Effectively select and apply current sense amplifiers to better manage power

06/25/2019

Power integrity and control are critical for portable, IoT and automotive equipment and systems because line or battery powered electronics need to monitor power supply current to control power distribution. Current sensing is the key to extending battery life, preventing overcurrent conditions, monitoring ground faults, and optimizing power control. The problem is that these measurements can be made accurately despite the high common mode voltage.


A current sense amplifier (CSA) or current shunt monitor is a differential amplifier IC designed to perform this critical measurement. The current measurement is based on calculating the voltage drop across the series shunt resistor used as the current sensor. The selection and placement of these shunts and associated current sense amplifiers is critical to proper power distribution and efficiency.


This article will describe the selection criteria for shunts and current sense amplifiers based on accuracy requirements and cost.


Resistance current detection

The easiest way to measure current is to insert a small resistor, also called a current shunt, in series with the current to be measured. Measure the voltage across the current sense resistor and calculate the current using Ohm's law based on the known value of the resistor. This method has the advantages of simplicity, low cost and linearity.


The choice of current sense resistor must include the accuracy of the resistor, the temperature coefficient of resistance (TCR) and the rated power. For a given current value, the resistance value determines the voltage drop across it. It also determines the power consumed by the sense resistor. Typically, the value of the sense resistor is a small fraction of ohms. A dedicated resistor can be used for this application. These resistors use metal components in the form of plates, foils or films, or deposited film or thick film hybrid components.


An example of a metal component surface mount shunt resistor is the Ohmite MCS3264R005FEZR current sense resistor (Figure 1). The surface mount device (SMD) is a two-terminal, 5 milliohm resistor rated at 2 watts and has a TCR of 50 ppm / °C.


Picture of Ohmite MCS3264R005FEZR Shunt Resistors

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Figure 1: Ohmite MCS3264R005FEZR is a metal component surface mount 5 milliohm shunt resistor. (Source: Ohmite)


The shunt resistor is also available in a four-terminal (Kelvin) configuration. In the Kelvin connection, current is supplied to a pair of source connection terminals. Two additional sensing connections (voltage leads) are placed in close proximity to the shunt resistor. Voltage lead placement avoids the voltage drop associated with the source leads or contacts. Since almost no current flows to the measuring instrument, the voltage drop in the sense leads is negligible. The Ohmite FC4TR050FER is a 50 milliohm four-terminal metal foil current shunt.


It is recommended to remember that due to the temperature coefficient of the resistor, the value of the sense resistor will vary with temperature. Selecting a resistor with a low TCR, using a resistor with a high power rating or using a heat sink minimizes the resistance change caused by temperature effects.


Current sense amplifier

The current sense amplifier is an application-specific integrated circuit differential amplifier designed to sense the voltage developed across the current shunt and output a voltage proportional to the measured current. The voltage across the current sense resistor is typically in the range of 1 to 100 millivolts, but can be multiplied by the nominal bus voltage potential. The CSA is designed to have a high common mode rejection ratio (CMRR) to eliminate the bus voltage in the output. These devices are designed to handle common-mode voltages that exceed their own supply voltage.


A simplified schematic of the current sense amplifier in Figure 2 shows a typical differential amplifier with inverting and non-inverting inputs and a single output.


Simplified schematic of a typical current sense amplifier

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Figure 2: Simplified schematic of a typical current sense amplifier. The gain is set by the ratio of resistors R2 to R1 and R4 to R3. (Source: Digi-Key Electronics)


The resistance value sets the gain of the CSA. The structure is symmetrical, R1 = R3 and R2 = R4. The gain is determined by the ratio of R2 to R1 and R4 to R3. In a typical CSA implementation, such as the high performance Texas Instruments INA210CIDCKR, R2 and R4 are 1 megohm, R1 and R3 are 5 kohms, and the gain is 200 volts/volt. This version of the amplifier has a gain accuracy of 0.5%. The IC's rated supply voltage is 2.7 to 26 volts, but the maximum common-mode input voltage is -3 to 26 volts regardless of the supply voltage. This is a key distinguishing feature of CSA. The input offset voltage is only 35 microvolts and the CMRR is typically 140 dB.


