dm00108908

UM1737
User manual
How to use the product evaluation board STEVAL-ISQ014V1 for
low-side current sensing with the TSZ121 operational amplifier
Introduction
This document describes how to use the product evaluation board STEVAL-ISQ014V1 for
low-side current sensing with the TSZ121 operational amplifier (op amp).
Power management mechanisms are found in most electronic systems and power
protection is of vital importance for them. One useful method of protecting an application is
by sensing the current. The low-side current sensing method consists of placing a sense
resistor between the load and the ground of the circuit. The resulting voltage drop is
amplified using the TSZ121.
This user manual describes how to accurately measure the current in your application and
describes the advantages of the low-side current sensing method. In addition, it provides:
• 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
May 2014
DocID025983 Rev 1
1/14
www.st.com
Contents
UM1737
Contents
1
Advantages of the low-side current sensing method . . . . . . . . . . . . . . 3
2
STEVAL-ISQ014V1 product evaluation board schematic . . . . . . . . . . . 3
3
How to choose the right components for your application . . . . . . . . . 4
4
Theoretical and practical measurements . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1
Theoretical measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.2
Practical measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5
Frequency behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
6
Bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
8
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2/14
DocID025983 Rev 1
UM1737
1
Advantages of the low-side current sensing method
Advantages of the low-side current sensing method
With the low-side current sensing method, the common mode voltage of the op amp is close
to ground, regardless of the voltage of the power source. Therefore, the current voltage that
is sensed can be amplified by a low input rail operational amplifier (there is no need for a
rail-to-rail input op amp).
The TSZ121 is a very accurate op amp, which allows you to measure the current through
your application precisely with a smaller shunt value. Thus, the dissipated power is also
reduced.
If you need to sense the current with the high-side sensing method, STMicroelectronics also
provides the appropriate products with the TSC series.
2
STEVAL-ISQ014V1 product evaluation board
schematic
Figure 2 shows the STEVAL-ISQ014V1 schematic. The voltage drop through the shunt
resistor Rs, created by the current Imeas, is amplified by the TSZ121.
Figure 2. STEVAL-ISQ014V1 schematic
,PHDV
9FF
5I
5J
&
,LES
9RXW
9LR
5V
76=
,LEQ
5J
5I
&I
1. Imeas = current, Rs = shunt resistor, Rg, Rf = resistors, Iibp, Iibn = input currents, C = capacitor
The TSZ121 is a very accurate op amp which operates from 1.8 V to 5.5 V. It has a rail-torail configuration on both its input and output. At 25 °C, it demonstrates the following
features:
•
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.
DocID025983 Rev 1
3/14
14
How to choose the right components for your application
3
UM1737
How to choose the right components for your
application
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 significant 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 mΩ. 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 good signal-to-noise ratio. Thus, the power
dissipation is limited and the shunt resistor price is reduced.
4/14
DocID025983 Rev 1
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 kΩ 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 lower 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
Rg2
Rf1
Vout = Rs × I × ⎛⎝ 1 – ----------------------------⎞⎠ × ⎛⎝ 1 + -----------⎞⎠ + Iibp ×
Rg2 + Rf2
Rg1
× Rf2
Rf1 ⎞
Rf1
⎛ Rg2
-----------------------------⎞ ⎛
- – Iibn × Rf1 – Vio × ⎛⎝ 1 + -----------⎞⎠
⎝ Rg2 + Rf2 ⎠ × ⎝ 1 + ---------Rg1⎠
Rg1
Equation 1 can be simplified as Equation 2 assuming that Rf2 = Rf1 = Rf and Rg2 = Rg1 =
Rg.
Equation 2
Rf
Rf
Vout = Rs × I × -------- – Vio × ⎛⎝ 1 + --------⎞⎠ + Rf × Iio
Rg
Rg
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 represents the input offset voltage. To obtain a
precision current sensing solution, the Vio should be as low as possible. For the TSZ121,
Vio is equal to 5 µV max which corresponds to a very high accuracy.
DocID025983 Rev 1
5/14
14
Theoretical and practical measurements
UM1737
Considering the errors due to resistor inaccuracy, Equation 3 is obtained.
Equation 3
Rf
Rg
ΔRf1 ΔRg1
ΔRs
ΔRf2 ΔRg2
Rf
Vout = Vth × ⎛ 1 + ----------- + -------------------- ⎛⎝ -------------- – ---------------⎞⎠ + -------------------- ⎛ -------------- – ---------------⎞ ⎞ + Rf × Iio – Vio ⎛ 1 + --------⎞
⎝
⎝
Rf + Rg ⎝ Rf2
Rg1
Rs Rf + Rg Rf1
Rg2 ⎠ ⎠
Rg⎠
Where:
Vth = Rs x I x (Rf/Rg)
∆R/R is the resistance tolerance. In the current application example, the tolerance of Rg and
Rf is 0.1 % and that of Rshunt is 1 %.
