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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?

For supplies greater than 100V, the on-board linear regulator at the INTVCC pin can be used in both high and low side configurations to provide power to the LTC2946 through an external shunt resistor. Figure 4a shows a high side power monitor with an input monitoring range beyond 100V in a high-side shunt regulator configuration. The LTC2946 ground is separated from the circuit ground through RSHUNT and clamped at 6.3V below the input supply. Due to the different ground levels, the LTC2946's I2C signals would need to be level shifted for communication with other ground referenced components; a current mirror would also be needed to measure the external voltage on the spare ADC input. Figure 4b shows the LTC2946 deriving power from a greater than -100V supply. Here, the low-side shunt regulator configuration allows operation by clamping the voltage at INTVCC to 6.3V above the input supply, which in this case is a negative rail. As shown in figure 4c, a shunt resistor is not required if the input supply and transients are limited to below -100V, where VDD measures the supply voltage at circuit ground with respect to the LTC2946 ground.

Figure 4a: LTC2946 Derives Power Through High-Side Shunt Regulator.

Figure 4b: LTC2946 Derives Power Through Low-Side Shunt Regulator in Low-Side Current Sense Topology.

Figure 4c: LTC2946 Derives Power from the Supply Being Monitored in Low-Side Current Sense Topology.

Digital convenience
Consistent with the flexible powering options, the LTC2946 includes a host of convenient digital features that simplifies designs. The most apparent digital feature is the integration of a digital multiplier and accumulator which provide users with 24bit power and 32bit energy and charge values, alleviating the host of polling voltage and current data and performing extra computations. The LTC2946 calculates power by multiplying 12bit measured current with 12bit measured voltage. In continuous mode, the differential sense voltage is measured to obtain the load current data. However, the voltage data can be selected between the supply voltage, positive sense voltage, or spare ADC input voltage. A 24bit power value is then calculated every time a current measurement is made. Lastly, energy and charge accumulators are incremented with power and current data and capable of storing several months worth of data at nominal current and power levels.

The LTC2946 has minimum and maximum registers for current, voltage, and power, which eliminate the need for continuous software polling and free the I2C bus and host to perform other tasks. In addition to detecting and storing min/max values, the LTC2946 has min/max limit registers that can be used to issue an alert in the event any of the limits are exceeded, again, eliminating the need for the microprocessor to constantly poll the LTC2946 and analyse data. The LTC2946 can also be configured to generate an overflow alert after a specified amount of energy or charge has been delivered or when a preset amount of time has elapsed. For an energy monitor, an alert response can be equally as valuable as minimum and maximum registers. Figure 5 shows how the LTC2946 generates an alert signal via software and hardware. Measured data is compared against user defined thresholds; over-voltage, under-voltage, over-current, undercurrent, overpower, and underpower thresholds can all be defined and simultaneously monitored. Then, a status register informs the user which parametric thresholds have been exceeded, while actual fault values are logged in another register and can be interrogated at a later time. A separate alert register allows users to select which parameters will respond in accordance with the SMBus alert response protocol, where the Alert Response Address (ARA) is broadcasted and the /ALERT pin is pulled low to notify the host of an alert event.

Figure 5: LTC2946 Fault Alert Generation.

The LTC2946 uses a standard I2C interface with very unique enhancements to communicate with the outside world. Nine I2C device addresses are available so multiple LTC2946s can be easily designed into the same system. All LTC2946 devices respond to a common address, which allows the bus master to write to several LTC2946s simultaneously, regardless of their individual address. A stuck-bus reset timer resets the internal I2C state machine to allow normal communication to resume in the event that I2C signals are held low for over 33ms (stuck bus condition). A split I2C data line conveniently eliminates the need to use I2C splitters or combiners for bidirectional transmission and receiving of data across an isolation boundary. Furthermore, the LTC2946-1 option has an inverted data output for use with inverting opto-isolator configurations.

Conclusion
The LTC2946 is a board level energy monitor that addresses a wide range of applications, providing users with a method to monitor current, voltage, power, energy, charge and time. Building blocks allow the LTC2946 to monitor 0V to 100V positive and negative rails. Users have a variety of biasing options due to independent high voltage monitoring and supply pins, and an onboard regulator to support beyond 100V supplies. The LTC2946'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 capable I2C interface.

About the author
Christopher Gobok is Product Marketing Engineer at Linear Technology Corporation.


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