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max6642 1c, smbus-compatible remote/ local temperature sensor with overtemperature alarm evaluation kit available general description the max6642 precise, two-channel digital temperaturesensor accurately measures the temperature of its own die and a remote pn junction and reports the tempera- ture data over a 2-wire serial interface. the remote pn junction is typically a substrate pnp transistor on the die of a cpu, asic, gpu, or fpga. the remote pn junction can also be a discrete diode-connected small- signal transistor. the 2-wire serial interface accepts standard system management bus (smbus), write byte, read byte, send byte, and receive byte commands to read the temperature data and to program the alarm thresholds. to enhance system reliability, the max6642 includes an smbus timeout. the temperature data format is 10 bit with the least significant bit (lsb) corresponding to +0.25?. the alert output asserts when the local or remote overtemperature thresholds are violated. a faultqueue may be used to prevent the alert output from setting until two consecutive faults have been detected.measurements can be done autonomously or in a sin- gle-shot mode. remote accuracy is ?? maximum error between +60? and +100?. the max6642 operates from -40? to +125?, and measures remote temperatures between 0? and +150?. the max6642 is available in a 6-pin tdfn package with an exposed pad. applications desktop computersnotebook computers servers thin clients test and measurement workstations graphic cards benefits and features ? integrated temperature sensor enables simultaneous dual temperature (remote and local) measurements ? remote accuracy 1c ? local accuracy 2c from +60c to +100c ? measures remote temperature up to +150c ? 0.25c resolution ? dual zone monitoring automates over-temperature alarms ? programmable remote/local temperature thresholds ? alert output ? small footprint ? 3mm x 3mm tdfn package with exposed pad ? low thermal mass reduces measurement latency ? smbus/i 2 c address hardwired ordering information part temp range pin-package MAX6642ATT90-t -40? to +125? 6 tdfn-ep* max6642att92-t -40? to +125? 6 tdfn-ep* max6642att94-t -40? to +125? 6 tdfn-ep* max6642att96-t -40? to +125? 6 tdfn-ep* max6642att98-t -40? to +125? 6 tdfn-ep* max6642att9a-t -40? to +125? 6 tdfn-ep* max6642att9c-t -40? to +125? 6 tdfn-ep* max6642att9e-t -40? to +125? 6 tdfn-ep* max6642 2200pf 0.1 f p dxp gnd sda sclk alert dataclock interrupt to p 47 10k each 3.3v v cc typical operating circuit part measured temp range top mark MAX6642ATT90-t 0? to +150? afc max6642att92-t 0? to +150? afd max6642att94-t 0? to +150? afe max6642att96-t 0? to +150? aff max6642att98-t 0? to +150? aew max6642att9a-t 0? to +150? afg max6642att9c-t 0? to +150? afh max6642att9e-t 0? to +150? afi selector guide t = tape and reel. * ep = exposed pad. pin configuration and functional diagram appear at end of data sheet. smbus is a trademark of intel corp. 19-2920; rev 5; 11/15 downloaded from: http:///
max6642 1c, smbus-compatible remote/ local temperature sensor with overtemperature alarm maxim integrated | 2 www.maximintegrated.com absolute maximum ratings electrical characteristics (v cc = +3.0v to +5.5v, t a = -40? to +125?, unless otherwise specified. typical values are at v cc = +3.3v and t a = +25 c.) (note 1) stresses beyond those listed under ?bsolute 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. all voltages referenced to gndv cc ...........................................................................-0.3v to +6v dxp.............................................................-0.3v to (v cc + 0.3v) sclk, sda, alert ..................................................-0.3v to +6v sda, alert current ...........................................-1ma to +50ma continuous power dissipation (t a = +70?) 6-pin tdfn (derate 24.4mw/? above +70?) .........1951mw esd protection (all pins, human body model) ................?000v junction temperature ......................................................+150? operating temperature range .........................-40? to +125? storage temperature range .............................-65? to +150? lead temperature (soldering, 10s) .................................+300? parameter symbol conditions min typ max units supply voltage v cc 3.0 5.5 v 0.25 ? temperature resolution 10 bits t rj = +60? to +100?, t a = +25 c to +85 c -1.0 +1.0 t rj = 0? to +125? -3.0 +3.0 remote temperature error v cc = 3.3v t rj = +125? to +150? -3.5 +3.5 ? t a = +60? to +100? -2.0 +2.0 local temperature error v cc = 3.3v t a = 0? to +125? -3.0 +3.0 ? supply sensitivity of temperatureerror ?.2 ?