Current Shunt Monitors (Rev. A) TEXAS MONITOR GUIDE
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Curren Shunt t Current Shunt Monitors Power Supply High-Side Sensing VOUT Load Low-Side Sensing www.ti.com/currentshuntmonitor VOUT 2014 Current Shunt Monitors What are Current Shunt Monitors? • Current shunt monitors are also referred to as current sense amplifiers. Current to be measured • Current shunt monitors are designed to monitor the current flow by measuring the voltage drop across a resistor placed in the current path. RSHUNT • Current sense amplifiers tend to be easier to design, more precise, less prone to noise and lower cost than magnetic current sensors. V Voltmeter Current to be measured Key Parameters Common Mode Range: This specification defines the DC voltage range at the input of an amplifier with respect to ground. Current shunt monitors are typically designed to accept common mode voltages well beyond the chip supply voltage. For example, the INA282 is capable of accepting a common mode voltage from -14V to +80V while running on a supply as low as 2.7V. Offset Voltage: The differential DC error at the input of the amplifier. Historically, to reduce the impact of amplifiers with high offsets, larger shunt resistors are used to increase the measured voltage drop. Today, TI is able to offer current sensing solutions with offsets as low as 10µV, enabling higher precision measurements at low currents and the use of smaller shunt resistances for improved system efficiency. CMRR (Common Mode Rejection Ratio): CMRR is the ability of the amplifier to reject signals common to the differential inputs. This is important in the ability to measure small signals superimposed upon a large voltage. TI’s portfolio offers solutions with CMRR as high as 140dB. Output Types Digital Output: Simple all-in-one solution integrating the ADC/MUX with programmable switching. Provides measurements in amps, volts and watts across the I2C interface for a complete power monitoring solution. Voltage Output: High precision, lowest power and industry’s smallest form factors. Fixed gain options ranging from 14V/V to 1000V/V. Variable gain may be set through an external resistor. VIN+ VBUS Power Register V IN+ V IN VOUT SDA V Current Register ADC I Voltage Register Alert Register I2 C Interface VREF SCL VIN- Alert Current Output: Variable gain set through external resistor. Highest bandwidth options. A0 A1 GND VIN+ VOUT VIN- Current Shunt Monitors 2 Texas Instruments 2014 Current Shunt Monitors Low-Side Measurements Advantages: • Typically only requires an op amp such as OPA335 • Straightforward, easy • Inexpensive Disadvantages: • Undesirable resistance in the load’s ground path • Cannot detect fault conditions (short/open circuits) • Requires precision external components to achieve and maintain high accuracy Low-side current sensing techniques connect the current sense element between the load and ground. When to choose low-side sensing: Always choose low-side sensing if the system can tolerate disturbances on the ground path. Power Supply 3kΩ +5V +5V LOAD - F S = 0.2V OPA335 + 50mV Shunt RS I2 C ADS1114 RPD (1) 49 .9kΩ 1kΩ PGA Gain = 16 256V FS –5V G=4 Note: (1) Pull-down resistor to allow accurate swing to OV. High-Side Measurements Advantages: • Eliminates ground disturbances associated with low-side sensing • Able to detect fault conditions Disadvantages: • Difficult to use standard op amp. Resistors must be precisely matched to obtain acceptable common mode rejection ratios (CMRR) • A 0.01% deviation in resistor value lowers the CMRR to 86dB approach • A 0.1% deviation in resistor value lowers the CMRR to 66dB • A 1% deviation in resistor value lowers the CMRR to 46dB • Must withstand very high, dynamic changes in common mode voltage High-side current sensing techniques connect the current sense element between the supply and the load. When to choose high-side sensing: • System cannot tolerate ground disturbance of low side sensing • System needs to be able to identify shorts to ground +5V Power Supply INA 210 +5V High-Side Sensing RS + - ADS1114 I2 C LOAD Low-Side Measurements With a High-Side Monitor Advantages over op amps as a low-side monitor: • Integrated gain resistors • Excellent matching that requires more expensive external precision resistors with an op amp approach • Integrated resistors approach reduces board space requirements Disadvantages over op amps: • Fixed gain settings reduce flexibility in maximizing the full-scale range of the following ADC stage Current Shunt Monitors High-side monitors are designed to accommodate input voltages that exceed the power supply voltage. However, many of our current shunt monitors have common-mode ranges that include or even go below ground. This makes them excellent low-side current shunt monitors as well. 3 Power Supply LOAD INA210 +5V 20mV Shunt RS + - ADS1114 I2 C PGA Gain = 1 4.096V FS Texas Instruments 2014 Current Shunt Monitors Total Error Total Error vs. Differential Input Voltage 10% • For small differential signals at the input, the error is dominated by the amplifier’s offset voltage. Low input offsets are critical to achieving accurate measurements at the low end of the dynamic range. • For large differential signals at the input, the error is dominated by the amplifier’s gain error. Total Error % 8% Offset; Gain Error 6% 10µV; 0.1% 10µV; 1% 4% 1mV; 1% 1mV; 5% 2% 0% 0 10 20 30 40 50 60 70 80 90 100 Differential Voltage (mV) Extending the Common Mode Range With additional circuitry, current shunts can be configured to operate beyond the specified common mode range by using one of the following techniques. Example 1: MOSFET and Zener Example 2: Current Follower RS 0.1Ω 3 4 V IN + VIN - 5kΩ 5kΩ + From Supply Load Up to 1A Load RG MOSFET rated to standoff supply voltage such as BSS84 for up to 50V 100Ω 5.1V Zener IC1 INA168 (200 µA/V) - Shunt R SHUNT 10kΩ C1 0.1µF 24V Tranzorb RL 100Ω Q1 BSS92 INA220 Power Register V VIN+ VIN- +5V Zener VIN Intermediate Power Bus RZ LOAD V BUS VS V+ INCP INCN INVG INVP VDD LMP92064 GND Current Shunt Monitors SPI Bus Digital Isolators A1 Voltage Register I Example 4: Isolated I2C Interface Isolated Power Converter R SENSE Two-Wire Interface Current Register ADC Data (SDA) Clock (SCL) A0 GND VOUT RL 50kΩ (1V at 1A) Example 3: Isolated SPI Interface –48V VS (Supply Voltage) 13.7kΩ 1 R1 330kΩ 10µF VBUS (Bus Voltage Input) OUT GND 2 0.1µF 35.7kΩ OPA333 V+ 5 DZ1 39V zener +3.3V to +5V SPI Bus INA220 RSENSE System Management Controller I 2V– C I2 C ISO1541 Supply +160 to +200V I2 C System Management Controller GND –48V 4 Texas Instruments 2014 Current Shunt Monitors Featured Products INA226: Highest precision solution on the market INA282: Widest common mode range + precision INA225: Programmable gain, zero-drift current sense amplifier • Integrated ADC and MUX with programmable sampling • Common mode range = -14V to 80V • 4-pin selectable gain settings • Offset (max) = 70µV • Bandwidth = 100kHz @ 100V/V • Common mode range = 0V to 36V • Offset drift (max) = 1.5µV/ºC • Offset (max) = 125µV • Offset (max) = 10µV • Gain error (max) = 1.4% • Gain error (max) = 0.50% • Gain error (max) = 0.1% • CMRR (typ) = 140dB • CMRR (typ) = 140dB • Lower cost alternative: INA193, LMP8601 • Lower cost alternative: INA219, INA230 INA210: Precision voltage output LMP8640: High bandwidth and high voltage INA300: Over current detector • Input/output response time = 10µs • Programmable threshold: 0 to 250mV • Hysteresis of 2, 5, or 10mV • Gain options: 50V/V, 100V/V, 200V/V, 500V/V, 1000V/V • Bandwidth = 950kHz • Common mode range: –0.3V to 26V • Offset (max) = 900µV • Offset (max) = 35µV • Gain error = 0.25% • Gain error (max) = 1% • Battery gauge: (Coulomb Counting) • CMRR (min) = 103dB • CMRR (typ) = 140dB • Power supplies • Lower cost alternative: LMP8645 • Inductive charging • Common mode range: -2V to 76V • Lower cost alternative: INA199 AMC1200: 4kV isolated amplifier INA216: Designed for portable battery powered applications • Offset (max) = 1.5mV • Gain options: 25V/V, 50V/V, 100V/V, 200V/V • Gain error (max) = 1% • Common mode range = 1.8V to 5.5V • Offset (max) = 75µV • Offset drift (max) = 10µV/K • CMRR (typ) = 108dB Applications: • Graphics cards • Desktops / laptops / servers • Tablets / E-books • Smartphones & feature phones • Basestations • Networking LMP8481: High voltage • Industrial automation • Bandwidth = 270kHz • Automotive • Common mode range = 4.0V to 76V • Medical • Offset (max) = 400µV • Motor control • Gain error (max) = 1.2% • Battery backups • CMRR (typ) = 124dB • Inverters INA3221: Triple-channel digital shunt and bus voltage monitor • Lower cost alternative: LMP8480 • Solar • Integrated ADC and MUX with programmable sampling LMP92064: Simultaneous sampling current/voltage monitor • Common mode range = 0V to 26V • 125ksamples/sec • Offset error (max) = 80µV • Bandwidth = 70kHz • Gain error (max): 0.5% • Offset (max) = 60µV • Quiescent current = 450µA • Gain error (max) = 0.75% • Gain error (max) = 0.2% • Quiescent current = 25µA • CMRR (typ) = 108dB • 0.76 x 0.76mm WCSP or QFN package available Current Shunt Monitors 5 Texas Instruments 2014 TI Worldwide Technical Support Internet TI Semiconductor Product Information Center Home Page support.ti.com TI E2E™ Community Home Page e2e.ti.com Product Information Centers Americas Phone +1(512) 434-1560 Brazil Phone 0800-891-2616 Mexico Phone 0800-670-7544 Fax Internet/Email +1(972) 927-6377 support.ti.com/sc/pic/americas.htm Europe, Middle East, and Africa Phone European Free Call International Russian Support 00800-ASK-TEXAS (00800 275 83927) +49 (0) 8161 80 2121 +7 (4) 95 98 10 701 Note: The European Free Call (Toll Free) number is not active in all countries. 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