If the accuracy of the resistors Rf1, Rf2, Rg1, and Rg2 = ԑ1 and the accuracy of
Rshunt = ԑ2, there is a maximum deviation of (2.ԑ1 + ԑ2) x Vth as shown in Equation 4.
Equation 4
Rf
Rg
ΔRs
ΔRf1 ΔRg1
ΔRf2 ΔRg2
Vth ⎛ ----------- + -------------------- ⎛ -------------- + ---------------⎞ + -------------------- ⎛ -------------- + ---------------⎞ ⎞ = Vth ( 2ε 1 + ε 2 )
⎝ Rs Rf + Rg ⎝ Rf1
Rg1 ⎠ Rf + Rg ⎝ Rf2
Rg2 ⎠ ⎠
Equation 3 can be simplified as Equation 5
Equation 5
Rf
Vout = Vth ( 1 + 2ε 1 + ε 2 ) + Rf × Iio – Vio ⎛⎝ 1 + --------⎞⎠
Rg
Figure 3 shows the theoretical behavior of the above defined application.
Vout_th, Vout_min, and Vout_max are referred to the left Y-axis.
Vout_min and Vout_max take into consideration the Voh and Vol errors due to input
currents, Vio, and resistor inaccuracy. In practice, the measured values should be between
the Vout_min and Vout_max curves.
Figure 3. Theoretical output voltage and error vs. Is/Imax
10000
10
Vout_max
1000
Vout_th
100
0.1
Maximum theoretical error on Vout
0.01
10
Vout_min
1E-3
100m
1
1
10
Isense/Isense_max (%)
6/14
DocID025983 Rev 1
100
Error on Vout (%)
Vout (V)
1
UM1737
Theoretical and practical measurements
The maximum theoretical error on the output voltage (shown in green) is referred to the right
Y-axis and is defined in Equation 6.
Equation 6
Max ( Vout_max – Vout_th , Vout_min – Vout_th )
Error ( % ) = ------------------------------------------------------------------------------------------------------------------------------------- × 100
Vout_th
Example:
If Imax = 1A with the same conditions on the resistors as shown in Figure 3, the following
applies:
Note:
•
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 %.
When Isense is used at 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 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.
Table 1. Maximum error on the measured value (depending on Imax) for the TSZ121
Maximum absolute error (%)
Imax Rs (Ω)
(A) ε = 1 %
Rg (Ω)
ε = 0.1 %
Rf (kΩ)
ε = 0.1 %
Isense/Imax Isense/Imax Isense/Imax Isense/Imax Isense/Imax
1%
3%
10 %
30 %
100 %
1
0.05
100
9.76
2.2
1.5
1.3
1.2
1.2
2
0.02
100
12.1
2.5
1.6
1.3
1.2
1.2
3
0.01
100
16.2
2.9
1.8
1.4
1.3
1.2
4
0.01
100
12.1
2.5
1.6
1.3
1.2
1.2
5
0.01
100
9.76
2.2
1.5
1.3
1.2
1.2
7
0.005
100
14
2.7
1.7
1.3
1.2
1.2
10
0.005
100
9.76
2.2
1.5
1.3
1.2
1.2
12
0.003
100
13.3
2.6
1.7
1.3
1.2
1.2
15
0.003
100
10.7
2.3
1.6
1.3
1.2
1.2
20
0.002
100
12.1
2.5
1.6
1.3
1.2
1.2
Rs is calculated for a maximum sense voltage of 50 mV. It represents 1 % of the
voltage drop for a 5 V voltage source.
DocID025983 Rev 1
7/14
14
Theoretical and practical measurements
4.2
UM1737
Practical measurements
This section summarizes the results of several 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 minimum 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
the TSZ121
10000
10
1000
Vout_max
Absolute error on Vout (%)
Vout (V)
1
Figure 5. Absolute error on the output voltage
vs. Isense/Imax using the TSZ121
Vout_opamps
0.1
0.01
Theoretical maximum error
100
Error on opamps output
10
1
0.1
Vout_min
1E-3
100m
1
10
100
0.01
100m
Isense/Isense_max (%)
8/14
1
10
Isense/Isense_max (%)
DocID025983 Rev 1
100
UM1737
Theoretical and practical measurements
Figure 6. Output voltage vs. Isense/Imax using
an alternative op amp
Figure 7. Absolute error on the output voltage
vs. Isense/Imax using an alternative op amp
10000
10
Vout_opamps
Absolute error on Vout (%)
1000
Vout (V)
1
0.1
Vout_max
0.01
Vout_min
1E-3
100m
1
10
100
10
1
0.1
0.01
100m
100
Theoretical maximum error
Isense/Isense_max (%)
Error on opamps output
1
10
100
Isense/Isense_max (%)
Figure 8 and Figure 9 show the error contribution of each parameter to overall error for case
1 and case 2.
