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mic281 low-cost ittybitty? thermal s ensor ittybitty is a trademark of micrel, inc. all other tra demarks are the property of their respective owners. micrel inc. ? 2180 fortune drive ? san jose, ca 95131 ? usa ? tel +1 (408) 944 - 0800 ? fax + 1 (408) 474 - 1000 ? http://www.micrel.com april 23 , 20 14 revision 2.0 general description the mic281 is a digital thermal sensor capable of measuring the temperature of a remote pn junction. it is optimized for applications favoring low cost and small size. the remote junction may be an inexpensive commodity transistor, e.g., 2n3906, or an embedded thermal diode such as found in intel pentium ? ii/iii/iv cpus, amd athlon ? cpus, and xilinx virtex ? fpgas. the mic281 is 100% software and hardware backward compatible with the mic280 and features the same industry-leading noise performance and small size. the advanced integrating a/d converter and analog front-end reduce errors due to noise for maximum accuracy and minimum guardbanding. a 2-wire smbus 2.0-compatible serial interface is provided for host communication. the clock and data pins are 5v- tolerant regardless of the value of v dd . they will not clamp the bus lines low even if the device is powered down. superior performance, low power, and small size make the mic281 an excellent choice for cost-sensitive thermal management applications. datasheets and support documentation are available on micrels web site at: www.micrel.com . features ? remote temperature measurement using embedded thermal diodes or commodity transistors ? accurate remote sensing: 3c max., 0c to 100c ? excellent noise rejection ? i 2 c and smbus 2.0-compatible serial interface ? smbus timeout to prevent bus lockup ? voltage tolerant i/os ? low power shutdown mode ? failsafe response to diode faults ? 3.0v to 3.6v power supply range ? available in ittybitty sot23- 6 p ackage applications ? desktop, server, and notebook computers ? set-top boxes ? game consoles ? appliances typical application downloaded from: http:///
micrel, inc. mic281 april 23 , 20 14 2 revision 2.0 ordering information part number marking ( 1 ) slave address ambient temp. range package mic281-0ym6 tb 00 100 1 000 x b C 55 to +125c sot23-6 mic281-1ym6 tb 01 100 1 001 x b C 55 to +125c sot 23 -6 mic2 81 -2ym6 tb 02 100 1 010 x b C 55 to +125c sot23-6 mic281-3ym6 tb 03 100 1 011 x b C 55 to +125c sot23-6 mic281-4ym6 tb 04 10 01 100 x b C 55 to +125c sot23-6 mic281-5ym6 tb 05 1001 101 x b C 55 to +125c sot23-6 mic281-6ym6 tb 06 1001 110 x b C 55 to +125c sot23-6 mic281-7ym6 tb 07 1001 111 x b C 55 to +125c sot23-6 note: 1. underbar (_) may not be to scale. pin configuration sot23-6 (m6) top view pin description pin number pin name pin function 1 vdd analog input: power supply input to the ic. 2 gnd ground return for all ic functions. 3 t1 analog input: connection to remote diode junction. 4 clk digital input: serial bit clock input. 5 data digital input/output: open-drain. serial data input/output. 6 nc no connection: must be left unconnected. downloaded from: http:///
micrel, inc. mic281 april 23 , 20 14 3 revision 2.0 absolute maximum ratings ( 2 ) power supply voltage (v dd ) ........................................ +3.8v voltage on t1 ....................................... C 0.3v to v dd + 0.3v voltage on clk, data ..................................... C 0.3v to 6v current into any pin .................................................. 10ma power dissipation, t a = +125c .............................. 109mw storage temperature (ts) ......................... C 65 c to +150 c esd ratings ( 4 ) human body model ..................................................... 1.5 kv machine model ............................................................. 200v soldering (sot23-6 package) vapor phase (60s) ........................................... 220 c +5 / -0 c infrared (15s) .................................................... 235 c +5 / -0 c operating ratings ( 3 ) power supply voltage (v dd ) ........................ +3.0v to +3.6v ambient temperature range (t a ) ............ C 55c to +125c package thermal resistance sot23-6 ( ? ja ) .................................................. 