SBOA203A June   2017  – September 2021 INA209 , INA219 , INA220 , INA220-Q1 , INA226 , INA226-Q1 , INA228 , INA228-Q1 , INA229 , INA229-Q1 , INA230 , INA231 , INA233 , INA234 , INA236 , INA237 , INA237-Q1 , INA238 , INA238-Q1 , INA239 , INA239-Q1 , INA260 , INA3221 , INA3221-Q1

 

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Devices that monitor and report system current can either provide an analog output that is proportional to the sensed current, or communicate the current to a host processor digitally. Use of a digital current sense amplifier is sometimes preferred because the integrated analog to digital conversion allows data to be sent directly to the host controller or processor. Interaction with a host processor requires a digital interface that can both allow sending and receiving of instructions as well as data. For current sensing applications, the most commonly used digital interfaces are I2C, SMBus, PMBus, and SPI. Each interface has different strengths and weaknesses; selecting the correct interface for a given application can allow better system optimization, faster system response time, and a reduction in software development time.

I2C, SMBus, and PMBus all utilize open drain clock and data lines that require pull-up resistors to an external power supply. SMBus and PMBus compatible devices feature an active low alert output to notify the host processor of fault conditions. I2C, SMBus, and PMBus devices commonly exist on the same physical bus; however, differences exist. Table 1-1 highlights some of the key differences among I2C, SMBus, and PMBUS devices.

I2C, SMBus, and PMBus devices can easily co-exist on the same bus since all logic low thresholds are at 0.4 V. Differences in the logic high thresholds usually are not an issue since the open-drain clock and data lines will go up to VDD when not held low. SMBus expanded on the frame work laid by I2C by adding a bus timeout requirement that prevents a device from holding data lines low for extended amounts of time. SMBus also clearly defined many different types of transaction protocols that support the transmission of data from the bit level to blocks of bytes. The SMBus and PMBus specifications are very similar because PMBus leverages the electrical characteristics and communications protocols as defined by the SMBus specification. PMBus incorporates the SMBus electrical specification, while also standardizing the address locations for common commands that are used in power systems. The address/command standardization allows one software driver to support many devices without the need to be completely rewritten to support new devices or devices from different manufacturers.

Table 1-1 Comparison of I2C, SMBus, and PMBus Interfaces
ParameterI2CSMBusPMBus
Electrical LevelsOutput Logic Low, VOL0.4 V, sinking 3 mA0.4 V, sinking 350 µA (Low Power) sinking 4 mA (High Power)Same as SMBus
Input Logic Low, VIL0.3 x Vdd0.8 VSame as SMBus
Input Logic High, VIH0.7 x Vdd2.1 VSame as SMBus
SpeedMinimum-100 kHzSame as SMBus
Maximum5 MHz400 kHz, 1 MHzSame as SMBus
Number of wires/pins2: SDA, SCL3: SDA, SCL, SMBALERT3-4: SDC, SCL, SMBALERT, CONTROL(optional)
Time-out requirementNoYesYes
Specified Transaction ProtocolsNoYesYes
Alert CapabilityNoYesYes
Address Resolution ProtocolNoYes, but optionalYes, but optional
CRC error checking supportNoYes, but optionalYes, but optional
Group Protocol supportNoYes, but optionalYes, required for PMBus
Standardized Commands / Register SetNoNoYes, both standard and device specific commands supported

SMBus adds support for dynamic address resolution, CRC checking to increase communications robustness, and group protocol that allows communication to multiple devices within a single transaction. Support for group protocol is optional for SMBus devices but is required for all PMBus devices.

Most digital current sense amplifiers available from Texas Instruments are compatible with both I2C and SMBus interfaces. For example, the INA228 supports the high-speed mode (up to 2.94 MHz) offered by I2C, but also features an Alert pin and error resolution protocols as defined by SMBus. The device can monitor and report the shunt voltage, bus voltage, die temperature, current, power, energy accumulation, and charge accumulation. The INA228 is available in a VSSOP-10 package, is an ultra-precise digital current monitor with a 20-bit delta-sigma ADC, has a gain error of only 0.05%, and a maximum offset voltage of 1 µV. Table 1-2 provides a summary of current sense devices that are compatible with I2C and SMBus.

