AN3222 STMICROELECTRONICS | Alldatasheet
Document overview
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Technical content
designed for low-side current sensing with the TS507 operational amplifier. drop is amplified using the TS507.
- 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
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).
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 µm Rg2 Rf2 Ip Vcc c Vout TS507 Rf1 Cf Rg1 Rs Vio In
Figure 3. Demonstration board: top view Figure 4. Demonstration board: bottom view
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 25°C it demonstrates the following features.
- Vio = 25 µV typ, 100 µV max
- AVD = 131 dB (typical Vcc = 5 V)
- GBP = 1.9 MHz
- Vol = 4 mV typ, 15 mV max, with RL = 10 kΩ Additional information on the TS507 can be found at: http://www.st.com/stonline/products/families/amplifiers_comparators/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 mΩ.. In the current example, Rshunt has been set to 30 mΩ. 3. Calculate the maximum power dissipation in the shunt resistor. To avoid 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 Ω and Voh = 4.985 V (TS507 at 25°C, Vcc = 5 V). Rf must therefore be lower than 16.6 kΩ 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 the amplifier. Selecting Rf in the E192 series leads to Rf = 16.2 kΩ. Imax Power_max voltage⁄ 5 W 5 V⁄ 1 A== = => Rshunt 1% 5 V⋅ Pmax Rshunt Imax 2⋅ 0.03 1 2⋅ 0.03 W== = Vout Rf Rg ⁄() V⋅ Rf Rg⁄() Rshunt I⋅⋅== Vout Voh => Rf Voh Rg⋅ Rf max 4.985 100⋅
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 Ω 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 operational amplifier and can be ignored for 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 = ε 1 and the accuracy of Rshunt = ε2, these inaccuracies create a maximum deviation of (2.ε1 + ε2)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 ⎛⎞ 1 Rf1 ⎛⎞ Ip+ Rg2 Rf2⋅ ⎛⎞ 1 Rf1 ⎛⎞ In Rf1 Vio 1 Rf1 Vout Rs I Rf Vth Rs I Rf Vout Vth 1 ΔRs ⎛⎞ Rg ⎛⎞++ +⎝⎠ ⎛⎞ Rf Iio Vio 1 Rf ⎛⎞–⋅+= ΔR Vth ΔRs ⎛⎞ Rg ⎛⎞++⎝⎠ ⎛⎞ Vth ε 2 Rf Vth ε 22 ε 1⋅+()= Vout Vth ε % Vth Rf Iio Vio 1 Rf
The error on output (in yellow) is represented from the right Y -axis and is defined as follows. Figure 5. Theoretical output voltage and error vs. Is/Imax => the error on the measured voltage is lower than 5% at 10% full scale of Isense.
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 mΩ, Rg = 100 Ω, Rf = 150 kΩ, max = 1 A
- Case 2: Rshunt = 10 mΩ, Rg = 100 Ω, Rf = 47,5 kΩ, max = 1 A
- Case 3: Rshunt = 30 mΩ, Rg = 100 Ω, Rf = 16,2 kΩ, max = 1 A
- Case 4: Rshunt = 100 mΩ, Rg = 100 Ω, Rf = 4,75 kΩ, 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. Y ou 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
Theoretical and practical measurements AN3222 12/18 Doc ID 17551 Rev 1 The following equation demonstrates this. Equation 7 For example: Y ou 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 Rf 1 Rf Rf Error Rf Case 3: Rf Error 162
6 Frequency behavior
This chapter provides different AC cases permitting the filtering of the measurements. The following equation demonstrates how to select the value of Cf to filter the oscillations. and on the right part a capacitor of 100nF has been added. A current of about 1 A is applied. Y ou can see that the signal is correctly filtered by the capacitor. Figure 12 shows another example with a lower Isense. Figure 11. Filtering: first example
you would obtain an even smoother response. with the spice model and, above all, checking the results on bench is recommended. Figure 12. Filtering: second example
7 Bill of materials
Table 2. Bill of materials
- 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.
- Only one shunt resistor is required, several footprints are available.
2512 Resistor 1% 30 m Ω (1) 1
8 Conclusion
This document provides the information necessary to develop your low-side current sensing application using the TS507. Y ou 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.
9 Revision history
Table 3. Document revision history 25-Oct-2010 1 Initial release.