AN13.19 Resistance Error Correction Author: Wayne Little Microchip Technology Inc. INTRODUCTION This application note describes the Resistance Error Correction feature available on many Microchip temperature sensor devices. OVERVIEW The information presented will show system designers that the Resistance Error Correction (REC) feature removes the need to compensate for series resistance in the thermal diode connection. Figure 1 shows a typical system and the remote diode-connected transistor could be a central processing unit (CPU) thermal diode or a discrete transistor located where the temperature must be measured. Remote Sensor System Controller with SMBus Interface DP Remote Diode-Connected Transistor DN SMBus Interface FIGURE 1: SMBus Block Diagram of Typical Temperature Sensing System. 2005-2014 Microchip Technology Inc. DS00001852A-page 1 AN13.19 POSITIVE TEMPERATURE OFFSET RESULTING FROM SERIES RESISTANCE Review of Temperature Sensing Method A typical temperature sensor forces two fixed currents (IF1 and IF2) into the thermal diode to measure temperature, as shown in Figure 2 below. The forward bias voltage (VF) of the diode is measured as each of the two fixed currents is sourced into the diode. In Figure 2, the value of VF measured at the DP/DN pins inside the chip is equivalent to the value of VBE at the remote diode-connected transistor. IF2 IF1 Remote Diode-Connected Transistor DP Voltage to Temperature Conversion VF VBE DN FIGURE 2: Two Current Sources. The difference between the two values of VF (VF2 – VF1 = VBE) is used to determine the temperature, as shown in Equation 1. EQUATION 1: I F2 kT V F2 – VF1 = --------- ln ------- I F1 q Where: k = Boltzmann’s constant T = Absolute temperature in Kelvin q = Electron charge η = Diode ideality factor DS00001852A-page 2 2005-2014 Microchip Technology Inc. AN13.19 Figure 3 shows that the relationship of VF2 – VF1 to temperature is linear. In this plot, the Ideality Factor (ƞ) is assumed to be 1.000 and the IF2/IF1 ratio is 17. The value of VF2 – VF1 will change to 244 µV when the temperature changes from 25°C to 26°C or from 100°C to 101°C. Positive Temperature Offset Resulting from Series Resistance In the real world, series resistance will be present in the path from the DP pin to the actual junction of the diode and back to the DN pin of the temperature sensor. Sources of series resistance include package leads, Printed Circuit Board (PCB) traces, other forms of interconnect and the physical structure of the remote diode itself. In Figure 4, all these sources of series resistance are combined and shown as RS. 95 (VF2 – VF1) = 0.2441 X Temperature 90 VF2 – VF1 (mV) 85 80 75 70 65 60 273 293 FIGURE 3: 313 333 Temperature (K) 353 373 VF2 – VF1 vs. Temperature. IF2 IF1 RS Remote Diode-Connected Transistor DP Voltage to Temperature Conversion VBE VF DN FIGURE 4: Block Diagram of Temperature Monitoring Circuit. When series resistance is present in the system, the VF value measured at the DP/DN pins inside the chip is no longer equivalent to the value of VBE. The VF value with series resistance is shown in Equation 2. This RS term will always induce a positive temperature measurement offset error. The reported temperature will be higher than the actual amount by the value obtained using Equation 3. EQUATION 2: EQUATION 3: V F = V BE + IF RS q I F2 – I F1 RS T OFFSET = -------- -------------------------------k I F2 ln ------- I F1 This means that a system could be operating at an acceptable temperature but the sensor would report that it is beyond critical temperature because of the series resistance. 