/v undervoltage lockout threshold uvlo falling edge of v cc disables adc 2.4 2.7 2.95 v undervoltage lockout hysteresis 90 mv power-on-reset (por) threshold v cc falling edge 1.5 2.0 2.4 v por threshold hysteresis 90 mv standby supply current smbus static 3 10 a operating current during conversion 0.5 1.0 ma average operating current 260 ? conversion time t conv from stop bit to conversion completion 106 125 143 ms conversion rate f conv 8h z high level 80 100 120 remote-diode source current i rj low level 8 10 12 ? alert v ol = 0.4v 1 output-low sink current v ol = 0.6v 4 ma output-high leakage current v oh = v cc 1 a downloaded from: http:///
max6642 1c, smbus-compatible remote/ local temperature sensor with overtemperature alarm maxim integrated | 3 www.maximintegrated.com note 1: all parameters tested at t a = +25?. specifications over temperature are guaranteed by design. note 2: timing specifications guaranteed by design. note 3: the serial interface resets when sclk is low for more than t timeout . note 4: a transition must internally provide at least a hold time to bridge the undefined region (300ns max) of sclk? falling edge. electrical characteristics (continued) (v cc = +3.0v to +5.5v, t a = -40? to +125?, unless otherwise specified. typical values are at v cc = +3.3v and t a = +25 c.) (note 1) parameter symbol conditions min typ max units smbus-compatible interface (sclk and sda) logic input low voltage v il 0.8 v logic input high voltage v ih v cc = 3.0v 2.2 v input leakage current i leak v in = gnd or 5.5v -1 +1 ? output low sink current i ol v ol = 0.6v 6 ma input capacitance c in 5p f smbus timing (note 2) serial clock frequency f sclk (note 3) 100 khz bus free time between stopand start condition t buf 4.7 ? start condition setup time 4.7 ? repeat start condition setuptime t su:sta 90% to 90% 50 ns start condition hold time t hd:sta 10% of sda to 90% of sclk 4 s stop condition setup time t su:sto 90% of sclk to 90% of sda 4 s clock low period t low 10% to 10% 4.7 ? clock high period t high 90% to 90% 4 s data setup time t hd:dat (note 4) 250 ? receive sclk/sda rise time t r 1 s receive sclk/sda fall time t f 300 ns pulse width of spike suppressed t sp 05 0 n s smbus timeout t timeout sda low period for interface reset 20 28 40 ms downloaded from: http:///
max6642 1c, smbus-compatible remote/ local temperature sensor with overtemperature alarm maxim integrated | 4 www.maximintegrated.com 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 0.01 0.1 1 10 100 standby supply current vs. clock frequency max6642 toc01 clock frequency (khz) supply current ( a) -4 -2-3 0 -1 1 2 remote temperature error vs. remote-diode temperature max6642 toc02 temperature ( c) temperature error ( c) 05 0 7 5 25 100 125 2n3906 -3 -1-2 10 2 3 0 125 local temperature error vs. die temperature max 6642 toc03 temperature ( c) temperature error ( c) 50 25 75 100 -1.5 -0.5-1.0 0.5 0 1.51.0 2.0 0.0001 0.01 0.1 0.001 1 10 100 temperature error vs. power-supply noise frequency max6642 toc04 frequency (khz) temperature error ( c) v in = 100mv p-p square wave applied to v cc with no bypass capacitor local error remote error 0 3020 10 40 50 60 70 80 90 100 0.001 0.1 0.01 1 10 100 temperature error vs. dxp noise frequency max6642 toc05 frequency (khz) temperature error ( c) local error remote error v in = ac-coupled to dxp v in = 100mv p-p square wave typical operating characteristics (v cc = 3.3v, t a = +25?, unless otherwise noted.) -6.0 -5.0 -4.0 -3.0 -2.0 -1.0 0 1.0 2.0 0.1 1 10 100 temperature error vs. dxp-gnd capacitance max6642 toc06 dxp-gnd capacitance (nf) temperature error ( c) downloaded from: http:///
