This application note assumes that the reader is familiar with hardware design and the functionality of the SMSC temperature sensor devices.

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1 AN 16.4 Using Anti-Parallel Diode (APD) with SMSC emperature Sensors 1 ntroduction his application note provides information on maintaining temperature measurement accuracy and noise immunity when using anti-parallel diodes (APD) with SMSC s APD emperature Sensors. his application note assumes that the reader is familiar with hardware design and the functionality of the SMSC temperature sensor devices. 1.1 eferences he following documents should be referenced when using this application note: Device datasheets (EMC1033/EMC1204/EMC1404/EMC1424/EMC2104) 1.2 Background hermal Diode emperature Measurement hermal diode temperature measurements are based on the change in forward bias voltage (ΔV ) of a diode when operated at two different currents: where: ΔV V _ HGH V _ ηk HGH k Boltzmann s constant absolute temperature in Kelvin [1] electron charge η diode ideality factor Figure 1.1, "hermal Diode emperature Measurement" shows a functional block diagram of the temperature measurement circuit. As shown, the SMSC temperature sensor incorporates switched capacitor technology that samples the temperature diode voltage at two bias currents and holds the difference voltage. he output of the switched capacitor sample and hold circuit interfaces to a single bit delta sigma analog to digital converter. his ADC runs at 100kHz sample freuency and its output is digitally filtered and averaged over 2048 samples generating a 11 bit output. Conversion time is approximately 20ms per temperature zone. he advantages of this architecture are superb linearity and inherent noise immunity. he linearity is directly attributed to the delta sigma ADC single bit comparator while the noise immunity is achieved by the digital averaging filter. SMSC AN 16.4 APPLCAON NOE evision 1.1 ( )

2 Using Anti-Parallel Diode (APD) with SMSC emperature Sensors HGH DP Diode DN nput Filter & Sampler ΔΣ ADC Figure 1.1 hermal Diode emperature Measurement 2 Using Anti-Parallel hermal Diodes for emperature Measurement 2.1 Anti-Parallel Diodes (APD) Unlike other traditional temperature monitoring devices which reuire two pins for each thermal zone, SMSC APD temperature sensors can monitor two separate temperature zones using only two pins. his is accomplished using two anti-parallel connected transistors as shown on the pins and in Figure 2.1, "Anti-Parallel Diode (APD) Example". his techniue maintains high accuracy while minimizing pin count and reducing board routing complexity. t is not compatible with substrate transistors (sometimes called thermal diodes or on-chip sense junctions), because the architecture of substrate transistors does not support reverse biasing. > D2 D1 ypical remote discrete NPN transistor i.e. 2N3904 Figure 2.1 Anti-Parallel Diode (APD) Example o measure two temperatures using an APD pair, the circuit first sources currents through the DP1 pin and measures the voltage cross D1. his is the same as normal non-apd measurement except that the parallel connected D2 is reverse biased (Figure 2.2, "APD emperature Measurement"). After calculating D1 s temperature, the D1/D2 Selection signal will switch the direction of the currents and force them through the DP2 pin, measuring D2 s temperature with D1 reverse biased. evision 1.1 ( ) 2 SMSC AN 16.4 APPLCAON NOE

3 Using Anti-Parallel Diode (APD) with SMSC emperature Sensors HGH D1/D2 Selection D2 D1 nput Filter & Sampler ΔΣ ADC Figure 2.2 APD emperature Measurement 2.2 Diode everse Saturation Current and Measurement Accuracy Since the ΔV is a function of the temperature () and bias currents ( HGH and ), as shown in Euation [1], different bias currents will generate different ΔV voltages at the same temperature. herefore when using an anti-parallel diode pair for temperature measurement, it is necessary to ensure that the measured temperature accuracy is not affected by the reverse saturation current of the reverse biased diode. Note: n semiconductor industry, a diode s reverse saturation current sometimes is also called the diode s leakage current Maximum Allowed Bias Current Error he SMSC temperature sensors are designed to monitor the remote temperature zones with the accuracy of /-1 C. o maintain this accuracy when using the anti-parallel diodes, the reverse saturation current of the reversed diode must be much smaller than 75 na, based on the calculations in the Appendix. SMSC AN evision 1.1 ( ) APPLCAON NOE

