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october 2010 doc id 17551 rev 1 1/18 AN3222 application note demonstration board user guidelines for low-side current sensing with the ts507 operational amplifier introduction this application note describes the steval-isq013v1, a demonstration board specifically designed for low-side current sensing with the ts507 operational amplifier. power management mechanisms are found in most electronic systems. current sensing is useful for protecting your applications. the low-side current sensing method consists in placing a sense resistor between the load and the circuit?s ground. the resulting voltage drop is amplified using the ts507. this document describes how to accurately measure the current in your applications. it provides: the advantages of the low-side current sense method. the schematics and layout of the demonstration board. a description of the ts507's main features. a method for selecting the most appropriate components for your application. theoretical and practical results. figure 1. demonstration board www.st.com
advantages of the low-side current sense method AN3222 2/18 doc id 17551 rev 1 1 advantages of the low-side current sense method the common-mode voltage is close to ground, despite the voltage of the power source. therefore, the current sense voltage can be amplified by a low-voltage operational amplifier (no vicm restriction).
AN3222 schematic and layout of the demonstration board doc id 17551 rev 1 3/18 2 schematic and layout of the demonstration board figure 2 represents the board?s schematics. figure 2. demonstration board schematics the demonstration board has the following features. board dimensions: 27 x 24 mm 2-layer pcb pcb thickness: 0.8 mm fr4 material copper thickness: 18 m rg2 rf2 ip vcc c vo u t t s 507 rf1 cf rg1 r s vio in figure 3. demonstration board: top view figure 4. demonstration board: bottom view
ts507 features AN3222 4/18 doc id 17551 rev 1 3 ts507 features the ts507 operates from 2.7 to 5.5 v. the device has a rail-to-rail configuration on both its input and output. at 25c it demonstrates the following features. vio = 25 v typ, 100 v max avd = 131 db (typical vcc = 5 v) gbp = 1.9 mhz vol = 4 mv typ, 15 mv max, with r l = 10 k additional information on the ts507 can be found at: http://www.st.com/stonline/prod ucts/families/amplifiers_compa rators/opamps/ ts507amp.htm
AN3222 selecting the components doc id 17551 rev 1 5/18 4 selecting the components depending on the type of application, various component values can be selected, such as rshunt or resistors for the amplifier gain. to select the correct component values: 1. find the maximum current. this is the maximum current that goes through the sensing resistor (the maximum current to sense in your system). for example: 2. find the correct shunt resistor. this value must be limited to avoid a significant voltage drop (such as 1%) and to limit power dissipation. it must, however, be high enough to obtain better accuracy. for example: vsense_max = 1% voltage, with voltage = 5 v and imax = 1 a rshunt imax vsense_max so rshunt must be lower than or equal to 50 m .. in the current example, rshunt has been set to 30 m . 3. calculate the maximum power dissipation in the shunt resistor. to avo i d damaging the sensing resistor, the shunt resistor has to sustain a suitable wattage. for example: in this case, a 1 w shunt resistor is sufficient. 4. choose the appropriate configuration gain. to avoid saturation: in the current configuration, rg = 100 and voh = 4.985 v (ts507 at 25c, vcc = 5 v). rf must therefore be lower than 16.6 k to avoid saturation of the ts507 at maximum currents. it is recommended to choose the highest possible rf to benefit from the output voltage capability of th e amplifier. selecting rf in the e192 series leads to rf = 16.2 k . imax power_max voltage ? 5 w 5 v ? 1 a === => rshunt 1% 5 v ? 1 a -------------------------- - pmax rshunt imax 2 ? 0.03 1 2 ? 0.03 w === vout rf rg ? () v ? rf rg ? () rshunt i ? ? == vout voh => rf voh rg ? rshunt imax ? ----------------------------------------- - < rf max 4.985 100 ? 0.03 1 ? ------------------------------- - 16.6k ==
selecting the components AN3222 6/18 doc id 17551 rev 1 to minimize the offset caused by the input currents, the feedback resistors must be minimized; the higher rf, the higher the error on iio (see equation 2 on page 7 ). as such, an rg of 100 must be considered (the lower rg, the lower rf). note that if the accuracy obtained is not sufficient, you can go back to step 2. and increase the rshunt value.
