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____________________________________________________ Precision Current Measurements on High-Voltage Power Supply Rails Scott Hill, Current Sensing Products Current is a signal that can provide valuable insight into how a system is operating. Under defined conditions, the amount of current required to perform a task is consistent making the current information a useful indicator to determine if the system is operating within expectations. There are multiple measurement methods and locations current is measured to evaluate this informative signal. Low-Side Sensing One current measurement location is in the return path to ground of a particular load or system. The device requirements for this location are minimal only requiring an amplifier capable of handling a commonmode signal reaching down to ground. Placing a small current sensing resistor (also called shunt resistor) in series with the system’s return path to ground, as shown in Figure 1, develops a voltage across the resistor proportional to the current. BATTERY VOUT = (ILOAD x RSHUNT) x (1+RF/RI) VOUT = VSHUNT x Gain LOAD VCM = 0V VCM > 0V Benefit Disadvantage Operational Amplifier + x Low Cost Accuracy, LowSide Difference Amplifier + + High-Side Low Gain, Cost Instrumentation Amplifier + x Accuracy, High Gain Low-Side, Cost Current Sense Amplifier + + High-Side, High Gain, Accuracy - One of the drawbacks to low-side sensing is the loss of the direct connection to the system ground for the load being monitored. As current passes through the shunt resistor the voltage developed across the component changes, as shown in Figure 2, causing the system reference to deviate from the ground potential of the monitored load. This varying reference connection can be problematic if the system is not able to accommodate the ground potential moving up and down proportionally to the system current. BATTERY + - Full-Scale VOUT RSHUNT 0A + RF VSHUNT RI Current LOAD RSHUNT ILOAD Table 1. Amplifiers For Current Sensing - VSHUNT (System Reference) Full-Scale 0V Figure 1. Low-Side Current Sensing Time (s) Many types of amplifiers are able to accommodate this low-side capability with an input voltage range down to ground. Standard operational amplifiers, difference amplifiers, instrumentation amplifiers and current sensing amplifiers are all capable of common-mode input ranges inclusive of ground. Table 1 provides an overview on how each of these four amplifier types compare for current sensing applications. SBOA165A – July 2016 – Revised December 2016 Submit Documentation Feedback Figure 2. Varying Load Reference In addition to the varying system ground, some fault conditions can be difficult to detect with a low-side measurement location. If a short-circuit condition results in current flowing through another path to ground other than through the shunt resistor, the event will not be detectable by the low-side amplifier. Precision Current Measurements on High-Voltage Power Supply Rails Scott Hill, Copyright © 2016, Texas Instruments Incorporated Current Sensing Products 1 www.ti.com Measuring current at the high-side of the load, or directly in series with the power rail being monitored and the remainder of the circuit, avoids both the varying system reference and alternate short-circuit path issues of low-side current measurements. The high-side location allows for measuring the entire system current so any excess current through unintended paths will be detected. Moving away from the low-side location eliminates the varying system ground due to current induced shunt voltage. A challenge associated with the high-side measurement location is the amplifier must interface with large input voltage rails such as high voltage batteries. A typical signal chain path for measuring current is to amplify the voltage developed across a current sensing resistor and direct that amplified signal to an Analog to Digital Converter. The input range of an ADC (whether discrete or integrated within a microcontroller) is relatively small compared to the voltage rails being monitored in communication and industrial equipment. Common-mode voltage requirements can exceed 60V requiring an amplifier capable of input signals far exceeding the low-voltage component's allowable input range. Current sense amplifiers are dedicated amplifiers developed specifically to accommodate these high voltage input levels while keeping the lower voltage components following the amplifier within their linear input range and protecting them from over-voltage conditions. The INA210 family of current sense amplifiers accommodate the requirement of monitoring high voltage power rails and interfacing with lower voltage components while being powered by a supply voltage as low as 2.7V as shown in Figure 3. In the event the system is placed into a shutdown or sleep state, many of the sub-regulated supply voltages are turned off. Low-voltage supplies powering the ADCs, microcontrollers, and signal path amplifiers can potentially be turned off during low power operating modes. However, batteries remain connected to the measurement circuitry even if the monitoring amplifiers are powered down. For this always-on scenario, a current sense amplifier's input circuitry is specifically designed to accommodate the entire input range independent of the device's supply voltage. The INA210 can withstand the full 26V input voltage at it's input pins regardless of whether a supply voltage is present or not, without being damaged. 2 VCM = 0V to 26V INA210 + - VOUT LOAD Figure 3. INA210: Dedicated Current Sensing Amplifier Alternate Device Recommendations For applications with lower performance requirements, using the INA199 still takes advantage of the benefits of the dedicated current sense amplifier. For higher voltage requirements, the INA240 provides an input common-mode voltage range reaching up to 80V and features enhanced PWM rejection circuitry for applications with large input voltage transitions such as motor control and switching power supplies. The INA301 current sense amplifier features an on-board comparator to perform over-current detection on chip. Table 2. Alternate Device Recommendations Device Optimized Parameter Performance Trade-Off INA199 Lower Cost VOS, Gain Error INA240 High VCM: -4V to +80V, Bandwidth Package: TSSOP-8 INA301 Signal Bandwidth, OnBoard Comparator Package: MSOP-8 Table 3. Related TI TechNotes SBOA160 High Precision, Low-Drift In-Line Motor Current Measurements SBOA162 Measuring Current To Detect Out-of-Range Conditions SBOA163 High-Side Motor Current Monitoring for Over-Current Protection SBOA167 Integrating The Current Sensing Signal Path Precision Current Measurements on High-Voltage Power Supply Rails Scott Hill, Current Sensing Products VS = 2.7V to 26V RSHUNT High-Side Sensing SBOA165A – July 2016 – Revised December 2016 Submit Documentation Feedback Copyright © 2016, Texas Instruments Incorporated IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, enhancements, improvements and other changes to its semiconductor products and services per JESD46, latest issue, and to discontinue any product or service per JESD48, latest issue. Buyers should obtain the latest relevant information before placing orders and should verify that such information is current and complete. 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