For a low Isense/Isense_max the error is mainly due to the saturation. When
Isense/Isense_max increases, the error is mainly caused by the Vio. After this, the most
significant error contribution is caused by the inaccuracy of the shunt resistor. Finally, when
Isense/Isense_max is high, close to or above the upper rail of the amplifier, the maximum
error contribution is the saturation.
Figure 8. Contribution of each parameter to
overall error for the TSZ121
Figure 9. Contribution of each parameter to
overall error for a 1 mv Vio op amp
100
100
TSZ121
1mV Vio op-amp
80
Error contribution (%)
Error contribution (%)
80
60
40
Saturation
Rs accuracy
Rf and Rg accuracy
Iio
Vio
20
60
Saturation
Rs accuracy
Rf and Rg accuracy
Iio
Vio
40
20
0
0
0.1
1
10
100
0.1
Isense/Isense_max (%)
1
10
100
Isense/Isense_max (%)
The error due to the AVD parameter has not been taken into consideration in this document,
because it is negligible. If the schematic gain equals 470 (gain used for the measurements)
and the AVD equals 120 dB, there is an inaccuracy of 0.047%.
Equation 7 demonstrates this.
DocID025983 Rev 1
9/14
14
Theoretical and practical measurements
UM1737
Equation 7
Rf
-------Vout
Rg
---------------- = --------------------------Rs × I
Rf
1 + -------Rg
1 + -----------------AVD
When:
Rf/Rg << AVD
Rf
-------Rg
Rf
Vout
---------------- = ------------ ≈ -------- ( 1 – ε )
Rg
1+ε
Rs × I
Rf
1 + -------Rg
Error ( % ) = – ------------------ × 100
AVD
Example:
Rf/Rg = 470
AVD = 120 dB (the minimum value for the TSZ121)
Rf
1 + -------Rg
Error = – ------------------ × 100 = – 0.047%
AVD
Clearly, the higher the gain of the schematics, the higher the inaccuracy, but in this
case the error is negligible.
10/14
DocID025983 Rev 1
UM1737
5
Frequency behavior
Frequency behavior
This section describes how the measurements are filtered.
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, the application is limited by the gain-bandwidth product or
by the slew rate of the amplifier. For test purposes, the same conditions as for DC
measurements are selected:
Rshunt = 10 mΩ, Rg = 100 Ω, and Rf = 47 kΩ.
This example deals with filtering the measurement of an AC current source which has
overlapping oscillations. The easiest way to achieve overlapping oscillations is to add a
capacitor e.g. see the Cf capacitor in Figure 2.
Figure 10 and Figure 11 show that the period of oscillations is T = 300 μs. In order to
efficiently filter 3.3 kHz, we can cut by a factor of ten earlier, and thus choose fc = 330 Hz.
Thus, choose:
1
1
Cf = ------------------------------- = ------------------------------------------------ = 10nF
2π × f c × R f
2π × 330 × 47000
In Figure 10, the measurement was performed without the capacitor.
In Figure 11 a capacitor of 10 nF was added. An AC current from 0.4 A to 1 A was then
applied. The signal is correctly filtered by the capacitor and the overlapping oscillations on
the current source are no longer visible on the op amp output voltage.
Figure 10. Filtering without the Cf capacitor
Figure 11. Filtering with Cf capacitor = 10 nF
2.5
5
2.5
5
2.0
4
2.0
3
1.5
3
1.5
2
1.0
2
1.0
1
Isense
0
0
10
Vout (V)
4
0.5
1
0.0
20
0
Isense
0
Time (ms)
Note:
10
Isense (A)
op-amp output
Isense (A)
Vout (V)
op-amp output
0.5
0.0
20
Time (ms)
Verification of this behavior with the spice model and, above all, checking the results on the
bench is recommended.