AN3222 theoretical and practical measurements doc id 17551 rev 1 7/18 5 theoretical and practical measurements 5.1 theoretical results cf helps to stabilize the oper ational amplifier an d can be ignored fo r the dc analysis. using figure 2 as reference for the components: equation 1 equation 2 this equation can be simplified assuming rf2 = rf1 = rf, and rg2 = rg1 = rg. only errors due to vio and iio remain. if we also consider the errors due to inaccuracies of the resistors, we obtain with a first-order limited development and with: equation 3 as you can see, with correct resistor matching (rf and rg) on the inputs, iib does not have any influence on vout. is the resistance tolerance. for example: 0.1% in our case for rg and rf, and 1% for rshunt. if the accuracy of the resistors rf1, rf2, rg1 and rg2 = 1 and the accuracy of rshunt = 2 , these inaccuracies create a maximum deviation of (2. 1 + 2 )vth as shown in equation 4 . equation 4 equation 3 can be simplified to become: equation 5 figure 5 on page 8 depicts the theoretical behavior of the above-defined application. vout_th , vout_min and vout_max are represented from the left y-axis. vout rs i 1 rg2 rg2 rf2 + ---------------------------- ? ?? ?? 1 rf1 rg1 ----------- + ?? ?? ip + rg2 rf2 ? rg2 rf2 + ----------------------------- ?? ?? 1 rf1 rg1 ----------- + ?? ?? in rf1 vio 1 rf1 rg1 ----------- + ?? ?? ? ? ? ? ? ? = vout rs i rf rg ------- - vio 1 rf rg ------- - + ?? ?? rf iio ? + ? ? ? = vth rs i rf rg ------- - ?? = vout vth 1 rs rs ----------- rf rf rg + -------------------- rf1 rf1 ------------- - rg1 rg1 --------------- ? ?? ?? rg rf rg + -------------------- rf2 rf2 ------------- - rg2 rg2 --------------- ? ?? ?? ++ + ?? ?? rf iio vio 1 rf rg ------- - + ?? ?? ? ? + = r r ------- - vth rs rs ----------- rf rf rg + -------------------- rf1 rf1 ------------- - rg1 rg1 --------------- - + ?? ?? rg rf rg + -------------------- rf2 rf2 ------------- - rg2 rg2 --------------- - + ?? ?? ++ ?? ?? vth 2 rf rf rg + -------------------- 2 1 rg rf rg + -------------------- 2 1 ? + ? + ?? ?? vth 22 1 ? + () = = vout vth % vth rf iio vio 1 rf rg ------- - + ?? ?? ? ? + ? + =
theoretical and practical measurements AN3222 8/18 doc id 17551 rev 1 vout_min and vout_max take into consideration voh, vol and errors caused by inaccuracies of the input currents, vio or the resistors. the measured values should therefore be between these curves. the error on output (in yellow) is represented from the right y-axis and is defined as follows. equation 6 figure 5. theoretical output voltage and error vs. is/imax example: if imax = 1 a, with the same conditions on the resistors as in figure 5 : ? isense = 0.1a, isense/imax = 10% => the error on the measured voltage is lower than 5% at 10% full scale of isense. ? isense = 0.5a, isense/imax = 50% => the error on the measured voltage is lower than 2% at 50% of the full-scale isense. note that when isense is used to its full scale, the offset caused by vio and the input bias current is limited and the errors on the output voltage converge towards the predicted value of 2 0.1% + 1% = 1.2% the accuracy of the measured value depends on the accuracy of the resistors rf, rg and rshunt but also on vsense_max and of course the amplifier. ta b l e 1 shows the maximum error for various configurations of the ts507 while applying the method described in this application note. error % () max vout_max vout_th ? , vout_min vout_th ? () vout_th ------------------------------------------------------------------------------------------------------------------------------- --- - 100 ? = 1 10 100 1000 10000 0.001 0.01 0.1 1 10 0.1 1 10 100 1000 error on vout (%) vout (v) isense/isense_max (%) low side current sensing ts507, vcc = 5 v, rf = 16.2 k, rg = 100 accuracy : 0.1% on rf and rg, 1% on shunt vout_th vout_min vout_max error%