max6642 1c, smbus-compatible remote/ local temperature sensor with overtemperature alarm maxim integrated | 5 www.maximintegrated.com detailed description the max6642 is a temperature sensor for localand remote temperature-monitoring applications. communication with the max6642 occurs through the smbus-compatible serial interface and dedicated alert pins. alert asserts if the measured local or remote temperature is greater than the software-programmedalert limit. the max6642 converts temperatures to digital dataeither at a programmed rate of eight conversions per second or in single conversions. temperature data is represented by 8 data bits (at addresses 00h and 01h), with the lsb equal to +1? and the msb equal to +128?. two additional bits of remote temperature data are available in the ?xtended?register at address 10h and 11h (table 2) providing resolution of +0.25 c. adc and multiplexer the averaging adc integrates over a 60ms period (each channel, typ), with excellent noise rejection. the multiplexer automatically steers bias currents through the remote and local diodes. the adc and associated circuitry measure each diode? forward volt- age and compute the temperature based on this volt- age. both channels are automatically converted once the conversion process has started, either in free-run- ning or single-shot mode. if one of the two channels is not used, the device still performs both measurements, and the user can ignore the results of the unused chan- nel. if the remote-diode channel is unused, connect dxp to gnd rather than leaving dxp open. the conversion time per channel (remote and internal) is 125ms. if both channels are being used, then each channel is converted four times per second. if the external conversion-only option is selected, then the remote temperature is measured eight times per sec-ond. the results of the previous conversion are always available, even if the adc is busy. low-power standby mode standby mode reduces the supply current to less than 10? by disabling the adc and timing circuitry. enter standby mode by setting the run bit to 1 in the config- uration byte register (table 4). all data is retained inmemory, and the smbus interface is active and listen- ing for smbus commands. standby mode is not a shut- down mode. with activity on the smbus, the device draws more supply current (see the typical operating characteristics ). in standby mode, the max6642 can be forced to perform adc conversions through theone-shot command, regardless of the run bit status. if a standby command is received while a conversion isin progress, the conversion cycle is truncated, and the data from that conversion is not latched into a tempera- ture register. the previous data is not changed and remains available. supply-current drain during the 125ms conversion peri- od is 500? (typ). in standby mode, supply current drops to 3? (typ). smbus digital interface from a software perspective, the max6642 appears as a set of byte-wide registers that contain temperature data, alarm threshold values, and control bits. a stan- dard smbus-compatible 2-wire serial interface is used to read temperature data and write control bits and alarm threshold data. the max6642 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 pin description pin name function 1v cc supply voltage input, +3v to +5.5v. bypass v cc to gnd with a 0.1? capacitor. a 47 series resistor is recommended but not required for additional noise filtering. 2 gnd ground 3 dxp combined remote-diode current source and adc input for remote-diode channel. place a 2200pf capacitor between dxp and gnd for noise filtering. 4 sclk smbus serial-clock input. may be pulled up to +5.5v regardless of v cc . 5 sda smbus serial-data input/output, open drain. may be pulled up to +5.5v regardless of v cc . 6 alert smbus alert (interrupt) output, open drain. alert asserts when temperature exceeds user-set limits. see the alert interrupts section. ? p exposed pad. internally connected to gnd. connect to a pcb ground pad for optimal performance. notintended as an electrical connection point. downloaded from: http:///
max6642 1c, smbus-compatible remote/ local temperature sensor with overtemperature alarm maxim integrated | 6 www.maximintegrated.com s address wr ack ack p data ack command 7 bits 1 8 bits 8 bits slave address: equiva-lent to chip-select line of a 3-wire interface data byte: data goes into the registerset by the command byte (to set thresholds, configuration masks, and sampling rate) write byte format s address address wr ack ack p s rd ack /// data command 7 bits 7 bits 8 bits 8 bits read byte format slave address: equiva-lent to chip select line command byte: selectswhich register you are reding from sp address wr ack ack command 7 bits 8 bits send byte format command byte: sends com-mand with no data, usually used for one-shot command sp address rd ack /// data 7 bits 8 bits receive byte format data byte: reads data fromthe register commanded by the last read byte or write byte transmission; also used for smbus alert response return address slave address: repeateddue to change in data- flow direction data