COVER ST RYR STORY


the same time the first ADC is measuring current. Temperature is both reported to the host and used internally by the LTC2947 to compensate for the


temperature drift of the internal current sense resistor, resulting in more accurate current measurements. Because of the 0V to 15V rail-to-rail operating range, this device is useful in many types of systems. Not only do 20V abs-max- rated supply and sense pins provide a lot of headroomfor 12V applications, but the zero volt sense monitoring capability is just as useful in monitoring current levels during short circuit or blac kout


situations. Fault current levels at zero volts can immediately indicate whether the power supply or load has gone bad without additional circuitry.


The LTC2947’s “secret sauce” when measuring power and energy really


resides in the third ADC, which multiplies current and voltage at a 5MHz sampling frequency, prior to any conversion averaging. You see, in typical power or energy monitoring ICs, on e or two ADC s are used to measure both current and voltage, and the results are multiplied to obtain power; however, because Δ∑ ADCs are normally used, the values being multiplied are inherently average current and average voltage, which will always contribute to some power error. Instead of multiplying averaged values, the LTC2947 multiplies raw (pre-decimation filter) readings of current and voltage,


also keeps track of the total accumulated time that has been used for integration, where the integration time base can be provided either by the 1%accurate internal clock or an external 100kHz to 25MHz time base.


Charge data can battery application


s where charge is just be especially useful in


one ofmany prerequisites to accurately determining the state of charge (SOC) of a battery.Moreover, energy data is proving to bemore common in everyday applications, as it allows for dynamic ing on static power ies.


readings for activit loading versus rely


DIGITA CONVENIENCE DIGITAL CONVENIENCE


The LTC2947 includes a host of convenient digital features that simplify designs. The most apparent digital feature is the integration of a


then converts the result. This enables this multiplier and accumulator which device to accuratelymeasure power in th e presence of current and voltage variations up to 50kHz – far beyond its conversion frequency. Thismight happen if, for example, power is drawn froma battery with significant impedance.


Figure 4 shows an example of a current and voltage waveformthat are changing phases over a 20µs interval, as well as how power would be calculated


differently in typical power or energy monitoring ICs versus the LTC2947. I n typical power or energy monitoring ICs, power is calculated as the average current multiplied by the average voltage. In the LTC2947, power is calculated as the average of multiplied samples (in this example, two samples are used). The 102W power calculation more closely resembles actual power, while the typical power or energy


monitoring IC’s 110W power calculation is a 7.8% error. The LTC2947 avoids this error and maintains accuracy with up to 50kHz signals.


Since charge is the amount of current consumed over time and energy is the amount of power consumed over time, the LTC2947 integrates current and power over time to calculate charge and energy flowing to or fromthe load. It


/ ELECTRONICS ELECTRONICS


provide users with 24-bit power and 48-bit energy and charge values, alleviating the host of polling voltage and current data and performing


extra computations. A separate 1.8V to 5.5V digital supply allows users to run logic levels at a voltage different from the supply being monitored. This device has minimumand


maximumregisters for current, voltage, power and temperature, which eliminate the need for continuous software polling and free the bus and host to perform other tasks. In addition to detecting and storing min/max values, the device has threshold registers that can be used to issue an alert in the event any of the thresholds are exceeded, again, eliminating the need for the


PLTC294 =102W Figure 4:


Example of ttypical power vs. LT


Example of Figure 4:


ypical power


calculation vs. LTC2947 power calculation


calculation vs L C294 power calculation


LTC2947=102W PTYPICAL =110W TYPICAL =1 Figure 2: Figure 2:


LTC2947 power/energy sense resistor


LTC2947 power/energy monitor wimonitor with integratedth integrated sense resistor


This device can also LTC2947 and analyse


generate an overflow specified amount of e


FEAT RE FEA ATUR E microprocessor to constantly poll the


be configured to data.


nergy or charge has alert after a


been delivered or when a preset amount of time has elapsed. For an energy


monitor, an alert response can be equally as valuable asminimumandmaximum registers, so a separate alert register is available and allows users to select which parameters will respond in accordance with the SMBus alert response protocol, where the Alert Response Address (ARA broadcasted and the /ALERT pin is pulled low to notify the host of an alert event. Users can pin-configure the LTC2947 2C interface


to communicate with to support a standard


Six I2 SPI or I the outside world. C device addresses are available so


multiple LTC2947s can be easily designed into the same system. A stuck-bus reset timer resets the internal I2C state


machine to allow normal communication 2C signals


to resume in the event that I


after stuck bus protec condition) for any rea


prevents a host from troubleshooting a bus


are held low for over 50ms (stuck bu s son; this sought- tion feature


stuck low, which manually


might result in a systemreset that is disruptive, costly and time consuming. The LTC2947 also provides a split I2 data line, which conveniently eliminates the need to use I2


C C splitters or combiners


for bidirectional transmission and receiving of data across an isolation boundary .


CONCLUSION CONCLUSION


The LTC2947 is a convenient board level energy monitor that integrates a 300µΩ sense resistor to eliminate common sense resistor challenges that tend to arise when measuring high currents. At any current level, three ADCs are designed to provide users with highly accurate readings of current, voltage, power, energy, charge, temperature and time. The ˗30A to 30A current range an d 0V to 15V voltage range allows it to work in a wide variety of applications,


including applications where bidirectional currents are present.


Figure 3 Figure 3:: LTC2947 block diagram LTC2947 block diagram


The LTC2947’s analogue prowess is equally matched by its host resource- reducing digital features, including a multiplier, accumulator, min/max registers, configurable alerts and a very 2C inte


capable SPI or I only 24mm2


of board


RA) is


area, the LTC2947 rface. Occupying


is the most sensible device in Linear Technology’s power monitoring portfolio to date.


www.linear.com 01628 477 066


Linear Technology (UK) Ltd. www.linear.com


ELEC


ELECTRONICS


| FEBRUARY 2017 13 FEBRUAR 201


13


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