230c/w electrical characteristics ( 5 ) v dd = 3.3v ; t a = 25c, unless noted. b old values indicate t min t a t max , unless noted ( 3 ) . symbol parameter condition min. typ. max. units power supply i dd supply current t1 open; clk = data = high; normal mode 0.23 0.4 ma shutdown mode; t1 open; note 7 clk = 100khz 9 a shutdown mode; t1 open; clk = data = high 6 a t por power-on reset time, note 7 v dd > v por 200 s v por power-on reset voltage all registers reset to default values; a/d conversions initiated 2.65 2.95 v v hyst power- on reset hysteresis voltage, note 7 300 mv temperature- to -digital converter characteristics accuracy, notes 7 , 8 , 9 0 c t d 100 c , 0 c < t a < 85 c; 3.15v < v dd < 3.45v 1 3 c C 40 c t d 125 c , 0 c < t a < 85 c; 3.15v < v dd < 3.45v 2 5 c t conv conversion time note 7 200 240 ms remote temperature input, t1 i f current into external diode, note 7 t1 forced to 1.0v, high level 192 400 a low level 7 12 a downloaded from: http:///
micrel, inc. mic281 april 23 , 20 14 4 revision 2.0 electrical characteristics ( 5 ) (continued) v dd = 3.3v; t a = 25c, unless noted. b old values indicate t min t a t max , unless noted ( 3 ) . symbol parameter condition min. typ. max. units serial data i/o pin, data v ol low output voltage, note 6 i ol = 3ma 0.3 v i ol = 6ma 0 .5 v v il low input voltage 3v v dd 3.6v 0.8 v v ih high input voltage 3v v dd 3.6v 2.1 5.5 v c in input capacitance note 7 10 pf i leak input current 1 a serial clock input, clk v il low input voltage 3v v dd 3.6v 0.8 v v ih high input voltage 3v v dd 3.6v 2.1 5.5 v c in input capacitance note 7 10 pf i leak input current 1 a serial interface timing t 1 clk (clock) period 2.5 s t 2 data in setup time to clk high 100 ns t 3 data out stable after clk low 300 ns t 4 data low setup time to clk low start condition 100 ns t 5 data high hold time after clk high stop condition 100 ns t to bus timeout 25 30 35 ms notes: 2. exceeding the absolute maximum ratings may damage the device. 3. the device is not guaranteed to function outside its opera ting ratings. final test on outgoing product is performed at t a = 25 c. 4. devices are esd sensitive. handling precautions are re commended. human body model, 1.5k ? in series with 100pf. 5. specification for packaged product only. 6. current into the data pin will result in self-heating of the device. sink current should be minimized for best accura cy. 7. guaranteed by design over the operating temperature ran ge. not 100% production tested. 8. accuracy specification does not include quantization noise, which may be up to 1/2 lsb. 9. t d is the temperature of the remote diode junction. testing is per formed using a single unit of one of the transistors liste d in table 4. downloaded from: http:///
micrel, inc. mic281 april 23 , 20 14 5 revision 2.0 timing diagram serial interface timing downloaded from: http:///
micrel, inc. mic281 april 23 , 20 14 6 revision 2.0 typical characteristics v dd = 3.3v; t a = 25 ? c, unless otherwise noted. downloaded from: http:///
micrel, inc. mic281 april 23 , 20 14 7 revision 2.0 functional description serial port operation the mic281 uses standard smbus write_byte and read_byte operations for communication with its host. the smbus write_byte operation involves sending the devices slave address (with the r/w bit low to signal a write operation), followed by a command byte and the data byte. the smbus read_byte oper ation is a composite write and read operation: the host ?rst sends the devices slave address followed by the command byte, as in a write operation. a new start bit must then be sent to the mic281, followed by a repeat of the slave address with the r/w bit (lsb) set to the high (read) state. the data to be read from the part may then be clocked out. these protocols are shown in figures 1 and 2. the command byte is eight bits (one byte) wide. this byte carries the address of the mic281 register to be operated upon. the command byte values corresponding to the various mic281 registers are shown in table 1. other command byte values are reserved, and should not be used. figure 1. write_byte protocol figure 2. read_byte protocol table 1. mic281 register addresses target register command byte value power-on default label description read write temp remote temperature result 01 h n/a 00 h (0 c) config configuration 03 h 03 h 80 h mfg_id manufacturer identification fe h n/a 2a h dev_id device and revision identification ff h n/a 0x h ( 10 ) note: 10. the lower nibble contains the die revision level (e.g., rev. 0 = 00 h). downloaded from: http:///
