring temperature with discrete remote sen-
sors, the use of smaller packages, such as a SOT23,
yields the best thermal response time. Take care to
account for thermal gradients between the heat source
and the sensor, and ensure that stray air currents
across the sensor package do not interfere with mea-
surement accuracy. When measuring the temperature
of a CPU or other IC with an on-chip sense junction,
thermal mass has virtually no effect; the measured tem-
perature of the junction tracks the actual temperature
within a conversion cycle.
Self-heating does not significantly affect measurement
accuracy. Remote-sensor self-heating due to the diode
current source is negligible. For the local diode, the
worst-case error occurs when autoconverting at the
fastest rate and simultaneously sinking maximum cur-
rent at the ALERT output. For example, at the minimum
delay between conversions, and with ALERT sinking
1mA, the typical power dissipation is VCCx 550µA +
0.4V x 1mA. Package θJAis about 150°C/W, so with
VCC= +5V and no copper PC board heat sinking, the
resulting temperature rise is:
∆T = 3.1mW x 150°C/W = +0.46°C
Even with these contrived circumstances, it is difficult
to introduce significant self-heating errors.
ADC Noise FilteringThe integrating ADC has inherently good noise rejec-
tion, especially of low-frequency signals such as
60Hz/120Hz power-supply hum. Micropower operation
MAX6655/MAX6656
Dual Remote/Local Temperature Sensors and
Four-Channel Voltage Monitors
