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Analog & mixed signal Fig. 3: a) Deriving power from the supply being monitored. b) Deriving power from a wide range secondary supply. c) Deriving power from a Low Voltage Secondary Supply. grates a digital multiplier to calculate a 24-bit power result, as well as an accumulator and oscillator to calculate 32-bit energy and charge results. All values, measurements, status and user configuration data are stored in I2C accessible registers. 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 useful in many different low voltage and high voltage sys tems. 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 12-bit ΔΣADC 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 25μV, 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. Energizing 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 minimize power consumption. 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. Digital convenience Consistent with the flexible powering options, the LTC2946 includes a host of convenient digital features that simplifies de- Fig. 4: a) Deriving power through high-side shunt regulator. b) Deriving power through low-side shunt regulator in low-side current sense topology. c) Deriving power from the supply being monitored in low-side current sense topology. 30 Electronic Engineering Times Europe October 2015 www.electronics-eetimes.com


EETE OCT 2015
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