Current Sensing Circuit Concepts and Fundamentals

1 2010-2011 Microchip Technology 1AN1332 INTRODUCTIONC urrent Sensing is a fundamental requirement in a widerange of electronic applications. Typical applications that benefit from Current sensinginclude: Battery life indicators and chargers Overcurrent protection and supervising circuits Current and voltage regulators DC/DC converters Ground fault detectors Linear and switch-mode power supplies Proportional solenoid control, linear or PWM Medical diagnostic equipment Handheld communications devices Automotive power electronics Motor speed controls and overload protectionThis application note focuses on the Concepts andfundamentals of Current Sensing circuits. It introducescurrent Sensing resistors, Current Sensing techniquesand describes three typical high-side Current sensingimplementations, with their advantages and disadvan-tages.

2 The other Current Sensing implementations arebeyond the scope of this application note and reservedfor subsequent Microchip Technology Incorporated sapplication Sensing RESISTOR DescriptionA Current sensor is a device that detects and convertscurrent to an easily measured output voltage, which isproportional to the Current through the measured path. There are a wide variety of sensors, and each sensoris suitable for a specific Current range andenvironmental condition. No one sensor is optimum forall applications. Among these sensors, a Current Sensing resistor is themost commonly used. It can be considered a Current -to-voltage converter, where inserting a resistor into thecurrent path, the Current is converted to voltage in alinear way of V = I R. The main advantages and disadvantages of currentsensing resistors include:a) Advantages:- Low cost- High measurement accuracy - Measurable Current range from very low to medium- Capability to measure DC or AC currentb) Disadvantages:- Introduces additional resistance into the measured Circuit path, which may increase source output resistance and result in undesirable loading effect- Power loss since power dissipation P=I2 R.

3 Therefore, Current Sensing resistors are rarely used beyond the low and medium Current Sensing :Yang ZhenMicrochip Technology Sensing Circuit Concepts and FundamentalsAN1332DS01332B-page 2 2010-2011 Microchip Technology CriteriaThe disadvantages mentioned previously could bereduced by using low-value Sensing , the voltage drop across the Sensing resistormay become low enough to be comparable to the inputoffset voltage of subsequent analog conditioningcircuit, which would compromise the measurementaccuracy. In addition, the Current Sensing resistor s inherentinductance must be low, if the measured Current has alarge high-frequency component. Otherwise, theinductance can induce an Electromotive Force (EMF)which will degrade the measurement accuracy as well. Furthermore, the resistance tolerance, temperaturecoefficient, thermal EMF, temperature rating and powerrating are also important parameters of the currentsensing resistors when measurement accuracy isrequired.

4 In brief, the selection of Current Sensing resistors is vitalfor designing any kind of Current monitor. The followingselection criteria can be used for resistance with tight tolerance, to create abalance between accuracy and Current capability and high peak powerrating to handle short duration and transientpeak inductance to reduce the EMF due to high-frequency temperature coefficient, low thermal EMFand high temperature capability, if there is awide temperature variationCURRENT Sensing TECHNIQUESThis section introduces two basic techniques forcurrent Sensing applications, low-side Current sensingand high-side Current Sensing . Each technique has itsown advantages and disadvantages, discussed inmore detail in the following Current SensingAs shown in Figure 1, low-side Current sensingconnects the Sensing resistor between the load andground.

5 Normally, the sensed voltage signal(VSEN=ISEN RSEN) is so small that it needs to beamplified by subsequent op amp circuits ( , non-inverting amplifier) to get the measurable outputvoltage (VOUT).FIGURE 1:Low-Side Current ) Advantages:- Low input Common mode voltage-Low VDD parts- Ground referenced input and output- Simplicity and low costb) Disadvantages:- Ground path disturbance- Load is lifted from system ground since RSEN adds undesirable resistance to the ground path- High load Current caused by accidental short goes undetectedIn a single-supply configuration, the most importantaspect of low-side Current Sensing is that the Commonmode input voltage range (VCM) of the op amp mustinclude ground. The MCP6H0X op amp is a goodchoice since its VCM is from VSS to VDD Considering the advantages, choose low-side currentsensing where short Circuit detection is not required,and ground disturbances can be Amps CircuitsISENISENVOUTLoadPower Supply 2010-2011 Microchip Technology 3AN1332 High-Side Current SensingAs shown in Figure 2, high-side Current sensingconnects the Sensing resistor between the powersupply and load.

6 The sensed voltage signal is amplifiedby subsequent op amp circuits to get the measurableVOUT. FIGURE 2:High-Side Current )Advantages:- Eliminates ground disturbance- Load connects system ground directly- Detects the high load Current caused by accidental shorts b)Disadvantages:- Must be able to handle very high and dynamic Common mode input voltages- Complexity and higher costs- High VDD partsIn a single-supply configuration, the most importantaspects of high-side Current Sensing are: The VCM range of the Difference amplifier must be wide enough to withstand high Common mode input voltages The Difference amplifier s ability to reject dynamic Common mode input voltagesThe MCP6H0X op amp is a good fit for high-sidecurrent Sensing , which will be discussed in more detailin the following Current Sensing IMPLEMENTATIONHigh-side Current Sensing is typically selected inapplications where ground disturbance cannot betolerated, and short Circuit detection is required, suchas motor monitoring and control, overcurrent protectionand supervising circuits, automotive safety systems,and battery Current monitoring.

