Welcome to the training module on TI current shunt monitors. This training module introduces the current sensing technology. We also discuss the featured current shunt monitors from TI.
A current sensor is the electronic circuits that monitor the current flow by measuring the voltage drop across a resistor placed in the current path. Although other technologies exist, such as magnetic, everything discussed in this module is limited to shunt resistor current measurement. The current sensor outputs either a voltage or a current that is proportional to the current through the measured path. A wide variety of applications benefit from the ability to measure current flow. Traditionally, current sensing was primarily for circuit protection and reporting. However, as technology advances, current sensing is becoming more and more important as a way to monitor performance. These applications include overcurrent protection and supervising circuits, overload and fault detection circuits, programmable current sources, linear and switch-mode power supplies, and battery chargers.
Low side current sensing techniques connect the current sensor element between the load and ground. Current is measured by looking at the voltage drop across a resistor placed between the load and ground. Low side current sensing is straightforward, easy, and rarely requires more than an op-amp to implement. It is also inexpensive and precise. However, it has some disadvantages. This kind of circuits will add undesirable resistance in the ground path and may require an additional wire to the load. If your application can tolerate the extra disturbance in the ground path, choose low side current sensing is almost the best option.
High side current sensing techniques connect the current sensor element between the supply and the load. Current is measured by looking at the voltage drop across a resistor placed between the supply and the load. The traditional approach for high-side current measurements has been the use of a differential amplifier, which is employed as a gain amplifier and a level shifter from the high side to ground. When low side sensing is not an option due to the added ground disturbance, it must turn to high side sensing. High side current sensing enables diagnostic systems to detect shorts to ground.
Current shunt monitors are a unique amplifier family that is solely dedicated to high side current sensing applications, and contains all the necessary functions needed to perform the measurement easily and economically. With a current-shunt monitor, the voltmeter has been replaced by a specially adapted instrumentation amplifier that amplifies the voltage developed across the shunt resistor. They provide a ground-referenced current- or voltage-source output that is proportional to the current of interest, and high common-mode rejection without the difficulty of resistor matching. These dedicated current sense amplifiers can accurately measure or control current for battery monitoring, power supply monitoring, battery charging and any other applications.
The INA210, INA211, INA212, INA213, and INA214 are voltage output current shunt monitors that can sense drops across shunts at common-mode voltages from -0.3V to 26V, independent of the supply voltage. Five fixed gains are available: 50V/V, 100V/V, 200V/V, 500V/V, or 1000V/V. The low offset of the Zero-Drift architecture enables current sensing with maximum drops across the shunt as low as 10mV full-scale. These devices operate from a single +2.7V to +26V power supply, drawing a maximum of 100mA of supply current.
The figure shows the basic connections of the INA210 – INA214. The input pins, IN+ and IN-, should be connected as closely as possible to the shunt resistor to minimize any resistance in series with the shunt resistance. Power supply bypass capacitors are required for stability. Applications with noisy or high impedance power supplies may require additional decoupling capacitors to reject power-supply noise. Unidirectional operation allows the INA210-INA214 to measure currents through a resistive shunt in one direction. This operation can be achieved to set the output at ground by connecting the REF pin to ground. Bidirectional operation allows the INA210-INA214 to measure currents through a resistive shunt in two directions. In this case, the output can be set anywhere within the limits of the reference.
An obvious and straightforward location for filtering is at the output of the INA210-INA214; however, this location negates the advantage of the low output impedance of the internal buffer. The only other option for filtering is at the input pins of the INA210-INA214; this location requires consideration of the ±30% tolerance of the input impedance. Using the lowest possible resistor values minimizes both the initial shift in gain and effects of tolerance.
While the INA210-INA214 series does not have a shutdown pin, its low power consumption allows powering from the output of a logic gate or transistor switch that can turn on and turn off the power-supply quiescent current. However, in current shunt monitoring applications. There is also a concern for how much current is drained from the shunt circuit in shutdown conditions. Evaluating this current drain involves considering the simplified schematic of the INA210-INA214 in shutdown mode shown in the figure.
