Single Chip Digitizes High-Side Power Measurements
In-circuit power measurements frequently involve a currentsense amplifier or a hot-swap controller and an analog-todigital converter (ADC). Optimally, the current sensing is done on the high side of the load to avoid false grounds. But one problem with that approach is the presence of a high common-mode voltage on the amplifier input. Another is the typical ADC’s limited input voltage range. In addition, the cost of the separate components adds up.
As an alternative, Linear Technology has developed the LTC4151, a high-side power monitor that measures current and input voltages from 7 to 80 V. Its internal 12-bit ADC continuously measures both high-side current and input voltage. Digitized power readings can be read via a two-wire I2C-compatible interface that also controls the chip’s operating mode.
In addition to load power, the data available on the interface includes the LTC4151’s input power data, as well as the value of a third low-voltage input, which can be used to measure a voltage from a thermistor or a fuse. Data can be reported continuously or in a nolatency single-snapshot mode for applications that only need to measure input power occasionally.
The default operating mode is continuous-scan (see the figure). In that mode, after power-up, the ADC continuously and sequentially measures the differential voltage between the SENSE+ and SENSE– pins, the input voltage, and the voltage applied to the ADIN pin. The ADC’s reference voltage is set internally. The ADC output is buffered by onboard registers and subsequently fed to an external controller through the serial bus.
In snapshot mode, the chip makes on-demand measurements and stores the results in internal registers until it receives a read request over the bus. In terms of accuracy, the maximum total unadjusted error (TUE) for the high-side current and voltage measurements is ±1.25% across the –40°C to 85°C industrial temperature range.
There are two versions of the chip. The LTC4151 has a dedicated shutdown pin, while the LTC4151-1 has data pins with an inverted output to drive optoisolators. Both chips are available today in MSOP-10 and 3- by 3-mm DFN-10 packages. Pricing starts at $2.60 in 1000-piece quantities.
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