Depending on the application, the more economical CSA option may be the Texas Instruments INA180B3IDBVR. The CSA has the same common-mode input voltage range and provides gains of 20, 50, 100 and 200 volts/volt. The gain accuracy is 1%, the CMRR is 100 dB, and the input offset voltage is 100 microvolts.


Current detection configuration

There are two current sensing topologies; high side and low side sensing. The high-end configuration places the sense resistor between the voltage source and the load, while the low-side test places the shunt between the load and ground (Figure 3).


High side and low side sensing

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Figure 3: High-side detection places the shunt (R SENSE) between the source and the load, while the low-side detection places it between the load and ground. (Source: Digi-Key Electronics)


The low side detection is referenced to ground with a low input common mode voltage. This makes current monitoring amplifiers and associated circuits easier, which can often translate into lower cost.


The disadvantage of a low side connection is that it raises the load above the ground. When the current value changes, the current flowing through the shunt resistor will raise or lower the system reference level. This can cause problems with the control loop. Furthermore, in this circuit configuration, the ground short of the voltage bus around the shunt resistor cannot be detected.


The advantage of the high-end topology is that the load and system reference voltages are fixed to ground, independent of the monitored current, and the bus short to ground can be easily detected.


On the lower side, there is a common mode voltage close to the bus voltage at the input of the measuring circuit. In addition to applying pressure to the current sense amplifier, in some applications it may be desirable to move the CSA output level down to near the system reference level.


Problems associated with high-end sensing have prompted the development of many CSA families. Both the INA180 and INA210 are new CSAs that handle common-mode voltages from -3 to 26 volts, independent of supply voltage. They are suitable for applications such as motor control, battery monitoring and power management. Applications with higher bus voltages can use other CSAs that provide input common-mode voltages up to 80 volts. For higher voltages, the CSA either uses an external component to isolate the amplifier from the common-mode voltage or an isolation amplifier.


Select the sense resistor value

Set the value of the sense resistor to ensure that the voltage drop across the resistor exceeds the expected bus current range, well above the CSA voltage offset and any additional vertical noise. The rated power of the sense resistor will be determined by the maximum bus current and the maximum voltage drop.


Taking the 12 volt bus as an example, the maximum current carrying capacity is expected to be 2 amps. If an INA210 CSA is used, the voltage drop across the shunt should be greater than the maximum offset voltage of 35 microvolts.


The common mode rejection ratio is in the range of 105 to 140 dB. Using a lower value (105 dB), the 12 volt bus potential (common mode voltage) will decay to approximately 67 microvolts. This will show the offset voltage output for the CSA, multiplied by the gain of the amplifier. This common mode residue shift is not due to the measured current, and in this case the residue is not a problem because it is less than 1% of the measured value.


The sense resistor value must be chosen to ensure that the voltage drop is much greater than the offset voltage. For a 2 volt unipolar swing at the output of the INA210 (gain of 200), the input should be 10 millivolts. This is significantly greater than the input voltage offset or the specified common mode residual. At a nominal maximum current of 2 amps, the sense resistor value should be 5 milliohms. The shunt should be rated for at least twice the nominal maximum power consumption of 20 milliwatts. The previously described Ohmite MCS3264R010FEZR works because it is rated at 2 watts.


Using the Texas Instruments TINA-TI program to simulate this configuration, we can see the DC and AC transmission characteristics of the circuit (Figure 4). The DC transfer function shows a linear response with a slope of 1 volt/ampere. This will produce a 2 volt output for a maximum current of 2 amps. The AC response has a bandwidth of 20 kHz.


Texas Instruments TINA-TI circuit simulation diagram

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Figure 4: Texas Instruments TINA-TI simulation circuit using a 5 milliohm current shunt, showing a linear DC transfer function with a 1 volt/ampere slope. (Source: Digi-Key Electronics)


in conclusion

The current sense amplifier is specifically designed to measure the bus current based on the voltage drop across the series shunt resistor. They are especially suitable for high side measurements where high common mode voltages are present. These amplifiers are easy to select and, if used properly, provide excellent results for power measurement, monitoring and control in electronic systems.