7 This section discusses three typical high-side currentsensing implementations, with their advantages anddisadvantages. Based on application requirements,one choice may be better than Op Amp Difference AmplifierFigure 3 shows a single op amp Difference amplifierthat consists of the MCP6H01 op amp and four externalresistors. It amplifies the small voltage drop across thesensing resistor by the gain R2/R1, while rejecting theCommon mode input voltage. FIGURE 3: Single Op Amp Difference Difference amplifier s Common mode rejectionratio (CMRRDIFF) is primarily determined by resistormismatches (R1, R2, R1*, R2*), not by the MCP6H0 Xop amp s CMRR. RSENISENISENVOUTLoadPower SupplyOp Amps CircuitsMCP6H01R1 VOUTR2R1*RSENISENRSEN << R1, R2 VOUTV1V2 R2R1------ VREF+=VREFR2*VDDLoadPower SupplyISENV1V2R1 = R1*, R2 = R2* AN1332DS01332B-page 4 2010-2011 Microchip Technology resistor ratios of R2/R1 and R2*/R1* must be wellmatched to obtain an acceptable CMRRDIFF.

8 However,the tight tolerance resistors will add more cost to DC CMRRDIFF is shown in Equation 1. EQUATION 1:Example 1 If R2/R1= 1 and TR= , then the worst case DC CMRRDIFF will be 54 dB. If R2/R1= 1 and TR= 1%, then the worst case DC CMRRDIFF will be only 34 , RSEN should be much less than R1 and R2in order to minimize resistive loading effect. TheDifference amplifier s input impedances, seen from V1and V2, are unbalanced. Note that the resistive loadingeffect and the unbalanced input impedances willdegrade the CMRRDIFF. The reference voltage (VREF) allows the amplifier soutput to be shifted to some higher voltage, withrespect to ground. VREF must be supplied by a low-impedance source, to avoid making CMRRDIFF worse. In addition, as shown in Figure 3, the input voltages (V1,V2) can be represented by Common mode input voltage(VCM) and Difference mode input voltage (VDM): V1=VCM+VDM/2 and V2=VCM+VDM/2 VOUT=(V1 V2) G+VREF=VDM G+VREF, where G = R2/R1In order to prevent VOUT from saturating supply rails, itmust be kept within the allowed VOUT range betweenVOL to VOH.

9 The VCM range of the Difference amplifier has beenincreased due to the resistor dividers made by R1, R2,R1* and R2*. In brief, the VDM and VCM of the Difference amplifiermust meet the requirements shown in Equation 2: EQUATION 2:Example 2 Refer to Figure 3 and assume that VDD=16V,VSS=GND, VREF= GND, R2/R1= 1, and the voltagedrop across RSEN is 200 mV. Thus, according to the MCP6H01 data sheet(DS22243), it is VCMRH = VDD = , VCMRL=VSS = Based on Equation 2, the acceptable VCM of theDifference amplifier is from to The advantages and disadvantages of Differenceamplifiers include:a) Advantages:- Reasonable Common mode rejection ratio (CMRRDIFF)- Wide Common mode input voltage range- Low-power consumption, low cost and simplicityb) Disadvantages:- Resistive loading effect - Unbalanced input impedances- Adjust the Difference amplifier s gain by changing more than one resistor valueCMRRDIFF201R2R1------+K------------ ---- log Where: TR= Resistor ToleranceK = Net Matching Toleranceof R2/R1 to R2*/R1*CMRRDIFF (dB) = Common Mode Rejection Ratio of Difference AmplifierK=4TR in the worst-caseWhere: G=R2/R1; Gain of Difference Amplifier VDM=V1 V2.

10 Difference Mode Input Voltage of Difference Amplifier VCM=(V1+V2)/2; Common Mode Input Volt-age of Difference AmplifierVOH= Op Amp High-Level OutputVOL= Op Amp Low-Level OutputVCMRH= Op Amp Common Mode Input Voltage High LimitVCMRL= Op Amp Common Mode Input Voltage Low LimitVCMVCMRLVREF 1R1R2------+ VDM2-----------+VOLVREF G-----------------------------VDM VOHVREF G------------------------------ VCMVCMRHVREF 1R1R2------+ VDM2----------- 2010-2011 Microchip Technology 5AN1332 Three Op Amp Instrumentation AmplifierThe three op amp instrumentation amplifier (3 op ampINA) is illustrated in Figure 4. It amplifies smallDifferential voltages and rejects large Common modevoltages. FIGURE 4:Three Op Amp Instrumentation 3 op amp INA s architecture includes the StageThe first stage is implemented by a pair of high-inputimpedance buffers (A1, A2) and resistors (RF and RG).