There is typically slightly more than 1MΩ impedance from each input of the INA210-INA214 to the OUT pin and to the REF pin. The amount of current flowing through these pins depends on the respective ultimate connection. If the REF pin is grounded, the calculation of the effect of the 1MΩ impedance from the shunt to ground is straightforward. As with any difference amplifier, the INA210-INA214 series common-mode rejection ratio is affected by any impedance present at the REF input. In systems where the INA210-INA214 output can be sensed differentially, such as by a differential input analog-to-digital converter (ADC) or by using two separate ADC inputs, the effects of external impedance on the REF input can be cancelled.
The INA210-INA214 series can be used in circuits subject to transients higher than 26V, such as automotive applications. Use only zener diode or zener-type transient absorbers or any other type of transient absorber has an unacceptable time delay. Start by adding a pair of resistors as shown in the figure as a working impedance for the zener. It is desirable to keep these resistors as small as possible, most often around 10Ω. Because this circuit is limiting only short-term transients, many applications are satisfied with a 10Ω resistor along with conventional zener diodes of the lowest power rating that can be found. In the event that low-power zeners do not have sufficient transient absorption capability and a higher power transzorb must be used, the most package-efficient solution then involves using a single transzorb and back-to-back diodes between the device inputs.
The INA219 is a digital current-shunt monitor with an I 2 C and SMBus-compatible interface. It provides digital current, voltage, and power readings necessary for accurate decision-making in precisely-controlled systems. The INA219 monitors both shunt drop and supply voltage, with programmable conversion times and filtering. A programmable calibration value, combined with an internal multiplier, enables direct readouts in amperes. An additional multiplying register calculates power in watts. The INA219 senses across shunts on buses that can vary from 0V to 26V. The device uses a single +3V to +5.5V supply, drawing a maximum of 1mA of supply current. The INA219 can be used without any programming if it is only necessary to read a shunt voltage drop and bus voltage with the default 12-bit resolution, 320mV shunt full-scale range, 32V bus full-scale range, and continuous conversion of shunt and bus voltage.
The INA219 offers compatibility with both I 2 C and SMBus interfaces. The device that initiates the transfer is called a master , and the devices controlled by the master are slaves . The bus must be controlled by a master device that generates the serial clock (SCL), controls the bus access, and generates START and STOP conditions. The master initiates a START condition by pulling the data signal line (SDA) from a HIGH to a LOW logic level while SCL is HIGH. All slaves on the bus shift in the slave address byte on the rising edge of SCL, with the last bit indicating whether a read or write operation is intended. During data transfer, SDA must remain stable while SCL is HIGH. Any change in SDA while SCL is HIGH is interpreted as a START or STOP condition. Once all data have been transferred, the master generates a STOP condition, indicated by pulling SDA from LOW to HIGH while SCL is HIGH.
The internal ADC is based on a delta-sigma (ΔΣ) front-end with a 500kHz (±30%) typical sampling rate. When the INA219 is in the normal operating mode it continuously converts the shunt voltage up to the number set in the shunt voltage averaging function. The device then converts the bus voltage up to the number set in the bus voltage averaging. The Mode control in the Configuration Register also permits selecting modes to convert only voltage or current, either continuously or in response to an event. All current and power calculations are performed in the background and do not contribute to conversion time; Power-Down mode reduces the quiescent current and turns off current into the INA219 inputs, avoiding any supply drain. ADC Off mode stops all conversions. Writing any of the triggered convert modes into the Configuration Register triggers a single-shot conversion.
These registers are volatile, and if programmed to other than default values, must be re-programmed at every device power-up. The Calibration Register makes it possible to set the scaling of the Current and Power Registers to whatever values are most useful for a given application. One strategy may be to set the Calibration Register such that the largest possible number is generated in the Current Register or Power Register at the expected full-scale point; this approach yields the highest resolution. The Calibration Register can also be selected to provide values in the Current and Power Registers that either provide direct decimal equivalents of the values being measured, or yield a round LSB number. After these choices have been made, the Calibration Register also offers possibilities for end user system-level calibration, where the value is adjusted slightly to cancel total system error.