2005-2014 Microchip Technology Inc. DS00001852A-page 3 AN13.19 Example with Series Resistance EQUATION 4: IF2 – IF1 R S = 170 A – 10 A 3 = 480 V For an IF ratio of 17, a 1° change in temperature equates to a 244 mV change in the VF2 – VF1 term. 5.00E-02 4.50E-02 4.00E-02 3.50E-02 3.00E-02 20 40 60 80 Temperature (°C) 100 FIGURE 6: Trace Resistance vs. Temperature (250 µm Traces, 0.5 oz Copper Plating). EQUATION 5: 480 V ------------------ = 1.96 244 V This series resistance term results in approximately a 2° error to the “real” temperature. The remote thermal diode is often connected to the temperature sensor using PCB traces. Figure 5 shows typical values of series resistance for PCB traces at room temperature. As the temperature of the PCB traces increases from 20°C to 60°C, the series resistance changes by approximately 32%. These small error terms should not be overlooked when designing systems with ±1°C accuracy components. The desired way to handle series resistance is to design with a Microchip temperature sensor that incorporates automatic resistance error correction. Resistance Error Correction 0.1000 7UDFH5HVLVWDQFHȍFP 5.50E-02 5HVLVWDQFHȍFP This example provides insight into the effects of series resistance on the detected temperature. Assume a value of IF1 = 10 µA and IF2 = 170 µA. A typical value of series resistance from a CPU data sheet is 3Ω. Microchip temperature sensor devices that include resistance error correction implement in the analog front end of the chip. Resistance Error Correction is an automatic feature that eliminates the need to characterize and compensate for the series resistance. The REC feature corrects for as much as 100 of series resistance. 0.0800 0.0600 0.5 oz 0.0400 0.0200 1 oz CONCLUSION 0.0000 0 0.2 0.4 0.6 Trace Width (mm) 0.8 1 FIGURE 5: Trace Resistance vs. Trace Width (at T = 25°C). In some systems, it may be practical to compensate for the error caused by series resistance by subtracting a constant offset value. However, this would require calculating a new offset value for each system because the total amount of series resistance added by the PCB traces will change depending on the physical properties of the board and on the dimensions of the traces. Moreover, changing the offset value often requires changing firmware that would otherwise not require a change. In addition, Figure 6 shows that the series resistance of PCB traces also changes over temperature. DS00001852A-page 4 In conclusion, using a Microchip temperature sensor with REC capabilities automatically eliminates temperature errors induced by the series resistance that is present in all systems. Microchip supplies a family of temperature sensors for a variety of applications. Several enhanced features, such as beta compensation and ideality configuration, are available. In addition, some devices are designed to work specifically with CPU thermal diodes. Please consult your Microchip representative or www.microchip.com for additional information. REFERENCES • Microchip Application Note 10.14 – “Using Temperature Sensing Diodes with Remote Thermal Sensors” (DS00001839) 2005-2014 Microchip Technology Inc. Note the following details of the code protection feature on Microchip devices: • Microchip products meet the specification contained in their particular Microchip Data Sheet. • Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the intended manner and under normal conditions. • There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our knowledge, require using the Microchip products in a manner outside the operating specifications contained in Microchip’s Data Sheets. Most likely, the person doing so is engaged in theft of intellectual property. • Microchip is willing to work with the customer who is concerned about the integrity of their code. • Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code protection does not mean that we are guaranteeing the product as “unbreakable.” Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of our products. Attempts to break Microchip’s code protection feature may be a violation of the Digital Millennium Copyright Act. If such acts allow unauthorized access to your software or other copyrighted work, you may have a right to sue for relief under that Act. Information contained in this publication regarding device applications and the like is provided only for your convenience and may be superseded by updates. It is your responsibility to ensure that your application meets with your specifications. MICROCHIP MAKES NO REPRESENTATIONS OR WARRANTIES OF ANY KIND WHETHER EXPRESS OR IMPLIED, WRITTEN OR ORAL, STATUTORY OR OTHERWISE, RELATED TO THE INFORMATION, INCLUDING BUT NOT LIMITED TO ITS CONDITION, QUALITY, PERFORMANCE, MERCHANTABILITY OR FITNESS FOR PURPOSE. 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