micrel, inc. mic281 april 23 , 20 14 8 revision 2.0 slave address the mic281 will only respond to its own unique slave address. a match between the mic281s address and the address speci?ed in the serial bit stream must be made to initiate communication. the mic281s slave address is ?xed at the time of manufacture. eight different slave addresses are available as determined by the part num ber. see table 2 and the ordering information tabl e. table 2. mic281 slave addresses part number slave address mic281-0y m6 100 1 000 b = 90 h mic281- 1y m6 100 1 001 b = 92 h mic281- 2y m6 100 1 010 b = 94 h mic281- 3y m6 100 1 011 b = 96 h mic281- 4y m6 100 1 100 b = 98 h mic281- 5y m6 100 1 101 b = 9a h mic281- 6y m6 100 1 110 b = 9c h mic281- 7y m6 100 1 111 b = 9e h temperature data format the least- signi?cant bit of the temperature register represents one degree centigrade. the val ues are in a twos complement format, wherein the most signi?cant bit (d7) represents the sign: ze ro for positive temperatures and one for negative temperatures. table 3 shows examples of the data format used by the mic281 for temperatures. table 3. digital temperature format temperature binary hex +127c 0111 1111 7f +125c 0111 1101 7d +25c 0001 1001 19 +1c 0000 0001 01 0c 0000 0000 00 C 1c 1111 1111 ff C 25c 1110 0111 e7 C 125c 1000 0011 83 C 128c 1000 0000 80 diode faults the mic281 is designed to respond in a failsafe manner to diode faults. if an inte rnal or external fault occurs in the temperature sensing circuitry, such as t1 being open or shorted to vdd or gnd, the t emperature result will be reported as the maximum full-scale value of +127c. note that diode faults will not be detected until the ?rst a/d conversion cycle is completed following power-up or exiting shutdown mode. shutdown mode setting the shutdown bit in the con?guration register will cause the mic281 to cea se operation. the a/d converter will stop and power consumption will drop to the i shdn level. no registers will be affected by entering shutdown mode. the last temperature reading will persist in the temp register. downloaded from: http:///
micrel, inc. mic281 april 23 , 20 14 9 revision 2.0 detailed register descriptions remote temperature result (temp) 8-bits, read only local temperature result register d[7] read-only d[6] read-only d[5] read-only d[4] read-only d[3] read-only d[2] read-only d[1] read-only d[0] read-only temperature data from adc. bit function operation d[7:0] measured temperature data for the remote zone. read only power-up default value: 0000 0000 b = 00 h = (0 c) ( 11 ) read command byte: 0000 0001 b = 01 h each lsb represents one degree centigrade. the values are in a twos complement bina ry format such that 0c is reported as 0000 0000b. see the temperature data format section for more details. note: 11. temp will contain measured temperature data after the c ompletion of one conversion. configuration register (config) 8-bits, read/write configuration register d[7] reserved d[6] reserved d[5] reserved d[4] reserved d[3] reserved d[2] reserved d[1] reserved d[0] write-o nly reserved shutdown (shdn) reserved bit function operation ( 12 ) d7 reserved always writes as zero; reads undefined d6 shutdown bit 0 = normal operation; 1 = shutdown d[5:0] reserved always writes as zero; reads undefined note: 12. any write to config will result in any a/d conversion in progr ess being aborted and the result discarded. the a/d will b egin a new conversion sequence once the write operation is complete. power-up default value: x0xx xxxx b (not in shutdown mode) command byte: 0000 0011 b = 03 h downloaded from: http:///
