Team player: I2C current and voltage monitoring takes an integrated approach
As today’s electronic designs continue to grow in complexity, managing power consumption and optimizing overall efficiency become even more important. Christopher describes the building blocks needed for accurate power supply voltage and current monitoring — crucial to conserving power and guaranteeing reliability in everything from industrial and telecom applications to automotive and consumer electronics.
A variety of components is necessary to monitor the power input to 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 mux must also be added to the list. An Analog-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 neighboring ICs. Because of the overall complexity and difficulty of finding suitable components, supply monitoring lends itself to an integrated solution.
Figure 1 shows the functional blocks needed to form a complete power monitoring system able to operate over a 7 V to 80 V range while monitoring current at the supply rail, its own supply voltage, and one additional voltage input. In developing such a system, making the sense resistor external adds flexibility and lets it accurately monitor currents ranging from milliamps to amps or more. Its ADC has 12-bit resolution and a Total Unadjusted Error (TUE) of 1 percent for voltage and 1.25 percent current. The external ADIN input TUE is just 0.75 percent. Digital communications are conducted over I2C, with a choice of nine device addresses.
Integrating all of the necessary functional blocks in a single-chip solution makes power monitoring practical in applications where a discrete solution is out of the question due to space or cost.
Meeting the needs of space-constrained applications
RAID systems, telecommunications, and industrial computer/control systems are among the applications that are complex, space-constrained, and low-voltage. To address these challenges, it is best to keep connections to the power monitoring solution simple and to have only a few connections. A small MS10 or tiny 3 mm x 3 mm DFN package can be used. Depending on the system, the monitoring IC could be located on the backplane or on a removable card. Figure 2 shows location on a removable card. The high-voltage I2C current and voltage monitor in Figure 2 is the LTC4151 from Linear Technology. It is monitoring the input current and voltage to a 12 V DC/DC converter. Here, the low- voltage input, ADIN, is used to measure the 5 V output of the converter, while a direct I2C connection is made to the microprocessor. The only required external components are a sense resistor, two bus pull-up resistors, and a resistive divider for 5 V measurement on ADIN.
Low side or high side?
Because of the inherent simplicity, low-side sensing, where the sense resistor is placed in series with the ground of the load, is an attractive means of monitoring supply current. It eliminates the need for a special amplifier by allowing the ADC to measure the sense resistor's voltage drop either directly or with a simple preamplifier. Unfortunately, few loads are truly floating in such a way as to permit opening the ground path. This scheme also presents a potential safety hazard: A failed or disconnected low-side sense resistor allows the load ground to rise to the full supply voltage.
For these reasons, high-side sensing is almost always preferred, yet difficult to accomplish. This is because the ADC must measure the drop across a sense resistor that is connected to the positive rail, often at a voltage far outside the reach of the ADC itself. In addition, a small sense resistor drop (20 mV/A in this case) is too small for a 12-bit converter, as most of the dynamic range would be wasted. To solve this problem, the team developing the LTC4151 at Linear opted for an amplifier capable of sensing a high positive rail while presenting its output to a ground-referenced ADC. This approach worked for measurement at a high-voltage rail. As a bonus, owing to the sense amplifier's gain of 25, the 12-bit converter does the work of a 16.5-bit converter.
The problems of high-side current monitoring are compounded as the supply voltage increases, making it useful to have a monitor that can maintain high precision for supplies from 7 V to 80 V. This range allows high-side current monitoring to encompass applications with 12 V, 24 V, or 48 V supply voltages, including servers, mass storage devices, and many other systems. Figure 3 shows the LTC4151 monitoring current, voltage, and temperature in a 48 V application. ADIN monitors temperature by measuring the voltage drop of a diode. The absolute maximum voltage of the supply pin and the two sense input pins is 90 V, which helps the IC survive high-voltage transients. This wide input voltage range allows direct connection to high-voltage supplies without needing a secondary supply.
The LTC4151 demonstrated accuracy that is more than sufficient for most applications and comparable, if not better, than the accuracy found on discrete solutions. When measuring current at the SENSE pins, the maximum TUE is ¬±1.25 percent. The full-scale current sense voltage is 81.92 mV, with 20 ¬µV/LSB resolution. When measuring voltage on VIN through the internal precision attenuator, the TUE is ¬±1 percent with a full-scale voltage of 102.4 V and 25 mV/LSB resolution, providing more than enough resolvability at both lower and higher voltages. Finally, when taking a voltage reading on ADIN, the TUE is ¬±0.75 percent with a full-scale voltage of 2.048 V and 25 mV/LSB resolution. These accuracy figures are all valid over the -40 ¬∞C to +85 ¬∞C industrial temperature range.
Some negativity is welcome
Some applications, especially in telecommunication systems, work off negative voltages and consume large amounts of current in cases where power monitoring might not be as straightforward. The monitor should be able to monitor both positive and negative voltages equally well. A shutdown pin, for example, can lower the quiescent current to 120 ¬µA at 12 V for low-power applications. For high-voltage negative applications, a second I2C data pin affords simple optoisolation. The use of optocouplers can allow the host controller to sit at a different ground level from the power monitor.
The LTC4151-1 connects to opto-couplers in a -48 V AdvancedTCA ap- plication. A split I2C data line, SDAI (data input) pin, and unique SDAO# (inverted data output) pin eliminate the need to use I2C splitters or combiners for bidirectional trans-mission and receiv-ing of data. (See Figure 4.) With all I2C signals clamped and pull-up resis-tors able to connect directly to the -48 V supply, the need for a separate pull-up supply is also eli-minated. Note that the voltage at VIN is measured on the upstream side of the sense resistor for greater accuracy, with the assumption that the quiescent current of the LTC4151 is negligible when compared to the load of a DC/DC converter, which is usually on the order of amperes for AdvancedTCA applications. Figure 4 also demonstrates how the ADIN pin can be used to measure the board temperature using a thermistor.
Regardless of whether or not an applica-tion requires isolation, there are monitor features that can prove convenient when reporting back to a polling host. For example the use of a stuck-bus reset timer that resets the internal I2C state machine allows nor-mal communication to resume in the event that I2C signals are held low for more than 33 ms (stuck bus condition). Otherwise, the LTC4151 can report data continuously or in a single snapshot mode. In continuous scan mode, the LTC4151 measures the voltage between the SENSE pins, at VIN, and at ADIN sequentially at a 7.5 Hz refresh rate. In snapshot mode, the host controller instructs the LTC4151 to perform a one-time measurement on any of the signals, useful for applications that only need to measure input power on occasion.
When faced with the challenge of solving a wide range of applications, traditional high-voltage current and voltage monitor-ing implementations using discretes and other power monitors tend to fall short in the areas of complexity, functionality, and performance. A ‚Äúteam‚Äù approach employing high-performance building blocks, including the internal sense amplifier, Delta Sigma ADC, and I2C interface, ensures that digital readings are accurate and precise. High-voltage applications can take advantage of the 90 V Abs Max rating, while all of the IC's flexibility applies to users monitoring negative voltages, including an isolation-friendly option. Designers can now spend less time on implementing a reliable power monitoring scheme.