UM1737 STMICROELECTRONICS | Alldatasheet
Document overview
- Manufacturer or author: Provided By ALLDATASHEET.COM(FREE DATASHEET DOWNLOAD SITE)
- PDF pages: 14
Technical content
Datasheet sections
- 1 Advantages of the low-side current sensing method
- 2 STEVAL-ISQ014V1 product evaluation board schematic
- 3 How to choose the right components for your application
- 4 Theoretical and practical measurements
- 4.1 Theoretical measurements
- 4.2 Practical measurements
- 5 Frequency behavior
- 6 Bill of materials
- 7 Conclusion
- 8 Revision history
low-side current sensing with the TSZ121 operational amplifier (op amp).
- the schematics of the STEVAL-ISQ014V1 evaluation board
- a method for selecting the most appropriate components for your application
- theoretical and practical results
Figure 1. STEVAL-ISQ014V1 product evaluation board
1 Advantages of the low-side current sensing method
provides the appropriate products with the TSC series.
2 STEVAL-ISQ014V1 product evaluation board
resistor Rs, created by the current Imeas, is amplified by the TSZ121. Figure 2. STEVAL-ISQ014V1 schematic
- Imeas = current, Rs = shunt resistor, Rg, Rf = re sistors, Iibp, Iibn = input currents, C = capacitor
- Vio = 5 µV (max)
- AVD = 135 dB
- GBP = 400 kHz
- Vol = 30 mV (max) with Rl = 10 kΩ Further details on this op amp can be found at www.st.com. ,PHDV ,LES ,LEQ 9LR 9RXW 9FF 76=
How to choose the right components for your application UM1737
3 How to choose the right components for your
Various component values can be selected for your application. They include:
- Rshunt
- resistors for the amplifier gain The four steps below describe how 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). Example Imax = Power_max/Voltage = 5 W/5 V = 1 A 2. Find the correct shunt resistor This value has to be limited to avoid a signif icant voltage drop (for example, 1 % of the application voltage) and to limit the power dissipation. It must, however, be high enough to obtain good accuracy. Example Vsense_max = 1 % voltage, with voltage = 5 V and Imax = 1 A Rshunt x Imax ≤ Vsense max => Rshunt ≤ (1 % x 5 V)/1 A So, Rshunt must be lower than or equal to 50 m Ω. In the current application example, Rshunt has been set to 10 mΩ. 3. Calculate the maximum power dissipation in the shunt resistor To avoid damaging the shunt resistor itself, the shunt resistor has to sustain a suitable wattage. Example: Pmax = Rshunt x I² = 0.01 x 1² = 0.01 W Another advantage of using the high accuracy TSZ121 op amp is that it allows you to amplify small signals while maintaining a go od signal-to-noise ratio. Thus, the power dissipation is limited and the shunt resistor price is reduced.
UM1737 Theoretical and practical measurements 4. Choose the appropriate configuration gain Vout = (Rf/Rg) x Rshunt x I To avoid saturation: Vout ≤ Voh => Rf < (Voh x Rg)/(Rshunt x Imax) In the current application configuration, Rg = 100 Ω and Voh = 4.970 V (TSZ121 at 25 °C, Vcc = 5 V). Therefore, Rf max = (4.970 x 100)/(0.01 x 1) = 49.7 kΩ Consequently, Rf must be lower than 49.7 k Ω to avoid saturation of the TSZ121 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 E24 series leads to an Rf of 47 kΩ. To minimize the offset caused by the input currents, the feedback resistors must be minimized. The higher the Rf, the higher the error due to Iio (see Section 4.1: Theoretical measurements). An Rg of 100 Ω must be considered (the lo wer Rg, the lower Rf) but, Rf should not be so low that the output saturation voltages cannot be increased. Note: If the accuracy obtained is not sufficient, go back to step 2 and increase the Rshunt value.