micrel, inc. mic281 april 23 , 20 14 10 revision 2.0 manufacturer id register (mfg_id) 8-bits, read only manufacturer id register d[7] read-only d[6] read-only d[5] read-only d[4] read-only d[3] read-only d[2] read-only d[1] read-only d[0] read-only 0 0 1 0 1 0 1 0 bit function operation d[7:0] identifies micrel, inc. as the manufacturer of the device read only. always returns 2a h power-up default value: 0010 10 10 b = 2a h read command byte: 1111 111 0 b = fe h die revision register (die_rev) 8-bits, read only die revision register d[7] read-only d[6] read-only d[5] read-only d[4] read-only d[3] read-only d[2] read-only d[1] read-only d[0] read-only mic281 die revision number bit function operation d[7:0] identifies the device revision number. read only power-up default value: [device revision number] h read command byte: 1111 1111 b = ff h downloaded from: http:///
micrel, inc. mic281 april 23 , 20 14 11 revision 2.0 application information remote diode section most small-signal pnp transistors with characteristics similar to the jedec 2n3906 will perform well as remote temperature sensors. table 4 lists several examples of such parts that micrel has tested for use with the mic281 . other transistors equivalent to these should also work well. table 4. transistors suitable for use as remote diodes vendor part number package fairchild semiconductor mmbt3906 sot- 23 on semiconductor mmbt3906l sot- 23 philips semiconductor smbt3906 sot- 23 samsung semiconductor kst3906- tf sot- 23 minimizing errors self-heating one concern when using a part with the temperature accuracy and resolution of the mic281 is to avoid errors induced by self-heating (v dd i dd ) + (v ol i ol ). in order to understand what level of error this might represent, and how to reduce that error, the dissipation in the mic281 must be calculated and its effects reduced to a temperature offset. the worst-case operating condition for the mic281 is when v dd = 3.6v.the maximum power dissipated in the part is given in the following equation: p d = [(i dd v dd )+(i ol(data) v ol(data) )] p d = [(0.4ma 3.6v)+(6ma 0.5v)] p d = 4.44mw r (j - a) of sot23-6 package is 230c/w, therefore the theoretical maximum self-heating is: 4.44mw 230c/w = 1.02c in most applications, the data pin will have a duty cycle of substantially below 25% in the low state. these considerations, combined with more typical device and application parameters, give a better system-level view of device self-heating. this is illustrated by the next equation . in any application, the best approach is to verify performance against calculation in the ?nal applic ation environment. this is especially true when dealing with systems for which some temperature data may be poorly de?ned or unobtainable except by empirical means. p d = [(i dd v dd )+(i ol(data) v ol(data) )] p d = [(0.23ma 3.3v)+(25% 1.5ma 0.15v)] pd = 0.815mw r (j - a) of sot23-6 package is 230c/w, therefore the typical self-heating is: 0.815mw 230c/w = 0.188c series resistance the operation of the mic281 depends upon sensing the v cb -e of a diode-connected pnp transistor (diode) at two different current levels. for remote temperature measurements, this is done using an external diode connected between t1 and ground. because this technique relies upon measuring the relatively small voltage difference resulting from two levels of current through the external diode, any resistance in series with the external diode will cause an error in the temperature reading from the mic281. a good rule of thumb is that for each ohm in series with the external transistor, there will be a 0.9c error in the mic281s temp erature measurement. it is not dif?cult to keep the series resistance well below an ohm (typically <0.1?), so this will rarely be an issue. filter capacitor selection it is usually desirable to employ a ?lter capacitor between the t1 and gnd pins of the mic281. the use of this capacitor is recommended in environments with a lot of high frequency noise (such as digital switching noise), or if long traces or wires are used to connect to the remote diode. the recommended total capacitance from the t1 pin to gnd is 2200pf. if the remote diode is to be at a distance of more than six- to -twelve inches from the mic281, using twisted pair wiring or shielded microphone cable for the connections to the diode can signi?cantly reduce noise pickup. if using a long run of shielded cable, remember to subtract the cables conductor - to -shield capacitance from the 2200pf total capacitance. downloaded from: http:///