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Current Shunt Monitors
Current Shunt Monitors <ul><li>Source: T EXAS I NSTRUMENTS </li></ul>
Introduction <ul><li>Purpose </li></ul><ul><ul><li>This training module introduces the current sensing technology. We also discuss the featured current shunt monitors from TI. </li></ul></ul><ul><li>Outline </li></ul><ul><ul><li>Basic current sensing technology </li></ul></ul><ul><ul><li>Analog current shunt monitors </li></ul></ul><ul><ul><li>Digital current shunt monitors </li></ul></ul><ul><li>Content </li></ul><ul><ul><li>16 pages </li></ul></ul><ul><li>Duration </li></ul><ul><ul><li>15 Minutes </li></ul></ul>
What is a Current Sensor? <ul><li>Current sensor: electronic circuits that monitor the current flow by measuring the voltage drop across a resistor placed in the current path. </li></ul>
Low Side Current Sensing <ul><li>Advantages: </li></ul><ul><li>Straightforward, easy, and rarely requires more than an op-amp to implement </li></ul><ul><li>Inexpensive and precise </li></ul><ul><li>Disadvantages: </li></ul><ul><li>Adds undesirable resistance in the ground path </li></ul><ul><li>May require an additional wire to the load that could otherwise be omitted </li></ul>
High Side Current Sensing <ul><li>Advantages: </li></ul><ul><li>Current sensor connected directly to the power source and can detect any downstream failure and trigger appropriate corrective action </li></ul><ul><li>No extra ground disturbance that comes with a low side current sensing design </li></ul><ul><li>Disadvantages: </li></ul><ul><li>Requires very careful resistor matching in order to obtain an acceptable common-mode rejection ratio (CMMR). </li></ul><ul><li>Must withstand very high, and often dynamic, common-mode voltages. </li></ul>
Current Shunt Monitors <ul><li>Integrating high side current shunt measurement capabilities into a single IC provides a vast amount of added benefits </li></ul>
INA21x Current Shunt Monitors <ul><li>FEATURES </li></ul><ul><li>Bi-directional current monitoring </li></ul><ul><li>Wide common-mode range: –0.3V to 26V </li></ul><ul><li>Offset voltage: ±35mV </li></ul><ul><li>Accuracy </li></ul><ul><ul><li>±1% Gain error (max over temperature) </li></ul></ul><ul><ul><li>0.5mV/°C Offset drift (max) </li></ul></ul><ul><ul><li>10ppm/°C Gain drift (max) </li></ul></ul><ul><li>Quiescent current: 100mA (max) </li></ul>
Transient Protection Using a Single Transzorb and Input Clamps Using Dual Zener Diodes
Digital Current Shunt Monitor <ul><li>Features </li></ul><ul><li>Senses bus voltages from 0V to +26V </li></ul><ul><li>Reports current, voltage, and power </li></ul><ul><li>16 programmable addresses </li></ul><ul><li>High accuracy: 1% (max) </li></ul><ul><li>Filtering options </li></ul><ul><li>Calibration registers </li></ul>
Bus Overview <ul><li>The master must address slave devices via a slave address byte. </li></ul><ul><li>Two address pins, A0 and A1. </li></ul><ul><li>Accessing a particular register on the INA219 is accomplished by writing the appropriate value to the register pointer. </li></ul><ul><li>When reading from the INA219, the last value stored in the register pointer by a write operation determines which register is read during a read operation. </li></ul><ul><li>To change the register pointer for a read operation, a new value must be written to the register pointer. </li></ul>Bus
Programming Power Measurement Engine <ul><li>Establish the parameters V BUS_MAX , V SHUNT_MAX and R SHUNT </li></ul><ul><li>Determine the maximum possible current </li></ul><ul><li>Choose the desired maximum current value </li></ul><ul><li>Calculate the possible range of current LSBs </li></ul><ul><li>Compute the Calibration Register value </li></ul><ul><li>Calculate the Power LSB </li></ul><ul><li>Compute the maximum current and shunt voltage values </li></ul><ul><li>Compute the maximum power </li></ul><ul><li>Compute corrected full-scale calibration value based on measured current </li></ul>Max_Expected_I ≤ I MAX Power_LSB = 20 Current_LSB Max_Current = Current_LSB x 32767 MaximumPower = Max_Current_Before_Overflow x V BUS_MAX
Additional Resource <ul><li>For ordering the current shunt monitors, please click the part list or </li></ul><ul><li>Call our sales hotline </li></ul><ul><li>For additional inquires contact our technical service hotline </li></ul><ul><li>For more product information go to </li></ul><ul><ul><li>http://focus.ti.com/paramsearch/docs/parametricsearch.tsp?family=analog&familyId=426&uiTemplateId=NODE_STRY_PGE_T </li></ul></ul>