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.
Linear Technology • www.linear.com