byte: reads fromthe register set by the command byte s = start conditionp = stop condition shaded = slave transmission/// = not acknowledged figure 1. smbus protocols smbclk a = start conditionb = msb of address clocked into slave c = lsb of address clocked into slave d = r/w bit clocked into slave ab cd e fg hi j smbdata t su:sta t hd:sta t low t high t su:dat t su:sto t buf lm k e = slave pulls smbdata line lowf = acknowledge bit clocked into master g = msb of data clocked into slave h = lsb of data clocked into slave i = slave pulls data line lowj = acknowledge clocked into master k = acknowledge clock pulse l = stop condition m = new start condition figure 2. smbus write timing diagram downloaded from: http:///
max6642 1c, smbus-compatible remote/ local temperature sensor with overtemperature alarm maxim integrated | 7 www.maximintegrated.com register was previously selected by a write byteinstruction. use caution when using the shorter proto- cols in multimaster systems, as a second master could overwrite the command byte without informing the first master. read temperature data from the read internal tempera- ture (00h) and read external temperature (01h) regis- ters. the temperature data format for these registers is 8 bits for each channel, with the lsb representing +1? (table 1). read the additional bits from the read extended tem- perature byte register (10h, 11h), which extends the data to 10 bits and the resolution to +0.25 c per lsb (table 2).when a conversion is complete, the main temperature register and the extended temperature register are updated. alarm threshold registers two registers store alert threshold values?ne each for the local and remote channels. if either measuredtemperature equals or exceeds the corresponding alert threshold value, the alert interrupt asserts unless the alert bit is masked. the power-on-reset (por) state of the local alert t high register is +70 c (0100 0110). the por state of the remote alert t high register is +120 c (0111 1000). diode fault detection a continuity fault detector at dxp detects an open cir- cuit on dxp, or a dxp short to v cc or gnd. if an open or short circuit exists, the external temperature registeris loaded with 1111 1111 and status bit 2 (open) of the status byte is set to 1. immediately after por, the status register indicates that no fault is present. if a fault is present upon power-up, the fault is not indicated untilthe end of the first conversion. diode faults do not set the alert output. alert interrupts the alert interrupt occurs when the internal or external temperature reading exceeds a high temperature limit(user programmed). the alert interrupt output signal is latched and can be cleared only by reading the statusregister after the fault condition no longer exists or by successfully responding to the alert response address. if smbclk ab cd e fg h i smbdata t su:sta t hd:sta t low t high t su:dat t hd:dat t su:sto t buf a = start conditionb = 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 j k f = acknowledge bit clocked into masterg = msb of data clocked into master h = lsb of data clocked into master i = acknowledge clock pulsej = stop condition k = new start condition figure 3. smbus read timing diagram temp ( c) digital output 130.00 1 000 0010 127.00 0 111 1111 126.00 0 111 1110 25 0 001 1001 0.00 0 000 0000 <0.00 0 000 0000 diode fault (short or open) 1 111 1111 table 1. main temperature register (high byte) data format fractional temp (?) digital output 0.000 00xx xxxx 0.250 01xx xxxx 0.500 10xx xxxx 0.750 11xx xxxx table 2. extended resolution temperature register (low byte) data format downloaded from: http:///
max6642 1c, smbus-compatible remote/ local temperature sensor with overtemperature alarm maxim integrated | 8 www.maximintegrated.com the alert is cleared by responding to the alert response address and the temperature fault conditionstill exists, alert is reasserted after the next tempera- ture-monitoring cycle. to clear alert while the tempera- ture is above the trip threshold, write a new high limit thatis higher than the current temperature. the alert out- put is open drain, allowing multiple devices to share acommon interrupt line. alert response address the smbus alert response interrupt pointer provides quick fault identification for simple slave devices like temperature sensors. upon receiving an alert inter- rupt signal, the host master can broadcast a receivebyte transmission to the alert response slave address (0001 100). following such a broadcast, any slave device that generated an interrupt attempts to identify itself by putting its own address on the bus. the alert response can activate several different slave devices simultaneously, similar to the i 2 c general call. if more than one slave attempts to respond, bus arbitra- a d d r ess po r st a t ef u n c t io n 00h 00h ( 0000 0000) read l ocal tem p er atur e 01h 00h ( 0000 0000) read r em ote tem p er atur e 02h n /a read status b yte 03h 10h ( 0001 0000) read confi g ur ati on b yte 05h 46h ( 0100 0110) + 70 c read l ocal hi g h l i m i t 07h 78h ( 0111 1000) + 120 c read r em ote hi g h l i m i t 09h n /a w r i te confi g ur ati on b yte 0bh n /a w r i te l ocal hi g h l i m i t 0d hn /a w r i te r em ote hi g h l i m i t 0fh n /a s i ng l e shot 10h 0000 0000 read r em ote extend ed tem p er atur e 11h 0000 0000 read i nter nal extend ed tem p er atur e fe h4 d h ( 0100 1101) read m anufactur er id table 3. command-byte assignments bit name por state function 7 (msb) mask1 0 a 1 masks off (disables) the alert interrupts. 6 stop/run 0 a 1 puts the max6642 into standby mode. 5 external only 0 a 1 disables local temperature measurements so that onlyremote temperature is measured. the measurement rate becomes 8hz. 4 fault queue 1 0: alert is set by a single fault. 1: two consecutive faults are required to set alert . 3 to 0 0000 reserved. table 4. configuration-byte bit assignments bit name por state function 7 (msb) busy 0 a 1 indicates the max6642 is busy converting. 6 lhigh 0 a 1 indicates an internal high-temperature fault. clearlhigh with a por or by reading the status byte. 5 0 reserved. 4 rhigh 0 a 1 indicates an external high-temperature fault. clearrhigh with a por or by reading the status byte. 3 0 reserved. 2 open 0 a 1 indicates a diode open condition. clear open with apor or by reading the status byte when the condition no longer exists. 1 to 0 0 reserved. table 5. status-byte bit assignments downloaded from: http:///
max6642 1c, smbus-compatible remote/ local temperature sensor with overtemperature alarm maxim integrated | 9 www.maximintegrated.com tion rules apply, and the device with the lower addresscode wins. the losing device does not generate an acknowledge and continues to hold the alert line low until cleared. (the conditions for clearing an alert vary depending on the type of slave device.)successful completion of the alert response protocol clears the interrupt latch. if the condition still exists, the device reasserts the alert interrupt at the end of the next conversion. command byte functions the 8-bit command byte register (table 3) is the master index that points to the various other registers within the max6642. the register? 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 local temperature data. single-shot the single-shot command immediately forces a new conversion cycle to begin. if the single-shot command is received while the max6642 is in standby mode ( run bit = 1), a new conversion begins, after which the device returns to standby mode. if a single-shot con-version is in progress when a single-shot command is received, the command is ignored. if a single-shot command is received in autonomous mode ( run bit = 0), the command is ignored. configuration byte functions the configuration byte register (table 4) is a read-write register with several functions. bit 7 is used to mask (disable) interrupts. bit 6 puts the max6642 into stand- by mode (stop) or autonomous (run) mode. bit 5 dis- ables local temperature conversions for faster (8hz) remote temperature monitoring. bit 4 prevents setting the alert output until two consecutive measurements result in fault conditions. status byte functions the status byte register (table 5) indicates which (if any) temperature thresholds have been exceeded. this byte also indicates whether the adc is converting and whether there is an open-circuit fault detected on the external sense junction. after por, the normal state of all flag bits is zero, assuming no alarm conditions are present. the status byte is cleared by any successful read of the status byte after the overtemperature fault condition no longer exists. slave addresses the max6642 has eight fixed addresses available. these are shown in table 6. the max6642 also responds to the smbus alert response slave address (see the alert response address section). por and uvlo to prevent ambiguous power-supply conditions from corrupting the data in