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
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www.st.com
Contents
UM1737
Contents
1
2
3
4
Advantages of the low-side current sensing method . . . . . . . . . . . . . . 3
STEVAL-ISQ014V1 product evaluation board schematic . . . . . . . . . . . 3
How to choose the right components for your application . . . . . . . . . 4
Theoretical and practical measurements . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1
4.2
Theoretical measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Practical measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5
6
7
8
Frequency behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
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Advantages of the low-side current sensing method
1
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
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-to-
rail 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.
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How to choose the right components for your application
UM1737
3
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.
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4.
Theoretical and practical measurements
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
4.1
Theoretical and practical measurements
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
⎠
Rf1
Rf1
⎛
Rg2
×
Rf2
⎞ × ⎛
1
+ ----------
⎞
–
Iibn
×
Rf1
–
Vio
× ⎛
1
+ ----------
⎞
----------------------------
-
-
⎠
-
⎝
Rg2
+
Rf2
⎠ ⎝
⎝
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.
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