Table 1-2 I2C/SMBus Compatible Devices
DeviceOptimized ParametersPerformance Trade-Off
INA209Internal analog comparators for critical over current detection26-V, 12-bit ADC, Large TSSOP-16 package
INA219Low pin count, SOT23-8 packageNo alert pin, 26-V, 12-bit ADC, VBUS tied to IN-
INA220Independent bus voltage measurementNo alert pin, 26-V, 12-bit ADC,VSSOP-10 package
INA226High accuracy, 16-bit ADC, Current / Power Monitor36-V, VSSOP-10 package
INA228Highset Accuracy, 85-V, 20-Bit ADC, Power / Energy / Charge / Temperature MonitorVSSOP-10 package
INA230High accuracy, 16-bit ADC, 3mm x 3mm QFN28-V, Higher offset and gain error than similar device INA226
INA231High Accuracy, 16-bit ADC, Smallest Package (WCSP-12), 1.8-V I2C interface28-V, Higher offset and gain error than similar device INA226
INA234Small DSBGA 8 package28-V, 12-bit ADC, One address pin for a maximum of 4 addresses
INA236High accuracy, 48-V, 16-bit ADC, small DSBGA 8 packageOne address pin for a maximum of 4 addresses
INA23785-V, 16-Bit ADC, Power / Energy / Charge / Temperature MonitorVSSOP-10 package, Higher offset and gain error than similar INA238 device
INA238High Accuracy, 85-V, 16-Bit ADC, Power / Energy / Charge / Temperature MonitorVSSOP-10 package, Higher offset and gain error than similar INA228 device
INA260Internal Shunt, high accuracy (total solution), 16-bit ADC36-V, Large TSSOP-16 package, Maximum current is 15A
INA32213 channel voltage and current monitor26-V, 12-bit ADC, Lower effective sample rate

Texas Instruments also has the INA233, which features a PMBus compatible interface and can monitor current, voltage, power, and energy. The INA233 is also available in a VSSOP-10 package, can monitor current very accurately with a gain error of only 0.1%, and has a maximum offset error of 10 µV. Table 1-3 provides a summary of current sense devices that are compatible with I2C, SMBus, and PMBus.

Table 1-3 I2C/SMBus/PMBus Compatible Devices
DeviceOptimized ParametersPerformance Trade-Off
INA233High accuracy, 16-bit ADC, Fast Sampling Rate, Energy Monitor / Power Accumulator36-V, VSSOP-10 package

Another digital interface that is used in current monitoring devices is the SPI interface. The SPI interface is a 4-wire interface that does not require external pull-up resistors like I2C and can operate at much higher clock frequencies. The pull-up resistors used in I2C limit the operational speed due to the RC time constant established by the value of the pull-up resistor and bus capacitance. One example is the INA229, which has the same current sensing specifications as the INA228, but allows data clock rates as high as 10 MHz. Table 1-4 provides a summary of current sense devices that are compatible with SPI.

Table 1-4 SPI Compatible Devices
DeviceOptimized ParametersPerformance Trade-Off
INA229Highset Accuracy, 85-V, 20-Bit ADC, Power / Energy / Charge / Temperature MonitorVSSOP-10 package
INA239High Accuracy, 85-V, 16-Bit ADC, Power / Energy / Charge / Temperature MonitorVSSOP-10 package, higher offset and gain error than similar INA229 device
Table 1-5 Adjacent Application Briefs
SBOA511Getting Started with Digital Power Monitors
SBOA167Integrating the Current Sensing Signal Path
SBOA179Integrated, Current Sensing Analog-to-Digital Converter
SBOA194Power and Energy Monitoring with Digital Current Sensors