4 Using Anti-Parallel Diode (APD) with SMSC emperature Sensors he emperature Dependence of everse Saturation Current A diode s reverse saturation current is a function of the diode s temperature. Driven by the same bias voltage, the reverse saturation current will be greater at a higher temperature, as shown in Figure 2.3, "emperature Dependence of everse Saturation Current". herefore, all data in this application note will be shown at 125 C. 1.5E-09 everse Saturation Current 0.7V Bias Voltage 1.0E E E emperature (C) Figure 2.3 emperature Dependence of everse Saturation Current everse Saturation Current est for Popular Commercial hermal Diodes everse saturation currents of 96 discrete NPN transistors (2N3904) from different vendors (Fairchild, On Semiconductor, Phillips and OHM) have been tested. he transistors were connected as a diode and tested using an Agilent 4156C Semiconductor Parameter Analyzer. Figure 2.4, "Fairchild Diode everse Saturation 125 C", Figure 2.5, "On Semiconductor Diode everse Saturation 125 C", Figure 2.6, "Philips Diode everse Saturation 125 C" and Figure 2.7, "OHM Diode everse Saturation 125 C" show the measured results at 125 C, when V is between 0.6V and 0.8V. Depending on vendors, the maximum reverse saturation current values are from 1.5nA to 5.8nA, which are much smaller than the 1 C error allowance of 75nA. n fact, the temperature errors caused by reverse saturation current are all smaller than 0.1 º C for the tested diodes, as shown in the figures. t is clear that the general transistor used for normal temperature measurement also can be used for APD temperature measurement while maintaining the same accuracy. evision 1.1 ( ) 4 SMSC AN 16.4 APPLCAON NOE

5 Using Anti-Parallel Diode (APD) with SMSC emperature Sensors 1.E-08 everse Saturation Current (A) 8.E-09 reverse saturation current for 0.1 C error 6.E-09 4.E-09 2.E-09 0.E e ve rse V (V) F01 F02 F03 F04 F05 F06 F07 F08 F09 F10 F11 F12 F13 F14 F15 F16 F17 F18 F19 F20 F21 F22 F23 F24 0.1C Err Figure 2.4 Fairchild Diode everse Saturation 125 C everse Saturation Current (A) 1.E-08 8.E-09 6.E-09 4.E-09 2.E-09 0.E00 reverse saturation current for 0.1 C error everse V (V) O01 O02 O03 O04 O05 O06 O07 O08 O09 O10 O11 O12 O13 O14 O15 O16 O17 O18 O19 O20 O21 O22 O23 O24 0.1C Err Figure 2.5 On Semiconductor Diode everse Saturation 125 C SMSC AN evision 1.1 ( ) APPLCAON NOE

6 Using Anti-Parallel Diode (APD) with SMSC emperature Sensors everse Saturation Current (A) 1.E-08 8.E-09 6.E-09 4.E-09 2.E-09 0.E00 reverse saturation current for 0.1 C error everse V (V) P01 P02 P03 P04 P05 P06 P07 P08 P09 P10 P11 P12 P13 P14 P15 P16 P17 P18 P19 P20 P21 P22 P23 P24 0.1C Err Figure 2.6 Philips Diode everse Saturation 125 C everse Saturation Current (A) 1.E-08 8.E-09 6.E-09 4.E-09 2.E-09 0.E00 reverse saturation current for 0.1 C error everse V (V) C Err 2.3 Layout Considerations Figure 2.7 OHM Diode everse Saturation 125 C Connecting the APD Diodes to SMSC emperature Sensors here are three recommended ways to connect the two APD diodes to a temperature sensor, as shown in Figure 2.8, "ecommended APD Diode Connections to emperature Sensors". evision 1.1 ( ) 6 SMSC AN 16.4 APPLCAON NOE

