26 June 2012
An integrated current and voltage monitor is enabling power monitoring in a range of applications
As electronic designs grow in complexity, managing power consumption and optimising overall efficiency become increasingly important. Accurate power supply voltage and current monitoring is crucial to conserving power and guaranteeing reliability in a range of applications.
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 convenient if the amplifier's 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 be added to the list. An a/d converter is next, with a precise reference and some means of interfacing to a microprocessor, while perhaps sharing I/O lines with neighbouring ics. Because of the overall complexity and difficulty of finding suitable components, supply monitoring lends itself to an integrated solution.
The LTC4151, which contains the functional blocks needed to form a complete power monitoring system (see fig 1), was developed to meet this need. It operates over a 7V to 80V range while monitoring current at the supply rail, its own supply voltage and one additional voltage input. For flexibility, the sense resistor is external, allowing a range of currents to be monitored more accurately. The a/d converter has 12bit resolution and a total unadjusted error (TUE) of 1% for voltage and 1.25% current. The external input TUE is 0.75%. Digital communications are conducted over I2C, with a choice of nine device addresses.
Because of its wide operating range, the LTC4151 is useful in a range of systems. By integrating the necessary functional blocks in a single chip solution, power monitoring is made practical where discrete solutions are out of the question, due to space or cost.
The LTC4151 is being used in a range of space constrained, low voltage applications. It requires only a few simple connections and is available in MS10 or 3 x 3mm DFN packages. Depending on the system, the monitoring ic could be located on the backplane or a removable card. In fig 2, the LTC4151 is monitoring the input current and voltage to a 12V dc/dc converter. Here, the low voltage input measures the converter's 5V output, with a direct i2c connection to the microprocessor. The only external components needed are a sense resistor, two bus pull up resistors and a resistive divider for 5V measurement on ADIN.
The LTC451 provides relatively straightforward connections while steering clear of the risks associated with low side sensing. Because of its inherent simplicity, low side sensing – where the sense resistor is placed in series with the ground of the load – is an attractive way to monitor supply current. It eliminates the need for a special amplifier by allowing the a/d converter to measure the sense resistor's voltage drop 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, since 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 generally preferred. However, it's difficult to accomplish because the a/d converter must measure the drop across a sense resistor connected to the positive rail, often at a voltage outside the converter's reach. In addition, a small sense resistor drop (20mV/A in this case) is too small for a 12bit converter, as most of the dynamic range would be wasted. A special amplifier is needed that can sense a high positive rail while presenting its output to a ground referenced a/d converter. The LTC4151 not only solves the problem of measurement at a high voltage rail, but due to the sense amplifier's gain of 25, it also does the work of a 16.5bit converter.
The problems of high side current monitoring are compounded as supply voltage increases. The LTC4151 maintains high precision for supplies from 7V to 80V and fig 3 shows the LTC4151 in a 48V 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 90V, which helps the ic survive high voltage transients. This input voltage range allows the device to be connected directly to high voltage supplies without the need of a secondary supply.
When measuring current at the SENSE pins, the maximum TUE is ±1.25%. The full scale current sense voltage is 81.92mV, with 20µV/lsb resolution; more than sufficient for most applications and comparable to, if not better than, the accuracy of discrete solutions. When measuring voltage on Vin through the internal precision attenuator, TUE is ±1% with a full scale voltage of 102.4V and a 25mV/lsb resolution. Finally, when taking a voltage reading on ADIN, TUE is ±0.75% with a full scale voltage of 2.048V and a 25mV/lsb resolution.
In some applications, especially telecoms systems which work off negative voltages and consume large amounts of current, power monitoring may not be straightforward. The LTC4151 can monitor both positive and negative voltages. While it features a shutdown pin to reduce quiescent current to 120µA at 12V for low power applications, the LTC4151-1 replaces this pin with a second i2c data pin that affords simple optoisolation, facilitating its use in high voltage negative applications. Optocouplers allow the host controller to sit at a different ground level from the power monitor. Fig 4 shows how the LTC4151-1 connects to optocouplers in a -48V ATCA application.
Regardless of whether an application requires isolation or not, the LTC4151 provides some convenient reporting features. The i2c interface features a stuck bus reset timer that resets the internal i2c state machine to allow normal communication to resume in the event that i2c signals are held low for more than 33ms. The LTC4151 can also report data continuously or in a single snapshot mode. In continuous scan mode, it measures the voltage between the SENSE pins, at VIN and at ADIN sequentially at a 7.5Hz refresh rate. In snapshot mode, the host controller instructs the LTC4151 to perform a one time measurement on any signal, ideal for applications that only need to measure input power on occasion. Together, the i2c interface and the two reporting modes make the LTC4151 suitable for use in digital power monitoring applications.
The LTC4151 provides a simple, yet effective, way to monitor current, voltage and temperature. High performance building blocks – including the internal sense amplifier, Delta Sigma a/d converter and i2c interface – ensure that digital readings are accurate and precise. High voltage applications can take advantage of the 90V Abs max rating, while the device's flexibility suits users monitoring negative voltages, including an isolation friendly option..
Christopher Gobok is a product marketing engineer with Linear Technology.
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