Current sensing configurations and topologies
Low-side configuration
The current is sensed between the ground and the load. The voltage seen on the current sensing input pins is close to ground. The Vicm in this case is the Ground.
The current is sensed between the ground and the load. The voltage seen on the current sensing input pins is close to ground. The Vicm in this case is the Ground.
High-side configuration
The current is sensed between the supply voltage and the load. The voltage seen on the current sensing input pins is close to the supply voltage. The Vicm in this case is the supply voltage.
The current is sensed between the supply voltage and the load. The voltage seen on the current sensing input pins is close to the supply voltage. The Vicm in this case is the supply voltage.
High-side advantage
High-side current sensing can detect a load short to ground
High-side current sensing can detect a load short to ground
High-side vs low-side configuration
Summary table of Pros and Challenge between high-side and low-side configuration
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Summary table of Pros and Challenge between high-side and low-side configuration
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Unidirectional or bidirectional sensing
Unidirectional operation
Unidirectional mode of operation allows the device to measure the current through a shunt resistor in one direction only. The output reference can be ground or Vcc and can be set by using Vref1 and Vref2 pins for adjustment. If no vref pin is present, it certainly means that the CS is unidirectional only.
Ground referenced
In this configuration Vref1 pin and Vref2 pin are connected to the ground. The output common mode voltage is then automatically set to GND when no current flows through the Rshunt resistance. This configuration allows the full-scale output in unidirectional mode. It allows a current to be measured as described in Figure 1.
Figure 1: Output reference to ground.
Vcc referenced
In this configuration Vref1 pin and Vref2 pin are connected to Vcc. The output common mode voltage is then automatically set to Vcc when no current flows through the Rshunt resistance. This configuration allows the full-scale output in unidirectional mode. It allows a current to be measured as described in Figure 2.
Figure 2: Output reference to Vcc.
Bidirectional operation
Bidirectional mode of operation allows the device to measure currents through a shunt resistor in two directions. The output reference can be set anywhere within the power supply range. If the output common-mode voltage is set at mid-range, the full-scale current measurement range is equal in both directions. This is achieved by connecting one Vref pin to Vcc and the other Vref pin to Gnd as described in Figure 3. It can be done as well connecting both Vref pins to Vcc / 2 voltage as described by Figure 4.
In case the current measurement is not equal in both directions, users can set the output in a non-symmetrical configuration, adjusting Vref according to the user's needs.
Split supply
The great advantage of this configuration is that the device can be used in bidirectional mode with an output common-mode voltage set at the middle of scale, with an accuracy of 0.1%, very good accuracy without any added external component or power supply. This configuration creates a midscale offset ratiometric to the power supply.
Figure 3: Split supply.
External supply
In this configuration, Vref1 pin and Vref2 pin are connected to a reference voltage. The output common-mode voltage is then automatically set to this reference voltage value when no current flows through the Rshunt resistance. This configuration adjusts the output offset as needed by the application. A DAC for calibration of the analog chain could also be used.
Figure 4: External supply.
Position of current sensing: pros and cons
| High side Amp High side current sensing Pro Short cut detection Cons Only global current information |  |
| Low side Amp High slew rate op amp Pro Simple & cheaper Cons Drive current ≠ Phase current |  |
| In- line Amp Dedicated high side current sensing Pro True phase current continuous Cons Support Common mode voltage transient |  |
Rsense selection
Case study by using the TSC2011 and TSC2012 bidirectional current sense amplifiers
The selection of the shunt resistor is a tradeoff between the dynamic range and power dissipation.
Generally, in high current sensing application, the main focus is to reduce as much as possible the power dissipation (I²R) by choosing the smallest value of shunt. It could be quite easy if a full-scale current to measure is small. In low current applications the Rsense value could be higher, to minimize the impact of the offset voltage on the circuit. Due to input bias current of several μA, the TSC2011 cannot measure the current in the same range, when the common mode voltage overpasses the power supply voltage (refer to section about theory of operation).
The tradeoff is mainly when a dynamic range of current to measure is large, meaning ability to measure with the same shunt value from low current to high current. Generally, the current full scale (Imax-Imin) defines the shunt value thanks to the full output voltage range, the gain of the TSC2011. The TSC2011 can work with a full scale ΔVout = 100 mV to Vcc - 100 mV with maximum gain accuracy of 0.3%.
At first order, the full current range to measure through Rsense can be defined by equation 1, just by taking the gain error and input offset voltage as inaccuracy parameters:
Equation 1
The Vsense parameter is defined in the electrical characteristics by following the equation 1.
Its purpose is to highlight that the product Rsense*TSC_gain is determined by the application, and that once one of these two parameters is selected, the maximum value of the second one can be calculated.
• If power the dissipation in the shunt is the key concern, RSense should be chosen as follows:
and then selecting the right gain. For example, for high current to sense, the TSC2012 can offer a gain of 100, as a result, a smaller shunt can be used limiting power losses. However, accuracy can be lower.
• Or alternativevly choosing the available product on the shelf and then size the shunt resistor value accordingly.