7 Using Anti-Parallel Diode (APD) with SMSC emperature Sensors Connection ype (a) his connection is recommended when the two APD diodes are placed close to each other, up to 12 inches away from the temperature sensor. Connection ype (b) his connection is recommended when the two APD diodes are not located closely, and both are away from the temperature sensor. Split the traces as close to the temperature sensor as possible. Connection ype (c) his connection can reduce the board routing complexity by using a single pair of traces or twisted cable. t is not recommended to split the traces or cables more than 0.4 inch away from the temperature sensor. he noise pickup characteristics of this configuration are too complex to give predictable results. emperature Sensor C1 wisted pairs or PCB traces up to 12 D1 D2 C2 C1: ypical 2200pF, /- 20% C2: pF, optional Connection ype (a) emperature Sensor PCB traces up to 0.4" D1 C2 C1 wisted pairs or PCB traces up to 12" C1: ypical 2200pF, /- 20% C2, C3: pF, optional D2 C3 Connection ype (b) emperature Sensor wisted pairs or PCB traces up to 12" total C1 D1 C2 D2 C3 C1: ypical 2200pF, /- 20% C2, C3: pF, optional Connection ype (c) Figure 2.8 ecommended APD Diode Connections to emperature Sensors SMSC AN evision 1.1 ( ) APPLCAON NOE

8 Using Anti-Parallel Diode (APD) with SMSC emperature Sensors PCB Layout Apply the following guidelines when designing the printed circuit board: 1. oute the remote diode traces on an outside layer of the PCB. 2. Place a ground plane on the layer immediately below the diode traces. 3. Keep the diode traces short. t is possible with careful layout to route the diode traces 8-12 inches, however, longer traces pickup more noise. 4. Keep the diode traces parallel, and the length of the two traces identical within 0.3 inches. 5. Use a diode trace width of 0.01 inches with a 0.01 inch spacing between traces. 6. f possible place a 0.01 inch wide ground guard trace on both side of the differential pair of diode traces at 0.01 inch spacing. he guard traces should be connected to the ground plane at least every 0.25 inch. Note: do not connect the guard traces to the CPU ground pins. hese pins may inject noise due to locally high currents. 7. Separate the diode traces from any other signal traces by at least inches, if the guard traces are not applicable for the PCB. 8. Keep the diode traces away from sources of high freuency noise such as power supply filtering or high speed digital signals. 9. When the diode traces must cross high speed digital signals, make them cross at a 90 degree angle. 10. Avoid joints of copper to solder that can introduce thermocouple effects. 11. Minimize use of vias and layer change. hese recommendations are illustrated in Figure 2.9, "outing the Diode races". 0.01" Spacing 0.01" Wide 0.01" Spacing 0.01" Wide 0.01" Spacing Ground Guard race Diode race Board Material Diode race Ground Guard race Via 0.25" Copper Ground Plane Via 0.25" Board Material Next PCB Layer Figure 2.9 outing the Diode races emote Diodes Connected by Cables When connecting remote diodes with a cable (instead of traces on the PCB) use shielded twisted pair cable. he shield should be attached to ground near the temperature sensor, and should be left unconnected at the sensor end. Belden 8451 or cables are good choices for this application. f shielded cable cannot be used, route the cables away from noise sources. evision 1.1 ( ) 8 SMSC AN 16.4 APPLCAON NOE

9 Using Anti-Parallel Diode (APD) with SMSC emperature Sensors 2.4 Capacitors on Diode races 3 Appendix n the board layout, provide pads to install a capacitor across the DP and DN traces as close to the temperature sensor package pins as possible. he value of the capacitor should not exceed 2200pF (/- 20%). n certain environments, installing a small capacitor (18pF - 100pF) at the diode can reduce the noise picked up by the PCB traces or connection cable. Especially for the connection type (c) in Figure 2.8, "ecommended APD Diode Connections to emperature Sensors", it is strongly recommended to put a small cap at each diode in addition to the 2200pF capacitor at the temperature sensors DP/DN pins. 3.1 Calculations for Maximum Allowed APD Diode everse Saturation Current f ΔV is the measured ADC result at temperature using single diode and ΔV is the result at the same temperature using APD diodes, then from Euation [1] on page 1, we can have: ΔV ηk HGH where: k Boltzmann s constant absolute temperature in Kelvin [2] electron charge η diode ideality factor and ΔV ηk HGH where: reverse saturation current of [3] reversed diode or ΔV ΔV ηk HGH HGH [4] From Euation [1], we also can get: ΔV ΔV ηk ' HGH ηk HGH [5] SMSC AN evision 1.1 ( ) APPLCAON NOE

