The leakage current of a BLE SoC at 25°C differs
significantly from that at 85°C or higher as
demonstrated in these supply current graphs
for the EFR32BG22 BLE SoC. Also evident in the
graphs is that supply current can depend a great
deal on the SoC clock frequency. Here the top
graph is for the EM0 active mode while the lower
graph is for the EM1 sleep mode. Both graphs
depict chip current when the internal dc-dc
converter is employed with a 3-V supply.eeworldonline.com | designworldonline.com 4 • 2020 DESIGN WORLD — EE NETWORK 35
EFR32BF22 current consumption in
active mode using dc-dc converterEFR32BF22 current consumption in
sleep mode using dc-dc converterSupply current (mA)Supply current (mA)Temperature (Degrees C)Temperature (Degrees C)001122334455-40-40-20-202020404060608080100100120120001 MHz1 MHz16 MHz16 MHz26 MHz26 MHz38 MHz38 MHzthe EFR32BG22 SoC from Silicon Labs
has a radio-receive current of 2.6 mA
and a transmit current of 3.5 mA at 0
dBm. Note these numbers only relate
to the SoC RF transceiver. At the SoC
level, these currents are slightly higher,
3.6 mA and 4.1 mA respectively. Relying
only on the radio numbers for the SoC
current drain is a common mistake. The
front-page of the device documentation
often must be validated with a thorough
analysis of the data-sheet.
Another example is the CPU power
consumption reported in microamps-
per-megahertz. This number can
become a decisive selection criterion
in the case of intensive compute
applications. It is typically reported in
the best-case scenario, which is often
the maximum frequency of the CPU.
In other words, the value shown in the
data-sheet could prove to be vastly
inaccurate when the SoC CPU works at
a different frequency than that specified
in the manufacturer’s documentation.
A third example is the deep-sleep
current, critical for battery-operated
end products. This number typically
ranges between hundreds of nanoamps
to a few microamps. It is essential
to ensure the deep- sleep current
numbers are associated with the size of
the RAM retained and include the real-
time-clock (RTC) current consumption.
The RTC is used to maintain the timing
necessary for proper BLE operation. In
the case of the EFR32BG22 SoC, the
front page of the data-sheet mentions
a deep-sleep current of 1.05 μA in EM3
mode with 8 kB of RAM retained and
the RTC running from the ULFRCO
(ultra-low-frequency RC oscillator) on-
chip module. The current consumption
section of the data-sheet provides
additional information.
Thus the lack of standardization
for power numbers in datasheets can
produce erroneous comparisons that
could ultimately lead to selecting the
wrong device.UNDERSTANDING APPLICATION
REQUIREMENTS
It is important to consider the
application requirements when
assessing BLE SoCs. Most suppliers tryto represent their numbers responsibly,
but it is impossible to treat all use cases
for a device that might serve in dozens
of different applications. This is where
knowledge of the end application
becomes critical.
Active and sleep currents are key
specifications when selecting a BLE
SoC. These current numbers must
be inserted into a model that closely
matches the application environment to
produce a fair estimate of the average
power consumption. Such models
typically include the ON/OFF duty-cycle,
knowing that a low duty cycle will favor
an SoC with the lowest deep-sleep
current. A high duty cycle will favor an
SoC with the lowest active current.
Another parameter could be
the ambient temperature of the
end product, understanding that
the leakage current of a BLE SoC at
25°C is significantly different from the
leakage at 85°C or higher. The leakage
current at a high temperature can be
a key selection criterion in industrial
applications such as sub-metering,
which need a guaranteed battery life at
high temperatures.
Another important element of
the application relates to the type of
battery technology used (in the context
of battery-operated end products).
The battery powers the on-chip dc-dc
converter integrated in the latest BLE
SoCs. Using the dc-dc converter will
significantly reduce the active current
consumption of the entire SoC. Some
sophisticated SoCs may integrate
separate dc-dc converters for the radio
and for the CPU. This practice provides
an optimized solution, but the trend is
clearly to have only one converter to
minimize the cost of the SoC.
Finally, it is also important to
understand how on-chip or off-chip
memories are used. A common
requirement for BLE end nodes is to
perform over-the-air (OTA) updates of
software. Depending on the size of the
image to be transferred, an external
flash device can be economical. But
its added power consumption and
potential for security problems can,
however, prove to be quite higher
than that when using on-chip flash.SELECTING BLE SoCS