AN3222 theoretical and practical measurements doc id 17551 rev 1 9/18 rs has been calculated for a maximum sense voltage of 50 mv (it represents 1% of the voltage drop for a 5 v voltage source). 5.2 practical results this section summarizes the results of four practical measurements ( figure 6 to figure 9 ). case 1: rshunt = 3 m , rg = 100 , rf = 150 k , max = 1 a case 2: rshunt = 10 m , rg = 100 , rf = 47,5 k , max = 1 a case 3: rshunt = 30 m , rg = 100 , rf = 16,2 k , max = 1 a case 4: rshunt = 100 m , rg = 100 , rf = 4,75 k , max = 1 a for each condition, five ts507 operational amplifiers have been measured with the same board. all resistors have an accuracy of 0.1% except for the shunt resistors, which have an accuracy of 1%. the left part of the figure shows the output voltage versus isense/imax. the maximum and minimum theoretical output voltages are shown in red and blue respectively. these have been calculated using equation 3 on page 7 . you can see that the output voltage of the operational amplifiers is as predicted between these two trends. the right part of the figure shows the absolute error on the output voltage versus isense/imax. the red trend shows the maximum theoretical error that can occur. as expected, all of our measurements are below this trend. the main error is due to vio: the lower vio is, the more accurate the results will be. table 1. maximum error on measured value depending on imax for ts507 imax rs ( )rg ( )rf ( ) isense/imax (%) = 1% = 0.1% = 0.1% 1 3 10 30 100 1 0.05 100 9k76 21 7.6 3.1 1.7 1.4 2 0.02 100 12k1 25.5 9.3 3.6 2 1.4 3 0.01 100 16k2 32 12 4.3 2.3 1.5 4 0.01 100 12k1 25.2 9.2 3.5 2 1.5 5 0.01 100 9k76 22 8.1 3.3 1.9 1.4 7 0.005 100 14k 30.7 11 4.1 2.2 1.5 10 0.005 100 9k76 21.9 8.1 3.3 1.9 1.4 12 0.003 100 13k5 30 10.8 4.1 2.2 1.5 15 0.003 100 10k9 24.2 8.9 3.5 2 1.4 20 0.002 100 12k1 27 9.7 3.8 2.1 1.5 max | error | (%)
theoretical and practical measurements AN3222 10/18 doc id 17551 rev 1 figure 6. case 1: rshunt = 3 m , rg = 100 , rf = 150 k , imax = 1 a 0.01 0.1 1 10 1 10 100 vout (v) isense/imax (%) vout vs isense/imax part0 part1 part2 part3 part4 vout_min vout_max 0.1 1 10 100 1000 1 10 100 error (%) isense/imax (%) absolute error on vout vs isense/imax err_max err0 err1 err2 err3 err4 figure 7. case 2: rshunt = 10 m , rg = 100 , rf = 47.5 k , imax = 1 a 0.01 0.1 1 10 1 10 100 vout (v) isense/imax (%) vout vs isense/imax vout_max vout_min part0 part1 part2 part3 part4 0.0001 0.001 0.01 0.1 1 10 100 1 10 100 error (%) isense/imax (%) absolute error on vo ut vs isense/imax (%) err_max err0 err1 err2 err3 err4 figure 8. case 3: rshunt = 30 m , rg = 100 , rf = 16.2 k , imax = 1 a 0.01 0.1 1 10 1 10 100 vout (v) isense/imax (%) vout vs isense/imax part0 part1 part2 part3 part4 vmax vmin 0.01 0.1 1 10 100 1 10 100 error (%) isense/imax (%) absolute error on vout vs isense/imax err0 err1 err2 err3 err4 err_max
AN3222 theoretical and practical measurements doc id 17551 rev 1 11/18 the graphs show that the higher rshunt is for the same isense_max, the more accurate the results are. this is due to the fact that if th e shunt resistor is small, the schematic gain is high, so vio is more amplified than with a bigger shunt resistor. figure 10 shows the error contribution of each parameter in the overall error for case 3. you can see that for a low isense/imax the maximum error is due to the saturation. then, when isense/imax increases, the maximum error is caused by the vio. after this, the most significant error is caused by the inaccura cy of the shunt resistor. to finish, when isense/imax is high, you are close to the upper rail of the amplifier, therefore the maximum error contribution is due to the saturation. figure 10. contribution of each parameter in the overall error the error due to the avd parameter has not been taken into consideration in these equations, but it is negligible. if the schematic gain equals 162 (in case 3) and the avd equals 100 db, there is an inaccuracy of 0.16%. figure 9. case 4: rshunt = 100 m , rg = 100 , rf = 4.75 k , imax = 1 a 0.01 0.1 1 10 1 10 100 vout (v) isense/imax (%) vout vs isense/imax part0 part1 part2 part3 part4 vout_min vout_max 0.001 0.01 0.1 1 10 100 110100 error (%) isense/imax (%) absolute error on vout vs isense/imax error_max err0 err1 err2 err3 err4 contribution of each parameter in the overall error 0% 20% 40% 60% 80% 100% 0.1 0.182 0.33 0.599 1.087 1.973 3.582 6.504 11.81 21.44 38.93 70.68 is/ismax (%) error (% ) saturation rs accuracy rf and rg accuracy iio vi o