4 Theoretical and practical measurements
4.1 Theoretical measurements
Cf can be ignored for the DC analysis. Equation 1 can be calculated using Figure 2 as a reference for the components. Equation 1 Equation 1 can be simplified as Equation 2 assuming that Rf2 = Rf1 = Rf and Rg2 = Rg1 = Rg. Equation 2 Thanks to the good matching of resistors Rf and Rg on the inputs, Iib has no affect on Vout. In Equation 2, the only error remaining is due to Vio and Iio. The Iio represents the input offset current (Iio = Iibp - Iibn) and the Vio re presents the input offset voltage. To obtain a precision current sensing solution, the Vio sh ould be as low as possible. For the TSZ121, Vio is equal to 5 µV max which corresponds to a very high accuracy. Vout Rs I × 1 Rg2 ⎛⎞× 1 Rf1 ⎛⎞× Iibp Rg2 Rf2× ⎛⎞ 1 Rf1 ⎛⎞×× Iibn Rf1 Vio 1 Rf1 Vout Rs I Rf
Y-axis and is defined in Equation 6.
- For Isense = 10 mA, Isense/Imax = 1 %, the maximum error on the measured voltage is lower than 7 %.
- For Isense = 70 mA to 1 A (7 % to 100 % of Imax), the maximum error on the measured voltage is lower than 2 %. Note: When Isense is used at its full scale, the offs et caused by Vio and the input bias current is limited and the errors on the output voltage converge towards the predicted value 2 x 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. Table 1 shows the maximum error for various configurations of the TSZ121 while applying the methods described in this user manual. Rs is calculated for a maximum sense voltage of 50 mV. It represents 1 % of the voltage drop for a 5 V voltage source. Error %() Max Vout_max Vout_th– Vout_min Vout_th–,()
Table 1. Maximum error on the measured value (depending on Imax) for the TSZ121
4.2 Practical measurements
- Case 1, use of the TSZ121 op amp which has a maximum Vio of 5 µV for Vcc = 5 V and Vicm = 2.5 V (see Figure 4 and Figure 5).
- Case 2, use of another op amp which has a maximum Vio of 1 mV (see Figure 6 and Figure 7) In both cases, five devices were measured on different boards using the following component values:
- Rshunt = 10 mΩ
- Rg = 100 Ω
- Rf = 47 kΩ
- Imax = 1 A All resistors had an accuracy of 0.1 % except for the shunt resistors which had an accuracy of 1 %. Figure 4 and Figure 6 show the output voltage versus Isense/Imax (100 % means that Isense = Imax = 1 A). The maximum and minimu m theoretical output voltage trends are shown in red and blue respectively. These have been calculated using Equation 3 . The output voltage of the op amps is, as predicted, between these two trends. Figure 5 and Figure 7 show the absolute error on the output voltage versus Isense/Imax. The red trend shows the maximum theoretical error that can occur. As expected, all ST measurements are below this trend. The main error contribution is due to the Vio i.e. the lower Vio is, the more accurate the results are. This is why it is important to use a very accurate op amp such as the TSZ121. Figure 4 and Figure 5 show that thanks to the TSZ121, it is possible to highly reduce the inaccuracy on the current measurement. This is especially true when there is a small signal that has to be amplified (because with the signal amplification, an error due to the amplification of the Vio is added).
Figure 4. Output voltage vs. Isense/Imax using Figure 5. Absolute error on the output voltage
Theoretical and practical measurements UM1737 Equation 7 When: Rf/Rg << AVD Example: Rf/Rg = 470 AVD = 120 dB (the minimum value for the TSZ121) Clearly, the higher the gain of the schematics, the higher the inaccuracy, but in this case the error is negligible. Vout Rf 1 Rf Vout Rf Error %() 1 Rf Error 1 Rf
5 Frequency behavior
This section describes how the measurements are filtered. capacitor e.g. see the Cf capacitor in Figure 2. efficiently filter 3.3 kHz, we can cut by a factor of ten earlier, and thus choose fc = 330 Hz. In Figure 10, the measurement was performed without the capacitor. the current source are no longer visible on the op amp output voltage. Figure 10. Filtering without the Cf capacitor F igure 11. Filtering with Cf capacitor = 10 nF
6 Bill of materials
7 Conclusion
the parameters described in Section 3. Table 2. Bill of materials
- To choose the correct component values, refer to Section 3. The default value has been chosen for a
8 Revision history
Table 3. Document revision history