memory and causing erratic behavior, a por voltage detector monitors v cc and clears the memory if v cc falls below 2.1 (typ). when power is first applied and v cc rises above 2.1 (typ), the logic blocks begin operating, although reads andwrites at v cc levels below 3v are not recommended. a second v cc comparator, the adc undervoltage lockout (uvlo) comparator prevents the adc from convertinguntil there is sufficient headroom (v cc = +2.7v typ). power-up defaults power-up defaults include: alert output is cleared. adc begins autoconverting at a 4hz rate. command byte is set to 00h to facilitate quick local receive byte queries. local (internal) t high limit set to +70?. remote (external) t high limit set to +120?. applications information remote-diode selection the max6642 can directly measure the die temperature of cpus and other ics that have on-board temperature- sensing diodes (see the typical operating circuit ) or they can measure the temperature of a discrete diode-connected transistor. effect of ideality factor the accuracy of the remote temperature measurements depends on the ideality factor (n) of the remote ?iode (actually a transistor). the max6642 is optimized for n = 1.008, which is the typical value for the intel pentium part no. suffix address MAX6642ATT90 1001 000 max6642att92 1001 001 max6642att94 1001 010 max6642att96 1001 011 max6642att98 1001 100 max6642att9a 1001 101 max6642att9c 1001 110 max6642att9e 1001 111 table 6. slave address downloaded from: http:///
max6642 1c, smbus-compatible remote/ local temperature sensor with overtemperature alarm maxim integrated | 10 www.maximintegrated.com iii. a thermal diode on the substrate of an ic is normallya pnp with its collector grounded. connect the anode (emitter) to dxp and the cathode to gnd of the max6642. if a sense transistor with an ideality factor other than 1.008 is used, the output data is different from the data obtained with the optimum ideality factor. fortunately, the difference is predictable. assume a remote-diode sensor designed for a nominal ideality factor n nominal is used to measure the tem- perature of a diode with a different ideality factor n 1 . the measured temperature t m can be corrected using: where temperature is measured in kelvin andn nomimal for the max6642 is 1.008. as an example, assume you want to use the max6642with 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: for a real temperature of +85? (358.15k), the mea- sured temperature is +82.91? (356.02k), an error of -2.13?. effect of series resistance series resistance in a sense diode contributes addition- al errors. for nominal diode currents of 10? and 100?, the change in the measured voltage due to series resistance is: ? v m = r s (100? - 10?) = 90? ? r s since +1? corresponds to 198.6?, series resistancecontributes a temperature offset of: assume that the diode being measured has a series resistance of 3 ? . the series resistance contributes an offset of: the effects of the ideality factor and series resistanceare additive. if the diode has an ideality factor of 1.002 and series resistance of 3 ? , the total offset can be cal- culated by adding error due to series resistance witherror due to ideality factor: 1.36? - 2.13? = -0.77? for a diode temperature of +85?.in this example, the effect of the series resistance and the ideality factor partially cancel each other. discrete remote diodes when the remote-sensing diode is a discrete transistor, connect its collector and base together. table 7 lists examples of discrete transistors that are appropriate for use with the max6642. the transistor must be a small-signal type with a rela- tively high forward voltage; otherwise, the a/d input voltage range can be violated. the forward voltage at the highest expected temperature must be greater than 0.25v at 10?, and at the lowest expected tempera- ture, the forward voltage must be less than 0.95v at 100?. large power transistors must not be used. also, ensure that the base resistance is less than 100 ? . tight specifications for forward current gain (50 < ?<150, forexample) indicate that the manufacturer has good process controls and that the devices have consistent v be characteristics. manufacturers of discrete transistors do not normallyspecify or guarantee ideality factor. this is normally not a problem since good-quality discrete transistors tend to have ideality factors that fall within a relatively narrow 3 0 453 1 36 ? ? =+ .. c c 90 198 6 0 453 ? = ? v v c c . . tt n n t t actual m nominal m m = ? ? ? ? ? ? = ? ? ? ? ? ? = 1 1 008 1 002 1 00599 .. ( . ) tt n n m actual nominal = ? ? ? ? ? ? 1 manufacturer model no. central semiconductor (usa) cmpt3906 rohm semiconductor (usa) sst3906 samsung (korea) kst3906-tf siemens (germany) smbt3906 zetex (england) fmmt3906ct-nd table 7. remote-sensor transistor manufacturers note: discrete transistors must be diode connected (base short- ed to collector). downloaded from: http:///