DocID025983 Rev 1
11/14
14
Bill of materials
6
UM1737
Bill of materials
Table 2. Bill of materials
Part
Footprint
Description
Value
Qty
TSZ121
SOT23-5
Very high accurate amplifier
TSZ121ILT
1
(1)
Rf
0603
0.1 % resistor
47 kΩ
2
Rg
0603
0.1 % resistor
100 Ω
2
Rs
9.4 x 9.1 mm
1 % 4-terminal shunt resistor
10 mΩ(1)
1
Cf
0603
Capacitor
33 nF
1
C1
0603
Decoupling capacitor
22 nF
1
1. To choose the correct component values, refer to Section 3. The default value has been chosen for a
power source of 5 V, sourcing a maximum current of 1 A
7
Conclusion
This document provides the information necessary to develop a low-side current sensing
application using the TSZ121. You can accurately measure current with a limited number of
components even if the sense current is noisy. Moreover, using the TSZ121 high precision
op amp allows you to reduce the shunt resistor value and to increase the schematic gain
without losing accuracy. Thanks to a lower shunt value, there is a lower dissipated power
within the shunt resistor. Consequently, the shunt resistor’s size can be reduced and the
customer can save money.
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 Section 3.
12/14
DocID025983 Rev 1
UM1737
8
Revision history
Revision history
Table 3. Document revision history
Date
Revision
23-May-2014
1
Changes
Initial release
DocID025983 Rev 1
13/14
14
UM1737
Please Read Carefully:
Information in this document is provided solely in connection with ST products. STMicroelectronics NV and its subsidiaries (“ST”) reserve the
right to make changes, corrections, modifications or improvements, to this document, and the products and services described herein at any
time, without notice.
All ST products are sold pursuant to ST’s terms and conditions of sale.
Purchasers are solely responsible for the choice, selection and use of the ST products and services described herein, and ST assumes no
liability whatsoever relating to the choice, selection or use of the ST products and services described herein.
No license, express or implied, by estoppel or otherwise, to any intellectual property rights is granted under this document. If any part of this
document refers to any third party products or services it shall not be deemed a license grant by ST for the use of such third party products
or services, or any intellectual property contained therein or considered as a warranty covering the use in any manner whatsoever of such
third party products or services or any intellectual property contained therein.
UNLESS OTHERWISE SET FORTH IN ST’S TERMS AND CONDITIONS OF SALE ST DISCLAIMS ANY EXPRESS OR IMPLIED
WARRANTY WITH RESPECT TO THE USE AND/OR SALE OF ST PRODUCTS INCLUDING WITHOUT LIMITATION IMPLIED
WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE (AND THEIR EQUIVALENTS UNDER THE LAWS
OF ANY JURISDICTION), OR INFRINGEMENT OF ANY PATENT, COPYRIGHT OR OTHER INTELLECTUAL PROPERTY RIGHT.
ST PRODUCTS ARE NOT DESIGNED OR AUTHORIZED FOR USE IN: (A) SAFETY CRITICAL APPLICATIONS SUCH AS LIFE
SUPPORTING, ACTIVE IMPLANTED DEVICES OR SYSTEMS WITH PRODUCT FUNCTIONAL SAFETY REQUIREMENTS; (B)
AERONAUTIC APPLICATIONS; (C) AUTOMOTIVE APPLICATIONS OR ENVIRONMENTS, AND/OR (D) AEROSPACE APPLICATIONS
OR ENVIRONMENTS. WHERE ST PRODUCTS ARE NOT DESIGNED FOR SUCH USE, THE PURCHASER SHALL USE PRODUCTS AT
PURCHASER’S SOLE RISK, EVEN IF ST HAS BEEN INFORMED IN WRITING OF SUCH USAGE, UNLESS A PRODUCT IS
EXPRESSLY DESIGNATED BY ST AS BEING INTENDED FOR “AUTOMOTIVE, AUTOMOTIVE SAFETY OR MEDICAL” INDUSTRY
DOMAINS ACCORDING TO ST PRODUCT DESIGN SPECIFICATIONS. PRODUCTS FORMALLY ESCC, QML OR JAN QUALIFIED ARE
DEEMED SUITABLE FOR USE IN AEROSPACE BY THE CORRESPONDING GOVERNMENTAL AGENCY.
Resale of ST products with provisions different from the statements and/or technical features set forth in this document shall immediately void
any warranty granted by ST for the ST product or service described herein and shall not create or extend in any manner whatsoever, any
liability of ST.
ST and the ST logo are trademarks or registered trademarks of ST in various countries.
Information in this document supersedes and replaces all information previously supplied.
The ST logo is a registered trademark of STMicroelectronics. All other names are the property of their respective owners.
© 2014 STMicroelectronics - All rights reserved
STMicroelectronics group of companies
Australia - Belgium - Brazil - Canada - China - Czech Republic - Finland - France - Germany - Hong Kong - India - Israel - Italy - Japan Malaysia - Malta - Morocco - Philippines - Singapore - Spain - Sweden - Switzerland - United Kingdom - United States of America
www.st.com
14/14
DocID025983 Rev 1
Similar pages