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Examining a 100V DC energy monitor

Posted: 18 May 2015 ?? ?Print Version ?Bookmark and Share

Keywords:AC? DC? energy monitor? resistor? ADC?

In today's power conscious world of electronics, "energy monitoring" and "power monitoring" are often used interchangeably, but, in reality, have slightly different meanings, applications and benefits. Energy is often defined as the amount of power consumed over time, measured in joules (J) or kilowatt-hours (kWh), whereas power is a constant rate of energy usage, measured in watts (W). Thus, while a power rating is typically used to indicate how much electricity a device will consume at a snapshot in time, energy confirms in hindsight how much electricity was actually consumed during a defined time period. So, while the "green" goal of an energy and power monitor may ultimately be the same, an energy monitor may be more useful in most applications, since it goes a step further by accounting for shifts in power levels over time.

With AC loads aside, energy monitoring is becoming more popular and is already established in a handful of DC load applications. Handheld, rack-mounted and in-line energy meters are widely available and can be used by people such as facility managers to track and allocate energy used by equipment or departments among many things. This may also include load profiling, where expected energy consumption patterns are compared to present usage and areas of concern are flagged based on deviations from modelled energy patterns. By sizing loads, users can determine how many lights, computers, batteries, etc. can be connected to a system at any time. Energy monitoring also has obvious usage in renewable energy applications, such as in the case of wind turbines or solar panels, and monitoring how much DC electricity is being generated. Similarly, electric bikes and vehicles can report energy use per mile and quantify the energy being extracted from or returned to a battery.

Although a discrete energy monitoring solution can be built using a microprocessor and a handful of other components, this incurs system overhead from continuous data polling to perform the calculations and analyse the data. A respectable energy monitoring IC provides a simple solution that alleviates the host of these burdensome tasks, where the combination of measured parameters, which include voltage, current, power and energy levels, provide instant insight into a system's health. Programmable threshold alerts may be all that is needed to provide early detection of a fault so preventative action can be taken before catastrophic events occur. Alternatively, systems can be optimised by simply understanding usage patterns; with this kind of information, valuable resources can be diverted accordingly, where overutilised widgets can offload tasks to underutilised widgets.

The energy monitor role model
Energy monitors can be built in many different ways, which isn't surprising considering that a variety of components are necessary to monitor energy usage in a system. To measure current, a sense resistor and amplifier are needed, and it is most convenient if the amplifier common-mode range extends to the positive supply rail and translates its output to ground. Precision resistive dividers are needed to measure voltage and, if there is more than one voltage to monitor, a multiplexer must also be added to the list. A multi-channel analogue-to-digital converter (ADC) comes next, with a precise reference and some means of interfacing to a microprocessor, while perhaps sharing I/O lines with neighbouring ICs. ADC conversions would need to be synchronised to the time base of the microprocessor so that time can be tracked. The microprocessor must also multiply voltage and current to obtain power calculations, and sum these power values over the period for which energy is to be calculated. If detection of minimum and maximum values or alerts are required for any of the parameters, additional code needs to be written and constantly executed. Because of the overall complexity and difficulty of finding suitable components, energy monitoring easily lends itself to an integrated solution.

By integrating all of the necessary functional blocks in a small 4mm x 3mm QFN or MSOP package, Linear Technology's LTC2946 is touted to make energy monitoring practical for a wide variety of applications where a discrete solution is out of the question due to space, complexity or cost. The LTC2946 operates on as little as 2.7V, but can monitor the voltage and current of any 0V to 100V rail, as well as its own supply voltage and one additional voltage input. An on-board shunt regulator provides support for supplies greater than 100V. For flexibility, the sense resistor is external, allowing the LTC2946 to accurately monitor currents ranging from milliamps to tens of amps or more. The ADC has 12bit resolution and a maximum total unadjusted error (TUE) of 0.4% for voltage and 0.6% for current. The additional ADC input (ADIN pin) TUE is also just 0.3% and can be used for monitoring auxiliary functions. The LTC2946 also integrates a digital multiplier to calculate a 24bit power result, as well as an accumulator and oscillator to calculate 32bit energy and charge results. All values, measurements, status and user configuration data are stored in I2C accessible registers.

Figure 1: Simplified LTC2946 block diagram.

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