theoretical and practical measurements AN3222 12/18 doc id 17551 rev 1 the following equation demonstrates this. equation 7 for example: you can see that the higher the gain of the schematics, the higher the inaccuracy. nevertheless, the typical avd for the ts507 equals 131 db. with this value, the error is divided by more than 35. vout vs ------------ - rf rg ------- - 1 rf rg avd ? --------------------------- - + ------------------------------------- - = when rf rg ------- - << avd: vout vs ------------ - rf rg ------- - 1 + ------------ rf rg ------- - 1 ? () == error rf rg avd ? --------------------------- - 100% ? = case 3: rf rg ------- - 162 avd = 100 db = error 162 10 5 --------- - 100% = 0.16% ? =
AN3222 frequency behavior doc id 17551 rev 1 13/18 6 frequency behavior this chapter provides different ac cases permitting the filtering of the measurements. to sense in a large bandwidth, the gain of the application must not exceed the capability of the amplifier. if the gain is too big, you will be limited by th e gain-bandwidth product or by the amplifier?s slew rate. as such, for test purposes, we have selected the same conditions as in case 4 ( figure 9 ). rshunt = 100 m rg = 100 and rf = 4.75 k the two first cases deal with the filtering of the measurement for a current source which should be constant. the easiest way to achieve it is to add the capacitor named cf in figure 2 . in figure 11 and figure 12 , you can see that the period of oscillations is t = 300 s, so the frequency equals 3.33 khz. the following equation demonstrates how to select the value of cf to filter the oscillations. equation 8 to efficiently cut this frequency, you have to cut one decade earlier, so you have to set cf to 100 nf. on the left part of figure 11 , the measurement has been performed without the capacitor, and on the right part a capacitor of 100nf has been added. a current of about 1 a is applied. you can see that the signal is correctly filtered by the capacitor. figure 12 shows another example with a lower isense. f 1 2 rf cf ?? --------------------------------- - so cf 1 2 frf ?? ------------------------------ - 1 2 3330 4750 ?? ----------------------------------------------- - 10 nf ==== figure 11. filtering: first example without capacitor with capacitor: cf = 100 nf vout and isense vs time 0 1 2 3 4 5 6 0.0e+00 2.0e-04 4.0e-04 6.0e-04 8.0e-04 ti me (s) vout_th (v) 0 0.3 0.6 0.9 1.2 1.5 1.8 isense (a) vout_th is ens e vout and isense vs time 0 1 2 3 4 5 6 0.0e +00 2.0e -04 4.0e-04 6.0e-04 8.0e-04 ti m e (s) vout (v) 0 0.3 0.6 0.9 1.2 1.5 1.8 isense (a) vout isense
frequency behavior AN3222 14/18 doc id 17551 rev 1 as you can see, the signal is filtered. if you increased the capacitor to 1 f instead of 100 nf you would obtain an even smoother response. this application note shows the theoretical and practical results, but verifying the behavior with the spice model and, above all, checking the results on bench is recommended. figure 12. filtering: second example without capacitor with capacitor: cf = 100 nf vout and isense vs time 0 0.1 0.2 0.3 0.4 0.0e+00 5.0e-04 1.0e-03 1.5e-03 time (s) vout (v) 0 0.05 0.1 0.15 0.2 isense (a) vout isense vout and isense vs time 0 0.1 0.2 0.3 0.4 0.0e+00 5.0e-04 1.0e-03 1.5e-03 time (s) vout (v) 0 0.05 0.1 0.15 0.2 isense (a) vout isense
AN3222 bill of materials doc id 17551 rev 1 15/18 7 bill of materials table 2. bill of materials part footprint description value qty ts507 sot23-5 precision amplifier ts507 1 rf 0603 resistor 0.1% 16.2 k (1) 1. to choose the correct component values, refer to chapter 4 . the default value has been chosen for a power source of 5 v, sourcing a maximum current of 1 a. 2 rg 0603 resistor 0.1% 100 2 rs (2) 2. only one shunt resistor is required, several footprints are available. 2512 resistor 1% 30 m (1) 1 9.4 x 9.1 mm resistor 1% 30 m (1) strap resistor 1% 30 m (1) cf 0603 capacitor 1 nf (1) 1 c 0603 decoupling capacitor 1 f 1
conclusion AN3222 16/18 doc id 17551 rev 1 8 conclusion this document provides the information necessary to develop your low-side current sensing application using the ts507. you can accurately measure current with a limited number of components even if the sense current is noisy. with the theoretical equations provided, you can easily predict the maximum error on the output voltage. to minimize errors, you must select the components correctly according to the parameters described in this application note.
AN3222 revision history doc id 17551 rev 1 17/18 9 revision history table 3. document revision history date revision changes 25-oct-2010 1 initial release.
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