Measure energy with 100V DC energy monitor

introduction

In today's world of electronic products focusing on power consumption, "energy monitoring" and "power monitoring" are often used interchangeably, but in fact, they are slightly different in meaning, applications, and advantages. Energy is usually defined as the amount of power consumed over a period of time, in units of "joules" (J) or "kilowatt hours" (kWh), and power is a constant rate of energy usage, which is expressed in "watts" (W ) As the unit. Therefore, the rated power is usually used to indicate how much real-time power the device will consume at a certain moment, and the energy is to confirm how much power is actually consumed in a specified time period after the fact. Therefore, although the "green" target of the energy monitor and the power monitor may eventually be the same, the energy monitor may be more useful in most applications because it takes into account the changes in the power level over time, and thus is more advanced. A step away.

Regardless of AC load, energy monitoring is becoming increasingly popular and has established a position in some DC load applications. Hand-held, rack-mounted and online energy meters are widely used. Facilities managers and others can use these energy meters to track and distribute the energy used by equipment or departments and many other systems. The use of energy meters can also include load analysis. In this application, the expected energy consumption pattern is compared with the current usage, and the area of interest is marked according to the degree of deviation from the energy consumption pattern model. By adjusting the size of the load, the user can decide how many lights, computers, batteries, etc. can be connected to the system in a certain period of time. The use of energy monitoring in renewable energy applications can also be seen everywhere, such as in applications such as wind turbine generators or solar panels to monitor how much DC power is generated. Similarly, electric bicycles and electric cars can use energy per kilometer and quantify the energy extracted or returned to the battery.

Although a microprocessor and a few other components can be used to form a discrete energy monitoring solution, this method requires continuous data polling in order to perform data calculation and analysis, which leads to increased system overhead. A very suitable energy monitoring IC provides a simple solution that reduces a lot of burdens caused by such heavy tasks. In this solution, the measured parameters including voltage, current, power and energy are combined, Can help real-time insight into the health of the system. Programmable threshold alarms are necessary to provide early fault detection to be able to take preventive actions before catastrophic events occur. In addition, just by understanding the scale of use, the system can be optimized. With this kind of information, valuable resources can be used elsewhere accordingly, and overburdened devices can offload tasks to underutilized devices.

Energy monitor function model

Energy monitors can be developed in many different ways. It’s not surprising to think about the various components necessary to monitor the energy usage in the system. In order to measure the current, a sense resistor and amplifier are needed. It is convenient to extend the common-mode range of the amplifier to the positive power rail and convert its output to ground. The resistor divider required to measure the voltage, and if more than one voltage is to be monitored, a multiplexer must be added to the list of required components. Next comes the multi-channel analog-to-digital converter (ADC), which has a reference and some way to connect to the microprocessor, and perhaps also share I/O connections with adjacent ICs. ADC conversions may need to be synchronized to the microprocessor time base so that time can be tracked. The microprocessor must also multiply the voltage and current to obtain the power value, and then calculate the sum of these power values for a period of time, and use these sum values to calculate energy. If you need to detect values and values or alarm signals for a certain parameter, you need to write additional codes and execute these codes continuously. Because finding suitable components is generally complex and difficult, energy monitoring is often suitable for integrated solutions.

A variety of applications cannot be considered discrete solutions due to space, complexity, or cost factors. Linear Technology’s LTC2946 integrates all necessary functional components in a small 4mm x 3mm QFN or MSOP package to make energy monitoring become Very practical for this kind of application. The LTC2946 operates at a voltage as low as 2.7V, but can monitor the voltage and current of any 0V to 100V rail as well as its own supply voltage and an additional voltage input. The built-in shunt regulator supports power supplies higher than 100V. For flexibility, an external sense resistor is used, allowing the LTC2946 to accurately monitor currents from milliamps to tens of amperes or more. The ADC has a 12-bit resolution, the total unadjusted error of voltage (TUE) is 0.4%, and the total unadjusted error of current is 0.6%. The TUE of the other ADC input (ADIN pin) is also only 0.3%, which can be used to monitor auxiliary functions. LTC2946 also integrates a digital multiplexer to calculate 24-bit power values, as well as accumulators and oscillators to calculate 32-bit energy and charge values. All values, measurement results, status and user configuration data are stored in registers accessible through I2C.


LTC2946 can be used in many complex and space-constrained applications, including RAID systems, telecommunications, transportation, solar monitoring systems, and industrial computer/control systems. Fortunately, this device requires only a few simple connections. Figure 2 shows that the LTC2946 is monitoring the input voltage and current of a 3.3V microprocessor while being powered by a 12V voltage. The only external components required are a sense resistor and 3 pull-up resistors.

Since the LTC2946 provides a wide rail-to-rail operating range, the device is useful in many different low-voltage and high-voltage systems. Not only the 100V rated power supply and detection pins provide a lot of space, such as in 48V or -48V applications, but the zero-volt detection and monitoring capability is also very suitable for monitoring the current value in the event of a short circuit or power failure. Without additional circuitry, the value of the fault current at zero volts can immediately indicate whether the power supply or load has a problem. The internal 12-bit ΔΣ ADC itself can average the input noise in the measurement window, so it is not a problem to work in a noisy environment. In the scan mode, the ADC continuously monitors the differential detection voltage, the power supply or positive detection voltage, and the standby ADC input voltage in sequence with a resolution of 25?V, 25mV, and 0.5mV. In continuous scan mode, the effective refresh rate of the conversion is up to 20Hz (depending on the frequency of internal calibration), although the user can also enter the snapshot mode to obtain the measured value of a single selectable input. High-voltage monitoring ICs usually have a relatively large quiescent current, so they are not suitable for use in applications where energy saving is the goal. However, the LTC2946 consumes only 0.9mA when monitoring the 48V rail and can be shut down to reduce power consumption to only 15?A.