10 Using Anti-Parallel Diode (APD) with SMSC emperature Sensors evision 1.1 ( ) 10 SMSC AN 16.4 APPLCAON NOE or Using Euation [4] and Euation [6], we have Let Δ 1 and solve for For HGH 170 ua, 10 ua and 400 K, -75nA where: Δ - [6] [7] [8] [9] [10] [11] Δ Δ Δ HGH k V V η Δ HGH HGH HGH k k η η HGH HGH HGH HGH HGH HGH HGH HGH 1 HGH HGH HGH *

11 Using Anti-Parallel Diode (APD) with SMSC emperature Sensors 80 AKAY DVE, HAUPPAUGE, NY (631) , FAX (631) Copyright 2007 SMSC or its subsidiaries. All rights reserved. Circuit diagrams and other information relating to SMSC products are included as a means of illustrating typical applications. Conseuently, complete information sufficient for construction purposes is not necessarily given. Although the information has been checked and is believed to be accurate, no responsibility is assumed for inaccuracies. SMSC reserves the right to make changes to specifications and product descriptions at any time without notice. Contact your local SMSC sales office to obtain the latest specifications before placing your product order. he provision of this information does not convey to the purchaser of the described semiconductor devices any licenses under any patent rights or other intellectual property rights of SMSC or others. All sales are expressly conditional on your agreement to the terms and conditions of the most recently dated version of SMSC's standard erms of Sale Agreement dated before the date of your order (the "erms of Sale Agreement"). he product may contain design defects or errors known as anomalies which may cause the product's functions to deviate from published specifications. Anomaly sheets are available upon reuest. SMSC products are not designed, intended, authorized or warranted for use in any life support or other application where product failure could cause or contribute to personal injury or severe property damage. Any and all such uses without prior written approval of an Officer of SMSC and further testing and/or modification will be fully at the risk of the customer. Copies of this document or other SMSC literature, as well as the erms of Sale Agreement, may be obtained by visiting SMSC s website at SMSC is a registered trademark of Standard Microsystems Corporation ( SMSC ). Product names and company names are the trademarks of their respective holders. SMSC DSCLAMS AND EXCLUDES ANY AND ALL WAANES, NCLUDNG WHOU LMAON ANY AND ALL MPLED WAANES OF MECHANABLY, FNESS FO A PACULA PUPOSE, LE, AND AGANS NFNGEMEN AND HE LKE, AND ANY AND ALL WAANES ASNG FOM ANY COUSE OF DEALNG O USAGE OF ADE. N NO EVEN SHALL SMSC LABLE FO ANY DEC, NCDENAL, NDEC, SPECAL, PUNVE, O CONSEQUENAL DAMAGES; O FO LOS DAA, POFS, SAVNGS O EVENUES OF ANY KND; EGADLESS OF HE FOM OF ACON, WHEHE BASED ON CONAC; O; NEGLGENCE OF SMSC O OHES; SC LABLY; ACH OF WAANY; O OHEWSE; WHEHE O NO ANY EMEDY OF BUYE S HELD O HAVE FALED OF S ESSENAL PUPOSE, AND WHEHE O NO SMSC HAS EN ADVSED OF HE POSSBLY OF SUCH DAMAGES. SMSC AN evision 1.1 ( ) APPLCAON NOE

4 Maintaining Accuracy of External Diode Connections

4 Maintaining Accuracy of External Diode Connections AN 15.10 Power and Layout Considerations for EMC2102 1 Overview 2 Audience 3 References This application note describes design and layout techniques that can be used to increase the performance and dissipate