FEAT RE FEA ATURE


COVER


COVER STORY THE SENSIBLE 30A ENERGY METER THAT


Chr stopher Gobok, senior productm et g engineer, Corporation explores the l test energym nitorwth an energymo


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applications are plentiful, there are very few energy monitoring ICs in the market. Many system designers have gotten by with using power monitoring ICs, such as Linear Technology’s 100V LTC2945 power monitor, and having a microprocessor keep track of both power and time while also calcu although complex coding


A lthough energy monitoring


tegrated 30A sense resistor, mo


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Christopher Gobok, senior product marketing engineer, mixed signal products at Linear Technology Corporation explores the latest energy monitor with an integrated 30A sense resistor, which makes gy monitoring a much more practical option for today’s most demanding applications


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currents, including power dissipation, accuracy, temperature drift and size issues. When measuring a full-scale current of 30A, the voltage drop over the LTC2947’s integrated sense resistor is only 10mV, causing a power dissipation of only about a quar when measuring a 6


A rail. In addition to ter Watt or 10mW


is not required, lating energy;


the main drawback with this solution is that computing resources are tied down. Linear Technology’s 100V LTC2946 energy monitor is a more elegant solution that provides direct energy measurements, where us ers have the flexibility to choose their own sense resistor; however, the challenge arises when you need to measure high currents. Enter Linear Technology’s LTC2947 energy monitor with an integrated 30A sense resistor, which makes energy monitoring extremely practical for today’s most demanding applications.


REV SITIINGREVISII NG THE SENSE RESIS SENSE RESISTOR


When designing in a power or energy monitoring IC that uses a sense resistor for its current sense element, calculating the required sense resistor is generally a straightforward task. You simply apply Ohm’s law – that is, you take the data converter’s full scale voltage and divide by the load current. Simple enough, until you start measuring double digit currents. Take the LTC2946 wide range I2


C,


of approximately 100mV . example, which has a full


power, charge and energy monitor for -scale voltage If this device is


sense resistor is required, used to measure a 30A ra


which is readily il, a 3.3mΩ


available, but it will have to dissipate 2.9W of power! Furthermore, because of the high power dissipation, the package will likely not be “standard” by any means and, therefore, fairly,.


for


example Vishay’s CSM3637P 3.3mΩ±1% 5W sense resistor, as shown in Figure 1a. Its 6mm x 3mm x 0.6mmbulk metal foil package practically makes it a 5W heatsink that’s difficult t o mount. Now, if the LTC2946 is used to


measure lower currents, say a 6A rail, then a 16mΩ sense resistor is required and the power dissipation would be a more acceptable 0.57W, for example Panasonic’s inexpensive ERJ


low power dissipation, the LTC2947 offers high dynamic range due to its low offset of only -9mA (or 2.7µV). Temperature compensated energy readings are guaranteed to be 1.2% accurate at room temperature or 1.5% accurate over the entire -40°C to 85°C operating temperature range .


Furthermore, this device is housed in a 4mmx 6mm 32-pin QFN package, making it very effective in space - constrained designs.


ENERGY MEASUREMENTS The LTC2947 measures many


ENERG MEASUREMEN


parameters, including current, voltage, power, charge, energy, temperature and time (see Figure 3). This device uses three Δ∑ ADCs, two of which measure voltage and current while the third ADC calculates power. In continuous mode, the ADCs continuously and simultaneously measure current, voltage, power and temperature and update corresponding internal registers every 100ms.


Figure 1a:


This device is housed in a tiny 3.2mmx 1.6mmx 0.65mm1206 package, which isn’t difficult to mount and wouldn’t add much to the overall board space. If anything, this makes for a 2.4% energy monitoring solution that works over the ˗40°C to 85°C operating temperature range, which may be too inaccurate for some applications; the 2.4% does not include any inaccuracy due to the thermocouple effects created by the


external connections (i.e., leads and traces) between the LTC 2946 and sense resisto r. Whether you want to measure a 30A rail or 6A rail, a simpler alternative to the LTC2946 would be the LTC2947 energy monitor, shown integrates a 300


µΩ sense resistor and in Figure 2, which


RJ8CW 16mΩ ±1%1Wsense resistor, shown in Figure 1b. 12 FEBRUAR 201 12 FEBRUARY 2017 | ELEC ELECTRONICS eliminates the headaches of using external sense resistors to measure high Figure 1a:


Figure 1a: he LT


The LTC2946 measuring energy of 30A rail using


energy of 30A rail using a 3.3m


Figure 1b: L C2


The LTC2946 measuring946 measuring energy of 6A rail usingf 6A rail using


Figure 1b: he LT


energy o a 16m


a 1 Ωsense resistor sense resistor a 3.3mΩsense resistorsense resistor L C2946 measuring Figure 1b: b:


A single shot mode triggers a single set of round-robin measurements. When no measurements need to be made, this solution can park itself in either shutdown mode, where total current consumption is reduced to less than 10µA, or idlemode , where all circuitry stays active and ready to go into continuous, single shot or shutdownmode.


The LTC2947’s 1.3% accurate energy measurements are really owed to the advanced way it calculates power. Unlike existing power monitors where power is multiplied at an ADC’s conversion rate, this device implements an interesting measurement scheme that results in maximumpower measurement accuracy. Each of the three ADCs are tailored for a specific task. The first ADC measures current from-30A to 30A and uses a continuous offset calibration to ensure that all input samples are averaged with equal weight and no samples are missed. The second ADC measures both internal temperature and differential voltage at


/ ELECTRONICS ELECTRONICS


IMPLEMENTS A UNIQUE MEASUREMENT SCHEME xed signal products at Linear Technology


THE SENSIBLE 30A ENERGY METER THAT IMPLEMENTS A UNIQUE MEASUREMENT SCHEME


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