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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?

The LTC2946 finds its way into many complex, space-constrained, applications including RAID systems, telecommunications, transportation, solar monitoring systems, and industrial computer/control systems. Fortunately, only a few simple connections need to be made to this device. Figure 2 shows the LTC2946 monitoring the input voltage and current of a 3.3V microprocessor, while being powered by 12V. The only required external components are a sense resistor and three pull-up resistors.

Because of the wide rail-to-rail operating range, the LTC2946 is said to be useful in many different low voltage and high voltage systems. Not only do 100V abs-max-rated supply and sense pins provide a lot of headroom, such as in 48V or -48V applications, but the zero volt sense monitoring capability is just as useful in monitoring current levels during short circuit or blackout situations. Fault current levels at zero volts can immediately indicate whether the power supply or load has gone bad without additional circuitry. The internal 12bit ADC inherently averages input noise over the measurement window, so operating in noisy environments is not a problem. In scan mode, the ADC continuously monitors the differential sense voltage, supply or positive sense voltage, and spare ADC input voltage sequentially with 25?V, 25mV and 0.5mV resolution respectively. Conversions have an effective refresh rate of up to 20Hz in continuous scan mode (depending on how often internal calibration is performed), although users can also enter a snapshot mode to take measurements of a single selectable input. In applications where conserving energy is the goal, high voltage monitoring ICs are usually expected to have high quiescent currents and therefore not usable in the application; however, the LTC2946 only consumes 0.9mA when monitoring a 48V rail and can be shut down to reduce power consumption to only 15?A.

Figure 2: LTC2946 High Side Energy Meter.

Energising the energy monitor
The LTC2946 can derive its power from a wide range of supplies, which drastically simplifies the design process for any application. Figure 3a shows the LTC2946 being used to monitor a supply that ranges from 4V to 80V. No secondary bias supply is needed since the VDD supply pin can be connected directly to the monitored supply. If the LTC2946 is used to monitor a supply that goes as low as 0V, it can derive power from a wide range secondary supply connected to VDD as shown in figure 3b. Similarly, if a low voltage supply as low as 2.7V is present, the LTC2946 can be configured as shown in figure 3c to minimise power consumption.

Figure 3a: LTC2946 Derives Power from the Supply Being Monitored.

Figure 3b: LTC2946 Derives Power from a Wide Range Secondary Supply.

Figure 3c: LTC2946 Derives Power from a Low Voltage Secondary Supply.

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