max6642 1c, smbus-compatible remote/ local temperature sensor with overtemperature alarm maxim integrated | 11 www.maximintegrated.com range. we have observed variations in remote tempera-ture readings of less than 2 c with a variety of dis- crete transistors. still, it is good design practice toverify good consistency of temperature readings with several discrete transistors from any manufacturer under consideration. adc noise filtering the integrating adc used has good noise rejection for low-frequency signals such as 60hz/120hz power-sup- ply hum. in noisy environments, high-frequency noise reduction is needed for high-accuracy remote mea- surements. the noise can be reduced with careful pcb layout and proper external noise filtering. high-frequency emi is best filtered at dxp with an external 2200pf capacitor. larger capacitor values can be used for added filtering, but do not exceed 3300pf because excessive capacitance can introduce errors due to the rise time of the switched current source. nearly all noise sources tested cause the temperature conversion results to be higher than the actual temper- ature, typically by +1? to +10?, depending on the frequency and amplitude (see the typical operating characteristics ). pcb layout follow these guidelines to reduce the measurement error of the temperature sensors: 1) connect the thermal-sense diode to the max6642 using two traces?ne between dxp and theanode, the other between the max6642? gnd and the cathode. do not connect the cathode to gnd at the sense diode. 2) place the max6642 as close as is practical to the remote thermal diode. in noisy environments, suchas a computer motherboard, this distance can be 4in to 8in (typ). this length can be increased if the worst noise sources are avoided. noise sources include crts, clock generators, memory buses, and isa/pci buses. 3) do not route the thermal diode lines next to the deflection coils of a crt. also, do not route thetraces across fast digital signals, which can easily introduce a 30? error, even with good filtering. 4) route the thermal diode traces in parallel and in close proximity to each other, away from any highervoltage traces, such as +12vdc. leakage currents from pcb contamination must be dealt with careful- ly since a 20m ? leakage path from dxp to ground causes about +1? error. if high-voltage traces areunavoidable, connect guard traces to gnd on either side of the dxp trace (figure 4). 5) route through as few vias and crossunders as pos- sible to minimize copper/solder thermocoupleeffects. 6) when introducing a thermocouple, make sure that both the thermal diode paths have matching ther-mocouples. a copper-solder thermocouple exhibits 3?/?, and it takes about 200? of voltage error at dxp to cause a +1? measurement error. adding a few thermocouples causes a negligible error. 7) use wide traces. narrow traces are more inductive and tend to pick up radiated noise. the 10-milwidths and spacing recommended in figure 4 are not absolutely necessary, as they offer only a minor improvement in leakage and noise over narrow traces. use wider traces when practical. 8) add a 47 ? resistor in series with v cc for best noise filtering (see the typical operating circuit ). 9) copper cannot be used as an emi shield; only fer- rous materials such as steel work well. placing acopper ground plane between the dxp-dxn traces and traces carrying high-frequency noise signals does not help reduce emi. twisted-pair and shielded cables use a twisted-pair cable to connect the remote sensor for remote-sensor distances longer than 8in or in very noisy environments. twisted-pair cable lengths can be between 6ft and 12ft before noise introduces excessive errors. for longer distances, the best solution is a shielded twisted pair like that used for audio micro- phones. for example, belden #8451 works well for dis- tances up to 100ft in a noisy environment. at the device, connect the twisted pair to dxp and gnd and the shield to gnd. leave the shield unconnected at the remote diode. for very long cable runs, the cable? parasitic capaci- tance often provides noise filtering, so the 2200pf capacitor can often be removed or reduced in value. minimum 10 mils10 mils 10 mils 10 mils thermal diode cathode/gnd thermal diode anode/dxp gnd gnd figure 4. recommended dxp pc traces downloaded from: http:///