micrel, inc. mic281 april 23 , 20 14 12 revision 2.0 layout considerations the following guidelines should be kept in mind when designing and laying out circuits using the mic 28 1. 1. place the mic281 as close to the remote diode as possible, while taking care to avoid severe noise so urces such as high frequency power transformers, crts, memory and data busses, and the like. 2. because any conductance from the various voltages on the pc board and the t1 line can induce serious errors, it is good practice to guard the remote diode's emitter trace with a pair of ground traces. these ground traces should be returned to the mic281's own ground pin. they should not be grounded at any other part of their run. however, it is highly desirable to use these guard traces to carry the diode's own ground return back to the ground pin of the mic281, thereby providing a kelvin connection for the base of the diode. see figure 3. 3. when using the mic281 to sense the temperature of a processor or other device which has an integral thermal diode, e.g., intel's pentium iii, connect the emitter and base of the remote sensor to the mic281 using the guard traces and kelvin return shown in figure 3. the collector of the remote diode is typically inaccessible to the user on these devices. 4. due to the small currents involved in the measurement of the remote diodes ?v be , it is important to adequately clean the pc board after soldering to prevent current leakage. this is most likely to show up as an issue in situations where water-soluble soldering ?uxes are used. 5. in ge neral, wider traces for the ground and t1 lines will help reduce susceptibility to radiated noise (wider traces are less inductive). use trace widths and spacing of 10mm wherever possible and provide a ground plane under the mic281 and under the connections from the mic281 to the remote diode. this will help guard against stray noise pickup. 6. always place a good quality power supply bypass capacitor directly adjacent to, or underneath, the mic281. this should be a 0.1f ceramic capacitor. surface-mount parts provide the best bypassing because of their low inductance. figure 3. guard traces/kelvin ground returns downloaded from: http:///
micrel, inc. mic281 april 23 , 20 14 13 revision 2.0 package information ( 13 ) 6-pin sot23 ( m6 ) note: 13. package information is correct as of the publication date. for updates and most current information, go to www.micrel.com . downloaded from: http:///
micrel, inc. mic281 april 23 , 20 14 14 revision 2.0 micrel, inc. 2180 fortune drive san jose, ca 95131 usa tel +1 (408) 944-0800 fax +1 (408) 474-1000 web http://www.micrel.com micrel makes no representations or warranties with respect to the accuracy or completeness of the information furn ished in this data sheet. this information is not intended as a warranty and micrel does n ot assume responsibility for its use. micrel reserves th e right to change circuitry, specifications and descriptions at any time without notice . no license, whether express, implied, arising by est oppel or otherwise, to any intellectual property rights is granted by this document. except as provided in micrels terms and conditions of sale for such products, micrel assumes no liability whatsoever, and micrel disclaims any express or implied warra nty relating to the sale and/or use of micrel products includ ing liability or warranties relating to fitness for a particular purpose, merchantability , or infringement of any patent, copyright or other intelle ctual property right. micrel products are not designed or authorized for use a s components in life support appliances, devices or syst ems where malfunction of a product can reasonably be expected to result in personal injury. l ife support devices or systems are devices or systems t hat (a) are intended for surgical implant into the body or (b) support or sustain life, and whose failure to perform can be reasonably expected to result in a significant injury to the user. a purchasers use or sale of micrel products for use in li fe support appliances, devices or systems is a purcha sers own risk and purchaser agrees to full y indemnify micrel for any damages resulting from such use or sale. ? 20 14 micrel, incorporated. downloaded from: http:///

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