current drawn by the application after stepping down the
supply voltage to 2.1 V. The higher the battery voltage,
the more power is saved due to the efficient step-down
conversion. At the typical LiSOCl2 battery terminal
voltage of 3. 6 V, overall current consumption goes down
by 30 percent compared to the direct battery connection.
Peak Power For Wireless Transmission
Besides the low IQ aspect, the sensor must communicate
the gathered and processed data to a base station. For
example, this can be a local data concentrator, which is
common for smart gas sensors in an apartment building.
Besides the wireless metering bus (wireless M-Bus),
this also can be the available global system for mobile
communications (GSM) infrastructure used for field
sensor nodes on motorway bridges.
A typical mode of operation is gathering and processing
data throughout the day, then transmitting the collected
data up to a few times a day. From a power perspective,
this means that a low average current consumption in the
range of microamperes is mostly needed, with a rare need
of higher currents for several milliseconds.
The amount of energy needed for data transmission
depends on the range and, therefore, the radio frequency
protocol. Widely used standards are wireless M-Bus and
A comparison of three common standards is
shown in Table 1. Each standard has a typical radio
amplifier power condition and the required current for
Table 1: Power properties comparison of several wireless
In some cases, currents up to 2.5A are required by
the radio amplifier. This amount of current cannot be
delivered by the battery types described. Even currents of
more than 5 mA should be avoided in order to not reduce
the lifetime of a LiSOCl2 bobbin-type battery.
Energy Buffering Concept
To enable pulsed-load operation as described, new power
management concepts need to be considered. Since the
battery cannot deliver the necessary current, the required
energy needs to be stored when the radio is inactive so it
can be used when the radio is active. To achieve this, a
new power concept needs to be designed to buffer energy
and decouple load peaks from the battery. An excellent
medium for buffering energy are storage capacitors
because of their high-energy density and large capacitance.
When using a switch-mode power supply (SMPS), a
capacitor can be efficiently charged with different voltage
than the battery. This can be done in a current-limited
operation, which then defines the load current for the
Once energy is stored in a capacitor, voltage is
converted to the desired value, for example 1.9 V for the
microcontroller SoC or 3. 7 V for the radio power amplifier.
This conversion takes energy from the buffer capacitor and
decouples the load from the battery (Figure 2).
Figure 2: Capacitor storage concept
When using a SMPS buffering power architecture, two
basic concepts to store energy apply:
Boost – storage – buck
Buck – storage – boost
Concept one steps-up the battery voltage to a higher
voltage and charges a capacitor. Then the voltage is
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