Increase the vitality of the energy monitor

LTC2946 can use a variety of power sources to obtain power, which greatly simplifies the design process of any application. Figure 3a shows that the LTC2946 is used to monitor a power supply ranging from 4V to 80V. No auxiliary bias power supply is needed, because the VDD power supply pin can be directly connected to the monitored power supply. If the LTC2946 is used to monitor a power supply that can be as low as 0V, then the device can use a variety of auxiliary power supplies connected to VDD to obtain power, as shown in Figure 3b. Similarly, if it is a low-voltage power supply as low as 2.7V, then the LTC2946

For power supplies higher than ±100V, the built-in linear regulator on the INTVCC pin can be used in high-side and low-side configurations to power the LTC2946 through an external shunt resistor. Figure 4a shows a high-side power monitor with an input monitoring range exceeding 100V and a high-side shunt regulator configuration. The ground of the LTC2946 is isolated from the circuit ground by RSHUNT and clamped to a voltage that is 6.3V lower than the input power supply. Due to the different ground levels, the I2C signal of the LTC2946 needs to be level-shifted to communicate with other ground-based components. In addition, a mirrored current is required to measure the external voltage on the standby ADC input. Figure 4b shows that the LTC2946 is powered by a power supply higher than -100V. Here, the low-side shunt regulator configuration allows the voltage at INTVCC to be clamped to 6.3V higher than the input power supply and work under such conditions, in which case the input power supply is the negative rail. As shown in Figure 4c, if the input power and transient voltage are limited to less than -100V, there is no need for parallel resistors, where VDD is the power supply voltage at the circuit ground relative to the LTC2946 ground.

Digital functions bring convenience

Like the flexible power supply options, the LTC2946 also includes many convenient and simplified design digital functions. The obvious digital function is the integrated digital multiplexer and accumulator, which can provide users with 24-bit power, 32-bit energy and charge values, which not only reduces the burden of polling a large number of voltage and current data, but also performs additional Computing tasks. The LTC2946 calculates power by multiplying the measured 12-bit current and the measured 12-bit voltage. In the continuous mode, the load current data is obtained by measuring the differential detection voltage. However, the voltage data can be selected between the power supply voltage, the positive detection voltage, or the backup ADC input voltage. After each current measurement, the 24-bit power value is calculated. , The power and current data are gradually increased in the energy and charge accumulator. In the case of normal current and power values, it can store several months of data.

For current, voltage and power, the LTC2946 has a sum register, so no software continuous polling is required, so that the I2C bus and the host can be released to perform other tasks. In addition to detecting and storing the value, the LTC2946 also has a limit value register, which can be used to issue an alarm when the limit value is exceeded, so the microprocessor does not need to continuously poll the LTC2946 and analyze the data. The LTC2946 can also be configured to generate an overflow alarm after a specified amount of energy or charge is provided, or when a preset time has passed. For an energy monitor, the alarm may be as important as the sum register. Figure 5 shows how the LTC2946 generates alarms through software and hardware. The measured data is compared with the user-defined threshold; the thresholds for overvoltage, undervoltage, overcurrent, undercurrent, overpower, and underpower can be defined and monitored at the same time. Then, the status register informs the user which parameter threshold has been exceeded, and the actual fault value is recorded in another register, which can be queried later. A separate alarm register allows the user to choose which parameters to respond to according to the SMBus alarm response protocol. The alarm response address (Alert Response Address) in the protocol is broadcasted and the /ALERT pin is pulled low to notify the host that an alarm event has occurred.

LTC2946 uses a standard I2C interface with unique enhanced functions to communicate with the outside world. There are 9 I2C device addresses available, so it is very easy to design multiple LTC2946s into the same system. All LTC2946 devices respond to a common address, which allows the main bus to simultaneously write several LTC2946s, regardless of their respective addresses. The blocking bus reset timer resets the internal I2C state machine to allow normal communication to resume when the I2C signal remains low for more than 33ms (blocking bus condition). Since there are split I2C data lines, there is no need to use an I2C splitter or integrator to achieve two-way transmission and receive data across isolation boundaries, which brings convenience. In addition, the LTC2946-1 version provides an inverted data output for use in negative output optoisolator configurations.

  in conclusion

LTC2946 is a general-purpose board-level energy monitor, suitable for a variety of applications, providing users with simple but very effective current, voltage, power, energy, charge and time monitoring methods. The high-performance basic building blocks allow the LTC2946 to easily monitor the positive and negative rails from 0V to 100V with the accuracy of similar devices. Since there are independent high-voltage monitoring and power supply pins, and the built-in voltage regulator supports power supplies exceeding 100V, users have various bias options. The analog function of the LTC2946 is as good as its main resource saving digital function, including multiplexers, accumulators, value registers, configurable alarms, and a very powerful I2C interface.