source: EDN article
Steve Taranovich -September 10, 2016
There are times that a circuit board designer will need to perform an accurate monitoring of a low voltage, high current application. Such power supplies are typically used to power FPGAs, ASICs and Processors. Usually the need was met by using some appropriate components, including a tiny sense resistor, and fashioning a custom design to perform the task.
Well use your design time wisely and focus upon other difficult design problems because Linear Technology Corporation has introduced the LTC2947 power and energy monitor for 0V to 15V DC supply rails. There are other power and energy monitoring ICs, but most use an external sense resistor to measure the current. The task of choosing a sense resistor is not easy, and it becomes far more daunting when dealing with high currents, where available sense resistors can dissipate too much power, occupy a lot of board space or have a large impact on measurement accuracy. The Power monitor functionality integrates a 300µΩ temperature-compensated sense resistor to alleviate these design tasks. See Figure 1.
Figure 1: The LTC2947, 30A Power/Energy Monitor with its integrated 300 µΩ sense resistor shown
This 24mm2 solution will provide up to 1.2% accurate energy readings at up to ±30A. When measuring a full-scale current of 30A, the voltage drop over the device’s integrated sense resistor is only about 9mV, causing power dissipation of approximately a quarter watt or about 10mW when measuring a 6A rail. Aside from low power dissipation, the monitor provides for high dynamic range enabled by its low offset of only 6mA (or 1.8µV).
The IC contains three integrated Δ∑ ADCs and an internal or external precision time base (crystal or clock) which enable accurate measurement of multiple parameters of not only current but voltage, power, charge, energy, temperature and time. All of the digital readings, including minimum and maximum values, are stored in registers accessible by a selectable I2C or SPI interface. An alert signal will notify the host when measurements exceed configurable warning thresholds, eliminating cumbersome polling. The device provides access to all the necessary parameters to accurately assess and manage board level energy consumption, and its rail-to-rail operating range is perfect for monitoring current levels during short-circuit or blackout situations without the need of any additional circuitry.
Why use three ADCs?
Each of the three ∆∑ ADCs performs a specific important function. See Figure 2.
Figure 2: Three ADCs shown here for maximum accuracy measurement over temperature.
The first one measures current from -30A to +30A using a continuous offset calibration which will ensure that all of the input samples are averages with equal weighting with no missed samples.
The second ADC measures internal temperature of the IC as well as differential voltage simultaneously with the first ADC measuring the current. The temperature is reported to the host and is used internally by the IC to compensate for the temperature drift of that internal current sense resistor—this gives very accurate measurements.
Linear Technology calls the third ADC its “secret sauce” when measuring power and energy. The ADC multiplies current and voltage at a 5 MHz sampling rate before any conversion averaging.
Current and voltage input filtering to the ADCs
In order to maintain full electrical performance of the ADCs for current, voltage and power measurements, some simple external filter components should be employed. See Figure 2a.
Figure 2a: ADC input filtering
The integrated RSENSE resistor
One of my favorite and critical features of this device design is that the sense resistor is integrated. This feature allows designers to bypass the intricate selection of an external resistor that needs to be small in order to reduce power losses, with a low tolerance and drift that will maximize the accuracy of the power supply monitor.
The 300 µOhm, temperature-compensated sense resistor being an internal one gives designers an overall 24mm2 footprint with up to 1.8% accurate reading of energy with currents as high as 30A—all this over the full operating temperature range. See Figure 3.
Figure 3: One of the key features of this power monitor IC is the integrated 300 µOhm sense resistor.
Calculating power
Let’s take a look at how the LTC2947 calculates power vs. traditional power/energy monitoring ICs. In the following case the current and voltage waveforms are changing phase over a 20µs period of time.
A traditional method would calculate power as the average current times the average voltage. The LTC2947 calculates power as the average of multiplied samples. In Figure 4 we see two samples being used, so the LTC2947 calculates power at 0.218W which more accurately approaches actual power where a traditional power/energy monitoring IC would generate an answer of 0.234W power which is a 7.3% error. The LTC IC avoids error and holds accuracy with signals as high as 50 kHz.
Figure 4: The traditional vs. LTC2947 power calculation example
Maintaining high accuracy
Now let’s take a look at some graphs with examples of the LTC power /energy monitor under different power supply conditions. See Figure 5.
Figure 5: The LTC2947 accuracy performance
The graph on the left in Figure 5 demonstrates the measurement of a 3.3V to 12 V power supply set of rails with as much as 30A of current. The Total Unadjusted Error (TUE) is near 0 %. The graph on the right shows the measurement of a 12 V 30A supply using and internal or an external clock, and the TUE does not typically exceed around 0.25%. Note: The TUE encompasses all of the ADC errors such as gain, offset and INL and is a very good indicator of overall measurement performance. These low TUE figures may even eliminate the need for calibration in some designs.
Demoboard and GUI
There is a DC2334 demoboard available with a user-friendly Windows GUI. See Figure 6.
Figure 6: The LTC2947 demoboard (DC2334) and GUI
The circuit designer is able to choose the clock source; view each measured parameter min and max value as well as alert status. Raw registry contents may also be viewed if the user is interested.
Designers will have the use of multiple controls to manipulate which include operating the LTC2947 in the continuous mode, where the ADCs will continuously and simultaneously measure parameters every 100 ms, or the single shot mode that will trigger s single set of round-robin measurements.
If no measurements need to be made, designers can place the IC in shutdown mode in which the total current consumed is under 10µA. They can also select idle mode in which all circuitry will remain active and available to go into continuous, single shot or idle mode.
I like this a great deal because it will save circuit designers precious development time.
Price, package and temperature range
The IC is specified over the commercial and industrial temperature ranges and is provided in a 32-lead 4mm x 6mm QFN package. 1,000-piece pricing starts at $5.95 each.