max6642 1c, smbus-compatible remote/ local temperature sensor with overtemperature alarm maxim integrated | 12 www.maximintegrated.com cable resistance also affects remote-sensor accuracy.for every 1 ? of series resistance, the error is approxi- mately 1/2?. thermal mass and self-heating when sensing local temperature, this device is intend- ed to measure the temperature of the pcb to which it is soldered. the leads provide a good thermal path between the pcb traces and the die. thermal conduc- tivity between the die and the ambient air is poor by comparison, making air temperature measurements impractical. because the thermal mass of the pcb is far greater than that of the max6642, the device follows temperature changes on the pcb with little or no per- ceivable delay. when measuring temperature of a cpu or other ic with an on-chip sense junction, thermal mass has virtually no effect; the measured temperature of the junction tracks the actual temperature within a conversion cycle. when measuring temperature with discrete remote sen- sors, smaller packages, such as sot23s, yield the best thermal response times. take care to account for ther- mal gradients between the heat source and the sensor, and ensure that stray air currents across the sensor package do not interfere with measurement accuracy. self-heating does not significantly affect measurementaccuracy. 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, with v cc = +5.0v, at an 8hz conversion rate and with alert sink- ing 1ma, the typical power dissipation is: 5.0v x 450? + 0.4v x 1ma = 2.65mw j-a for the 6-pin tdfn package is about +41?/w, so assuming no copper pcb heat sinking, the resultingtemperature rise is: ? t = 2.65mw x 41?/w = +0.11? even under nearly worst-case conditions, it is difficult tointroduce a significant self-heating error. downloaded from: http:///
max6642 1c, smbus-compatible remote/ local temperature sensor with overtemperature alarm maxim integrated | 13 www.maximintegrated.com functional diagram max6642 mux remote local adc 2 control logic smbus readwrite 8 8 address decoder 7 sr q dxp sclk sda register bank command byte remote temperature local temperature alert threshold alert response address alert v cc diode fault top view 12 gnd 3 65 4 dxp sdasclk ep* *exposed pad connected to gnd. v cc tdfn-ep alert max6642 pin configuration chip information process: bicmos package information for the latest package outline information and land patterns (foot-prints), go to www.maximintegrated.com/packages . note that a ?? ?? or ??in the package code indicates rohs status only.package drawings may show a different suffix character, but the drawing pertains to the package regardless of rohs status. package type package code outline no. land pattern no. 6 tdfn-ep t633-2 21-0137 90-0058 downloaded from: http:///
maxim integrated cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a maxim integrated product. no circuit patent licenses are implied. maxim integrated reserves the right to change the circuitry and specifications without notice at any time. the parametric values (min and max limits) shown in the electrical characteristics table are guaranteed. other parametric values quoted in this data sheet are provided for guidance. maxim integrated and the maxim integrated logo are trademarks of maxim integrated products, inc. 2015 maxim integrated products, inc. | 14 for pricing, delivery, and ordering information, please contact maxim direct at 1-888-629-4642, or visit maxim integrateds website at www.maximintegrated.com. max6642 1c, smbus-compatible remote/ local temperature sensor with overtemperature alarm revision history revision number revision date description pages changed 0 8/03 initial release 1 10/08 added missing ep description to ordering information and pin description , removed the transistor count on page 12, and corrected some minor style issues 1, 5, 9, 10, 12 2 7/09 corrected errors in figures 2 and 3 6, 7 3 10/09 corrected error in package information table 13 4 4/15 revised benefits and featres section 1 5 11/15 updated package code 13 downloaded from: http:///

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